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Learn A Levels, GCSE, KS3 & KS2 2x Faster with Seneca

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Learn A Levels, GCSE, KS3 & KS2 2x Faster with Seneca

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Exam question practice with video solutions

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Exam question practice with video solutions

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Use the past paper tab to find the video solutions - made by an ex student at CSF.

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OCR (A) A-level Chemistry Revision - PMT

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Revision for OCR (A) Chemistry AS and A-Level, including summary notes, worksheets and past exam questions for each topic and paper.

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Useful for organic chemistry revision

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Helping you to understand Chemistry

I have removed the Google search box because it was giving problems. Follow this link to find out how you can still search Chemguide using keywords.

NEW! Chemistry for 14 - 16 year old students doing courses such as GCSE in the UK. I am still writing this section, but everything on the site so far should work.

Support pages for CIE (Cambridge International) A level students and teachers.

© Jim Clark 2009 (last modified August 2017)

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chemguide: helping you to understand Chemistry - Main Menu

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Main menu of a site aimed to help advanced level chemistry students to understand chemistry

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Find out more about the chemical elements

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Tables charting the chemical elements have been around since the 19th century - but this modern version has a short video about each one.

We've done all 118 - but our job's not finished. Now we're updating all the videos with new stories, better samples and bigger experiments.

Plus we're making films about other areas of chemistry, latest news and occasional adventures away from the lab.

All these videos are created by video journalist Brady Haran, featuring real working chemists from the University of Nottingham.

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The Periodic Table of Videos - University of Nottingham

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Tables charting the chemical elements have been around since the 19th century - but this modern version has a short video about each one. We've done all 118 - but our job's not finished. Now we're updating all the videos with new stories, better samples and bigger experiments.

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Titration level 1 Titration level 2 Titration level 3 Titration level 4

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Aspirin level 1 Aspirin level 2 Aspirin level 3 Aspirin level 4

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Cisplatin is a chemotherapy drug used to treat testicular, ovarian, bladder, head and neck, lung and cervical cancer.

You will be given cisplatin in a chemotherapy day unit or during a stay in hospital. A chemotherapy nurse will give it to you. Cisplatin can be given in combination with other cancer drugs or with radiotherapy.

During treatment you usually see a cancer doctor, a chemotherapy nurse or a specialist nurse, and a specialist pharmacist. This is who we mean when we mention doctor, nurse or pharmacist in this information.

Before or on the day of treatment, a nurse or person trained to take blood (phlebotomist) will take a blood sample from you. This is to check that your blood cells are at a safe level to have chemotherapy.

You will see a doctor or nurse before you have chemotherapy. They will talk to you about your blood results and ask you how you have been feeling. If your blood results are okay, the pharmacist will prepare your chemotherapy. Your nurse will tell you when your treatment is likely to be ready.

Your nurse usually gives you anti-sickness (anti-emetic) drugs before the chemotherapy.

Cisplatin is given into a vein (intravenously). The nurse will put:

a short thin tube into a vein in your arm or hand (cannula)

a fine tube under the skin of your chest and into a vein close by (central line)

a fine tube into a vein in your arm and through into a vein in your chest (PICC line).

Your nurse will give the cisplatin as a drip (infusion) that is attached to your cannula or line. They will usually put the drip through a pump. This will give you the treatment over a set time. Cisplatin can affect the kidneys. You will be given extra fluids through the drip before and after the cisplatin. You may also have a drug called mannitol. The fluids and mannitol can help prevent kidney damage. Your nurse will explain how long it will take to have the extra fluids. They will also give you advice about how much fluid to drink for the next 24 hours.

We explain the most common side effects of this treatment here. We also include some less common side effects. You may get some of the side effects we mention, but you are unlikely to get all of them.

You may also have some side effects that we have not listed here. Always tell your doctor, nurse or pharmacist about any side effects you have.

Your doctor can give you drugs to help control some side effects. It is important to take them exactly as your nurse or pharmacist explains. This means they will be more likely to work for you. Your nurse will give you advice about managing your side effects. After your treatment is over, most side effects start to improve.

Some people may have side effects while they are being given the chemotherapy or shortly after they have it:

Allergic reaction

Some people have an allergic reaction while having this treatment. Signs of a reaction can include:

feeling hot or flushed

a skin rash

itching

shivering

feeling dizzy

a headache

feeling breathless

swelling of your face or mouth

pain in your back, tummy or chest.

Your nurse will check you for signs of a reaction during your treatment. If you feel unwell or have any of these signs, tell them straight away. If you do have a reaction, it can be treated quickly.

Sometimes a reaction can happen a few hours after treatment. If you get any signs or feel unwell after you get home, contact the hospital straight away.

The drug leaks outside the vein

If the drug leaks outside the vein, it can damage the surrounding tissue. This is called extravasation. Extravasation is not common but if it happens it is important to treat it quickly. Tell your nurse straight away if you have any stinging, pain, redness or swelling around the vein.

If you get any of these symptoms after you get home, contact the doctor or nurse straight away on the contact telephone number they gave you.

Risk of infection

This treatment can reduce the number of white blood cells in your blood. These cells fight infection.

If the number of white blood cells is low, you are more likely to get an infection. A low white blood cell count is called neutropenia.

If you have an infection, it is important to treat it as soon as possible. Contact the hospital straight away on the 24-hour contact number you have if:

your temperature goes over 37.5°C (99.5°F)

you suddenly feel unwell, even with a normal temperature

you have symptoms of an infection.

Symptoms of an infection include:

feeling shivery

a sore throat

a cough

diarrhoea

needing to pass urine often.

It is important to follow any specific advice your cancer treatment team gives you.
The number of white blood cells will usually return to normal before your next treatment. You will have a blood test before having more treatment. If your white blood cell count is low, your doctor may delay your treatment for a short time.

Bruising and bleeding

This treatment can reduce the number of platelets in your blood. Platelets are cells that help the blood to clot.

Tell your doctor if you have any bruising or bleeding that you cannot explain. This includes:

nosebleeds

bleeding gums

tiny red or purple spots on the skin that may look like a rash.

Some people may need a drip to give them extra platelets. This is called a platelet transfusion.

Feeling sick

You may feel sick in the first few days after this treatment. Your doctor will give you anti-sickness drugs to help prevent or control sickness. Take the drugs exactly as your nurse or pharmacist tells you. It is easier to prevent sickness than to treat it after it has started.

If you feel sick, take small sips of fluids and eat small amounts often. If you continue to feel sick, or if you vomit more than once in 24-hours, contact the hospital as soon as possible. They will give you advice and may change the anti-sickness drug to one that works better for you.

Effects on the kidneys

Cisplatin can affect how your kidneys work. You will have blood tests before and during treatment to check this.

Before and after each treatment, your nurses will give you extra fluids through a drip. This is to protect your kidneys. It is also important to drink at least 2 litres (3½ pints) of fluids each day.

If you pass less urine than usual, tell your nurse.

Diarrhoea

This treatment may cause diarrhoea. Diarrhoea means passing more stools (poo) than is usual for you, or having watery or loose stools. If you have a stoma, it will be more active than usual.

If you have diarrhoea:

try to drink at least 2 litres (3½ pints) of fluids each day

avoid alcohol, caffeine, milk products, high-fat foods and high-fibre foods

contact the hospital for advice.

Effects on the heart beat

Cisplatin can cause fast, irregular or slow heart beats. Tell your doctor if you notice any changes.

Rarely, cisplatin can cause more serious heart problems. Contact a doctor straight away if you:

have pain or tightness in your chest

feel breathless or dizzy

feel your heart is beating too fast

feel your heart is beating too slowly.

Other conditions can cause these symptoms, but it is important to get them checked by a doctor.

Numb or tingling hands or feet (peripheral neuropathy)

This treatment affects the nerves, which can cause numb, tingling or painful hands or feet. You may find it hard to fasten buttons or do other fiddly tasks.

Tell your doctor if you have these symptoms. They sometimes need to lower the dose of the drug. The symptoms usually improve slowly after treatment finishes, but for some people they may never go away. Talk to your doctor if you are worried about this.

Blood clot risk

Cancer and some cancer treatments can increase the risk of a blood clot. Symptoms of a blood clot include:

pain, redness or swelling in a leg or arm

breathlessness

chest pain.

If you have any of these symptoms, contact a doctor straight away.

A blood clot is serious, but can be treated with drugs that thin the blood. Your doctor or nurse can give you more information.

Other medicines

Some medicines can affect how this treatment works or be harmful when you are having it. Always tell your cancer doctor about any drugs you are taking or planning to take, such as:

medicines you have been prescribed

medicines you buy in a shop or chemist

vitamins, herbal drugs and complementary therapies.

Tell other doctors, pharmacists or dentists who prescribe or give you medicines that you are having this cancer treatment.

You can visit the electronic Medicines Compendium (eMC) for more detailed information about your treatment.

Vaccinations

Your doctor or nurse may talk to you about vaccinations. These help reduce your risk of getting infections.

Doctors usually recommend that you have a flu jab, which is an inactivated vaccination. People with weak immune systems can have this type of vaccination.

If your immune system is weak, you need to avoid live vaccinations such as shingles. Your cancer doctor or GP can tell you more about live vaccinations.

Fertility

Some cancer drugs can affect whether you can get pregnant or make someone pregnant. If you are worried about this, it is important to talk with your doctor before you start treatment.

Medical and dental treatment

If you need medical treatment for any reason other than cancer, always tell the doctors and nurses that you are having cancer treatment. Give them the contact details for your cancer doctor so they can ask for advice.

If you think you need dental treatment, talk to your cancer doctor or nurse. Always tell your dentist you are having cancer treatment.

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Cisplatin (chemotherapy drug) - treatment and side effects

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Cisplatin is a chemotherapy drug used to treat testicular, ovarian, bladder, head and neck, lung and cervical cancer. You will be given cisplatin in a chemotherapy day unit or during a stay in hospital. A chemotherapy nurse will give it to you. Cisplatin can be given in combination with other cancer drugs or with radiotherapy.

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Why we need to ensure we synthesis the correct isomers

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Thalidomide changed our relationship with new medicines for ever.

It took five years for the connection between thalidomide taken by pregnant women and the impact on their children to be made. Not only did thalidomide change people's lives, but it resulted in tighter drug testing and reporting of side-effects.

Thalidomide is a drug that was developed in the 1950s by the West German pharmaceutical company Chemie Grünenthal GmbH. It was originally intended as a sedative or tranquiliser, but was soon used for treating a wide range of other conditions, including colds, flu, nausea and morning sickness in pregnant women.

During early testing, researchers at the company found that it was virtually impossible to give test animals a lethal dose of the drug (based on the LD50 test). Largely based on this, the drug was deemed to be harmless to humans. Thalidomide was licensed in July 1956 for over-the-counter sale (no doctor's prescription was needed) in Germany.

Thalidomide in the marketplace

Around the world, more and more pharmaceutical companies started to produce and market the drug under license from Chemie Grünenthal. By the mid-1950s, 14 pharmaceutical companies were marketing thalidomide in 46 countries under at least 37 different trade names.

In 1958, thalidomide was produced in the United Kingdom by The Distillers Company (Biochemicals) Ltd, under the brand names Distaval, Tensival, Valgraine and Asmaval. Their advertisement claimed that

Distaval can be given with complete safety to pregnant women and nursing mothers without adverse effect on mother or child.

The drug was prescribed for a range of conditions including pneumonia, colds and flu and for relieving the symptom of nausea often experienced in early pregnancy.

One country that did not approve thalidomide for marketing and distribution was the USA, where it was rejected by the Food and Drug Administration.

Pharmacologist Frances Oldham Kelsey turned down several requests from the distributing company who did not provided clinical evidence to refute reports of patients who developed nerve damage in their limbs after long-term thalidomide use. This prevented the drug thalidomide from ever being used in the United States.

Thalidomide and pregnancy

In the 1950s, scientists did not know that the effects of a drug could be passed through the placental barrier and harm a foetus in the womb, so the use of medications during pregnancy was not strictly controlled. And in the case of thalidomide, no tests were done involving pregnant women.

As the drug was traded under so many different names in 49 countries, it took five years for the connection between thalidomide taken by pregnant women and the impact on their children to be made. A UK Government warning was not issued until May 1962.

One reason why researchers and doctors were slow to make this connection was due to the wide range of changes to foetal development. Limbs, internal organs including the brain, eyesight and hearing could all be affected.

Later, they found that the impact on development was linked to when during pregnancy the drug was taken, and effects only occurred between 20 and 37 days after conception. After that, thalidomide had no effect on the foetus.

Another reason why it took so long to establish the link to thalidomide was that some of the damage caused by the drug was very similar to certain genetic conditions that affect the upper or lower limbs.

The thalidomide scandal

The first time the link between thalidomide and its impact on development was made public was in a letter published in The Lancet from an Australian doctor William McBride, in 1961.

The drug was formally withdrawn by Chemie Grünenthal on 26 November 1961 and a few days later, on 2 December 1961, the UK distributors followed suit. However, it remained in many medicine cabinets under many different names.

In the few short years that thalidomide was available, it's estimated that over 10,000 babies were affected by the drug worldwide. Around half died within months of being born. The thalidomide babies who survived and their families live with the effects of the drug.

The Thalidomide Society was formed in 1962 by the parents of children affected by the drug thalidomide. The original aim of the Society was to provide mutual support and a social network as well as to seek compensation.

In 1968 Chemie Grünenthal was brought to trial in Germany. The company settled the case out of court and arrangements were made to compensate German victims. No one was found guilty of any crimes.

The same year, the British licensee, the Distillers Company, also reached a compensation settlement with the UK victims of the drug. In the UK, payments from Distillers, as well as government compensation, were administered by the Thalidomide Trust.

In 1972, a highly publicised campaign led by the Sunday Times newspaper helped to secure a further settlement for children affected by thalidomide in the UK.

The consequences of thalidomide

Thalidomide forced governments and medical authorities to review their pharmaceutical licencing policies. As a result, changes were made to the way drugs were marketed, tested and approved both in the UK and across the world.

One key change was that drugs intended for human use could no longer be approved purely on the basis of animal testing. And drug trials for substances marketed to pregnant women also had to provide evidence that they were safe for use in pregnancy.

The easy, over-the-counter access to thalidomide prompted many countries to improve their classification and control of medicines. In the UK the 1968 Medicines Act, passed as a result of the thalidomide scandal, made distinctions between prescription drugs, drugs only available in pharmacies and drugs available for general sale.

The Yellow Card Scheme was set up for doctors to share previously unknown side effects of medications they prescribed. The Scheme has now widened so anyone can report a side effect.

In the UK thalidomide is only prescribed by a doctor under strict controls. Women taking thalidomide are required to use two forms of birth control and regular pregnancy tests. Men are required to use contraception when taking thalidomide. People who are prescribed thalidomide undergo counselling and are talked through the risks.

Uses of thalidomide today

This was not the end of the thalidomide story. In 1964 a leprosy patient at Jerusalem's Hadassah University Hospital was given thalidomide when other tranquilisers and painkillers could not help him. His doctor, Jacob Sheskin, noticed the drug had an effect on the patient's leprosy symptoms.

Within three days the leprosy had gone and skin lesions healed. When the patient stopped taking the thalidomide the leprosy returned. The drug seemed to be able to suppress the disease, although it was not a cure.

As a result, the World Health Organisation (WHO) ran a clinical trial on the use of thalidomide for leprosy in 1967. And after more positive results, thalidomide was used as a treatment for leprosy in many countries.

More recently, it has been used successfully to control some AIDS-related conditions, and as a targeted cancer drug for treating cancers such as multiple myeloma.

But the renewed use of thalidomide remains controversial because of its past history.

Find out more

A Leslie Florence, 'Is Thalidomide to Blame?', British Medical Journal, 2 (1960), p 1954

T Stephens and R Brynner, Dark Remedy: The Impact of Thalidomide and its Revival as a Vital Medicine, 2001

B Gault and H Rogers, Look, No Hands! The Inspiring Story of Brian Gault, 2000

The Insight Team of the Sunday Times, Suffer the Children: The Story of Thalidomide, 1979

A Daemmrich, 'A tale of two experts,' Social History of Medicine, 15/1 (2002), p 137-158

H Sjöstrom and R Nilsson, Thalidomide and the Power of the Drug Companies, 1972

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It took five years for the connection between thalidomide taken by pregnant women and the impact on their children to be made. Not only did thalidomide change people's lives, but it resulted in tighter drug testing and reporting of side-effects.

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ChemSci Pick of the Week
Scientists in the UK have investigated the phenomenon of shape-selectivity in shape-shifting molecules - molecules that adopt different configurations depending on their environment.
Most synthetic materials we make and use - such as drugs, plastics or dyes - are based on molecules with fixed structures. Molecules are made up of atoms joined to one another by a series of chemical bonds, and although many molecules can bend or twist to some extent, the atoms are held together in essentially fixed arrangements.

Dr Paul McGonigal from Durham University investigates a class of molecules that have 'shapeshifting' properties. In other words they can totally change their shape in response to their environment. Dr McGonigal's group have looked at the specific scenario where the molecules are dissolved a solution, and then crystallise out into different formations. They have discovered a shape-shifting molecule that can crystallise into an unexpected conformation.

He explains: "The shapeshifting molecules described in this paper are like an unruly game of musical chairs. When the molecules are dissolved, it's like the music is playing - the atoms in the structure switch places with one another and the overall shape of the molecule changes back and forth."

"For now, we have investigated what happens when the molecules form crystals, which is like the music stopping. Just like the players taking their seats, the atoms are no longer mobile and the overall structure becomes fixed. And just as it is difficult to predict where players will end up in a game of musical chairs, our experiments gave unexpected results. By studying these outcomes, we have started to learn about the factors that control this game of molecular musical chairs, i.e., what controls the preference for one of the rearranged molecules over another."

What makes this more interesting is that in the case of these special molecules, the environment can cause the less-favoured configuration of the molecule to form. Normally one configuration is more stable than another, but in this instance the less stable configuration can form, given the right conditions.

Crystallisation is one obvious way in which the shape of a molecule can become fixed. However there are other possibilities. For example a drug molecule may adapt its structure to match the active site on an enzyme. Dr McGonigal's work relates to the rather simple case of crystals, but he believes that this could lead to greater understanding of more complex environments.

"With more time, we will be able to apply this knowledge to understand how the molecules respond in more complex, messy environments (e.g. inside a biological cell), that are more likely to encountered in real life."

This article is free to read in our open access, flagship journal Chemical Science: Aisha Bismillah et al., Chem. Sci., 2018, Advance Article. DOI: 10.1039/C8SC04303E. You can access all of our ChemSci Picks in this article collection.

Read more like this

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Scientists in the UK have investigated the phenomenon of shape-selectivity in shape-shifting molecules - molecules that adopt different configurations depending on their environment. Most synthetic materials we make and use - such as drugs, plastics or dyes - are based on molecules with fixed structures.

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Two scientists have been awarded the 2020 Nobel Prize in Chemistry for developing the tools to edit DNA.

Emmanuelle Charpentier and Jennifer Doudna are the first two women to share the prize, awarded for their work on the technology of genome editing.

Their discovery, known as Crispr-Cas9 genetic scissors, is a way of making specific and precise changes to the DNA contained in living cells.

They are the first women to share the prize without a male collaborator.

The winners will share the prize money of 10 million krona (£861,200; $1,110,400).

Commenting on her win, Prof Emmanuelle Charpentier, from the Max Planck Unit for the Science of Pathogens in Berlin, said she was emotional on learning about the award.

"When it happens, you're very surprised, and you think it's not real. But obviously it's real," she said.

During Emmanuelle Charpentier's studies of the bacterium Streptococcus pyogenes, she discovered a previously unknown molecule called tracrRNA. Her work showed that tracrRNA is part of the organism's immune defences.

This system, known as CRISPR/Cas, disarms viruses by cleaving their DNA.

In 2011, the same year she published this work, Charpentier began a collaboration with Prof Doudna, from the University of California, Berkeley, to recreate the bacterium's genetic scissors in a test tube. They also worked on simplifying the scissors' molecular components so they were easier to use.

In their natural form, the bacterial scissors recognise DNA from viruses. But Charpentier and Doudna showed that they could be reprogrammed to cut any DNA molecule at a predetermined site. Cutting the DNA then allows the code of life to be rewritten.

Commenting on the discovery, biological chemist Pernilla Wittung-Stafshede, said: "The ability to cut DNA where you want has revolutionised the life sciences."

Chemist Claes Gustafsson added: "We can edit any genome, we can ask all kinds of questions," adding that it could be harnessed to treat genetic diseases.

On being one of the first two women to share the prize, Prof Charpentier said: "I wish that this will provide a positive message specifically for young girls who would like to follow the path of science...and to show them that women in science can also have an impact with the research they are performing."

She continued: "This is not just for women, but we see a clear lack of interest in following a scientific path, which is very worrying."

Swedish industrialist and chemist Alfred Nobel founded the prizes in his will, written in 1895 - a year before his death.

Follow Paul on Twitter.

Previous winners of the Nobel Prize in Chemistry

2019 - John B Goodenough, M Stanley Whittingham and Akira Yoshino share the prize for their work on lithium-ion batteries.

2018 - Discoveries about enzymes earned Frances Arnold, George P Smith and Gregory Winter the prize

2017 - Jacques Dubochet, Joachim Frank and Richard Henderson were awarded the prize for improving images of biological molecules

2016 - Jean-Pierre Sauvage, Fraser Stoddart and Bernard Feringa shared the prize for the making machines on a molecular scale.

2015 - Discoveries in DNA repair earned Tomas Lindahl and Paul Modrich and Aziz Sancar the award.

2014 - Eric Betzig, Stefan Hell and William Moerner were awarded the prize for improving the resolution of optical microscopes.

2013 - Michael Levitt, Martin Karplus and Arieh Warshel shared the prize, for devising computer simulations of chemical processes.

2012 - Work that revealed how protein receptors pass signals between living cells and the environment won the prize for Robert Lefkowitz and Brian Kobilka.

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Genome editing wins Nobel chemistry prize

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Two scientists have been awarded the 2020 Nobel Prize in Chemistry for developing the tools to edit DNA. Emmanuelle Charpentier and Jennifer Doudna are the first two women to share the prize, awarded for their work on the technology of genome editing.

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Helen Sharman trained as a Chemist before becoming Britain's first astronaut.

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"We should push forward not only our own individual boundaries, but also the boundaries of what humans believe is possible. People are the biggest limitations in our own lives. There's a huge amount we can do and we should make the best use of our lives for the benefit of the world."

Helen Sharman CMG OBE

Helen Sharman became the first British astronaut in May 1991 when she launched on a Soyuz spacecraft to spend eight days orbiting the Earth, most of that time on the Mir Space Station.

After the Soviet Space Agency reached an agreement with a company set up to manage the British Mission, Helen was one of two Britons selected for astronaut training out of almost 13,000 applicants. The programme, named Project Juno, was a co-operative arrangement and would enable Britain to send one of its people into space for the first time.

Helen underwent a rigorous selection process, which focussed on psychological and medical assessments, including tolerance to high g and motion sickness, technical understanding, practical skills and the ability to learn a foreign language. This was followed by 18 months of intensive training in Star City near Moscow, where learning to speak Russian and getting to know the cosmonauts' families were part of her new way of life, along with preparing for weightlessness, learning how to cope inside a cramped space capsule and how to deal with a return to Earth that might end up in the sea.

During the launch, Helen carried out certain spacecraft operations. Once in space, Helen's tasks included medical, agricultural and chemical experiments, materials' testing, Earth observation work and operating an amateur radio link with British school students. She also took some seeds into space with her that she brought back to Earth for British school students to use as part of a UK-wide experiment to investigate the effects of space travel on the seeds as compared with a control sample.

Coping with risk was a daily activity and teamwork was a vital element in the success of the Mission. Fitted in with other activities were media interviews and a telephone conversation with President Mikhail Gorbachev.

Helen was not quite 28 years old when she became an astronaut. Born in Sheffield, Helen attended local schools and read Chemistry at the University of Sheffield. She started her career in industry immediately after graduating, working in research and development for GEC in Hammersmith, London and later for Mars Confectionery in Slough. After her return from space, Helen spent many years communicating science and its benefits by speaking, presenting on radio and television and by organising science events for the public. More recently, she has worked as a manager at the National Physical Laboratory in Teddington and at Kingston University London. Currently, Helen is the Operations Manager for the Chemistry Department at Imperial College London.

Helen has not returned to Space although, like every other astronaut, she would love to be up there again, experiencing the weightlessness, the camaraderie and the views.

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"We should push forward not only our own individual boundaries, but also the boundaries of what humans believe is possible. People are the biggest limitations in our own lives. There's a huge amount we can do and we should make the best use of our lives for the benefit of the world."

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Practice and further reading on shapes of molecules

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Author: J. M. McCormick*

Last Update: January 31, 2011
Introduction
Dalton's Atomic Theory revolutionized chemistry by explaining chemical properties in terms of small, indivisible pieces of matter called atoms that are linked together to form polyatomic species (both ions and molecules). As chemists explored the properties of the polyatomic species, it became clear that they have size and shape and that shape is particularly important in explaining their physical properties and why and how chemical reactions occur. A summary of some of the commonly observed polyatomic shapes (also known as structures or geometries) are shown in Table 1 arranged by the number of atoms around the "central atom." These are idealized structures; real molecules seldom exhibit these idealized shapes. However, the ideal shapes are good starting points toward understanding how the spatial arrangement of atoms in polyatomic species affect their properties and chemistry.

Table 1. Summary of idealized common structures of molecules and polyatomic ions.

Chemists often find it convenient to build a model of a molecule or polyatomic ion to help them visualize its actual shape, much in the same as an architect will build a model of a building to help him or her see its structure. The complexity of the model a chemist will use is dictated by the answer sought. And the explanation as to why a molecule or polyatomic ion has a certain structure depends on the theory used.

The simplest model is a Lewis dot structure which gives us an approximate picture of the bonding interactions that hold the polyatomic species together. It does not, however, give an accurate picture of structure or the electrons' energies. Because Lewis dot structures assume that all valence electrons are paired and a chemical bond requires two electrons, it is limited to simple compounds of the main group elements (s and p blocks) and some transition metal ions (those with d ⁰ and d ¹⁰ configurations). A Lewis dot structure can be used to obtain a rough picture of structure by using Valence Shell Electron Pair Repulsion (VSEPR) theory.

IIn VSEPR theory we assume that the electron pairs, whether in bonds or in lone pairs, take up space and repel each other. The final arrangement of the atoms is determined in VSEPR theory by the jostling of the electrons for space. VSEPR theory gives reasonable predictions of structure and can explain why certain polyatomic species are distorted relative to an ideal geometry. It suffers from the same limitations as Lewis dot structures. And the structures that VSEPR predicts are really only valid in the gaseous state because it ignores interactions between species.

VSEPR theory only predicts structure and cannot be used, by itself, to describe the places where electrons are allowed to be (i. e., the molecular orbitals). Valence Bond theory allows us to take a VSEPR structure (or a real structure) and get a rough idea of how the electron density is distributed in bond. As with the other models a number of simplifying approximations have been made and we still cannot get energies for a polyatomic species. The hybrid orbitals that are used to explain bonding in Valence Bond theory do give us a reasonable approximation of how the electrons distribute themselves for many chemical species (the molecular orbitals). For relatively simple systems, like compounds formed by elements of the first two periods, the combination of Lewis dot, VSEPR and Valence Bond theories is sufficient to explain most of their chemistry.

The most complete theory is Molecular Orbital (MO) theory which considers the energies of the atomic orbitals and how well the atomic orbitals on different atoms overlap. In MO theory it is the balance between orbital energy and overlap that ultimately determines structure. MO theory can give a very accurate description of structure, electron arrangement and molecular energies, but it is neither quick nor simple to apply. So unless a detailed picture of molecular structure is required, one of the simpler models is used.

This exercise will give you practice in using Lewis dot structures, VSEPR and Valence Bond theory models. We will also be exploring the ramifications of molecular shape and how to describe shape in mathematical terms. NOTE! Your instructor may use this exercise in any one of a number of ways. Be sure that you understand what he or she wants you do before coming to laboratory.
Experimental
Do not write this exercise up in your notebook. There are worksheets for the eight groups of molecules and ions (click on the links below to obtain these worksheets in PDF format, print them out and bring them to laboratory).

In this exercise you will be using a traditional model kit to build models of a number of molecules and polyatomic ions. Most of the balls in the kit have only one hole drilled in them. These are used to model atoms that have only one connection to another atom where we can ignore the disposition of any lone pairs (i. e., terminal atoms). There are balls drilled for the ideal angles for tetrahedral, trigonal bipyramidal and octahedral geometries. Your kit may have red balls, representing oxygen, drilled with only two holes instead of four because the positions of the lone pairs have already been taken into account. See Table 2 for the correspondence between ball color and element.

Table 2. Key to the atoms in the molecular model kit. Any color may be used to represent any element not listed, as long as the number of bonds and the disposition of the lone pairs match.

The stiff, gray connectors are used for single bonds and the flexible, gray connectors are used for multiple bonds. Do not use the short white connectors (they are hard to get out). Because the connector representing bonds are the same size and the balls used to model the atoms are not to scale, the models give inexact representations of the molecules and ions. But, they are useful for observing the connectivity and the three-dimensional arrangement of atoms in space.

If directed by your instructor, before coming to laboratory draw Lewis dot structures for each of the species on the worksheets (use a pencil!). Show all contributing resonance structures, where appropriate (use the back of the page, if necessary), and consider formal charges as needed. Click here to review Lewis dot structures.

Predict the electron pair geometry (also known as the "stereochemical formula"), and the molecular geometries for each chemical species using VSEPR. Below the picture of each molecule write the name of the geometry (e. g. linear, trigonal planar, etc.). Although you do not need to name the molecular shape for molecules and ions with more than one "central atom", you should be able to indicate the molecular geometry about each "central atom." Click here to review VSEPR theory.

During lab construct a molecular model, using the kit provided, for each species listed in the tables. Do not make models for species where resonance is important (e. g., species in Group B and Group E) because these models will mislead you into thinking that there are double and single bonds in these species where there are not. Verify that your predicted geometries are the same as what you obtain with the models. Correct any errors in your table, and use the models as an aid to fill in any blank spaces. When writing the electron pair or molecular geometry for a species with a double or triple bond, just draw two or three straight lines, not curved lines.

After you have made a model check to see if it is included in Group H. If it is, examine its symmetry and use the table in Group H to find the different ways the molecule or ion can be rotated, reflected or inverted to give a configuration that is identical to the starting configuration. Fill in the Group H table.

Predict whether the molecule is "polar" or "non-polar". You do not need to indicate polarity for species with an entry of "-" in the polarity column. For ions this is because the electrostatic forces involved are much stronger than forces involving dipoles.
Results and Analysis
Click here to download a worksheet of questions to answer after you have finished this exercise.
Conclusions
There are no conclusions to write for this exercise.
References

1. Click here to obtain this file in PDF format. Note that hyperlinks are not active in the pdf version.

2. Zumdahl, S. S. Chemical Principles, 4th Ed.; Houghton-Mifflin: New York, 2002, chapters 13 and 14.

3. Gillespie, R. J. and Popelier, P. L. A. Chemical Bonding and Molecular Geometry: from Lewis to Electron Densities; Oxford University Press: New York, 2001.

4. Gray, H. B. Electrons and Chemical Bonding; W. A. Benjamin: New York, 1964.

5. Ballhausen, C. J. and Gray, H. B. Molecular Orbital Theory; Benjamin-Cummings: Reading, MA, 1964.

6. Pauling, L. The Nature of the Chemical Bond and the Structure of Molecules and Crystals; Cornell University Press: Ithaca, NY, 1960.

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Author: J. M. McCormick* Last Update: January 31, 2011 Introduction Dalton's Atomic Theory revolutionized chemistry by explaining chemical properties in terms of small, indivisible pieces of matter called atoms that are linked together to form polyatomic species (both ions and molecules).

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6 questions about hydrogen fuel cell cars: you're just a click away from the answers

Electric vehicles can deliver a real contribution to climate change mitigation, provided the entire value chain is sustainable.

Crash tests have shown that the hydrogen tanks are not damaged in a collision and that no hydrogen escapes.

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Hydrogen fuel cell technology: the customer's choice

Each driver has different wants and needs when it comes to mobility. Oliver Zipse, CEO of BMW AG, puts it like this: "For us, the central questions are: Which kinds of propulsion and technology will our customers want in the future? And how do we realize their preferences with the maximum possible climate protection?" That's why BMW will continue to focus on a range of different propulsion concepts - the classic combustion engine, fully electric vehicles and plug-in hybrids, and more research into hydrogen fuel cell cars.

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Hydrogen cars, fuel cells, etc.: what you need to know | BMW.com

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Ligand EDTA and its anticoagulant properties

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Ligand EDTA and its anticoagulant properties

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What went wrong in the trial of OJ Simpson?

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So...What Went Wrong?
Evidence Collection
From the beginning, there were issues involving evidence collection. An important bloody fingerprint located on the gateway at Nicole Brown's house was not properly collected and entered into the chain of custody when it was first located. Although it was documented in his notes by Detective Mark Fuhrman, one of the first to arrive on the scene, no further action was taken to secure it.

The detectives who took over Fuhrman's shift apparently were never aware of the print and eventually, it was lost or destroyed without ever being collected. Other items of evidence were also never logged or entered into the chain of custody, which gave the impression that sloppy forensic collection had been carried out at the scene.

The prosecution had expert witnesses who testified that the evidence was often mishandled. Photos were taken of critical evidence without scales in them to aid in measurement taking. Items were photographed without being labeled and logged, making it difficult, if not impossible, to link the photos to any specific area of the scene. Separate pieces of evidence were bagged together instead of separately, causing cross-contamination. Wet items were also packaged before allowing them to dry, causing critical changes in evidence. Police even used a blanket that came from inside the house to cover Nicole Brown's body, contaminating the body and anything surrounding it. Beyond poor evidence collection techniques, sloppy maneuvering at the scene caused more bloody shoe prints to be left behind by LAPD than by the perpetrator.

Securing the Evidence
Throughout the investigation, there were issues with how evidence was secured. There was about 1.5 mL of O.J. Simpson's blood assumed missing from a vial of evidence. The LAPD could not counter the idea of "lost blood" because there was no documentation of how much reference blood was taken from Simpson as evidence. The person who drew the blood could only guess he had taken 8 mL; only 6 mL could be accounted for by the LAPD.

To add to the problem, the blood was not immediately turned over as evidence but was carried around for several hours before it was entered into the chain of custody, allowing for speculation of when and how the 1.5 mL of blood may have disappeared.

The security of LAPD storage and labs was also brought under scrutiny when it was discovered that some pieces of evidence had been accessed and altered by unauthorized personnel. Simpson's Bronco was entered at least twice by unauthorized personnel while in the impound yard; Nicole Simpson's mother's glasses had a lens go missing while it was in the LAPD facility.

A Question of Planted Evidence
Not only were there many claims that the evidence was mishandled in the police lab but there were also claims that evidence was planted at the crime scene. Because the police department did not have proper collection documents regarding Simpson's blood, it was argued that the police planted Simpson's missing blood on critical evidence and in critical areas of the murder scene.
The defense team stated that EDTA was found in the samples of blood that were collected at the crime scene. EDTA is a blood fixer (anticoagulant) used in labs and mixed with collected blood. If evidence with Simpson's blood showed traces of EDTA, the defense claimed, then that blood had to have come from the lab, which meant that it was planted.

However, EDTA is also a chemical found naturally in human blood and chemicals such as paint. At the time, tests were not readily available to differentiate between natural and contaminant EDTA or the differences in the level of EDTA in blood. Some believe that the positive EDTA results may have been due to contamination of the equipment used to run the tests.

A Question of Character
Detective Fuhrman was discredited by the prosecution when he was alleged to be a racist and accused of planting evidence. When asked if he had falsified police reports or planted evidence in the Simpson case, he invoked his 5th Amendment rights against self-incrimination. Fuhrman was accused of planting critical evidence, contaminating it with Simpson's blood, and falsifying police records. In Fuhrman's book, he stated that at one point he was even accused of killing Nicole Brown and Ron Goldman himself. This put anything he touched in the investigation under scrutiny.

Understanding Forensic Science
A major hurdle that the prosecution team failed to overcome was the lack of knowledge and understanding regarding forensics, specifically the relatively new science of DNA. The jurors agreed that the DNA testimony was difficult to appreciate since expert witnesses were not able to put their evidence in terms that the jury could understand.

This inability to understand key evidence made the evidence essentially useless; even some seasoned lawyers found the scientific testimonies to be incomprehensible. It is reported that the DNA evidence showed that the chance that some of the blood found near the bodies came from anyone but Simpson was 1 in 170 million. The chance that blood found on Simpson's sock could be from someone other than Nicole Brown was 1 in 21 billion. Blood samples found inside of Simpson's Bronco, which was discovered outside Simpson's home the next day, were equally matched to Simpson and both victims. Such evidence should have resulted in an open and shut case by today's standards but was not made clear enough to understand at the time.

What happened in the trial of O.J. Simpson that led to his acquittal?
The role of the jury is to listen to both sides of the case (prosecutor and defense). The jurors have to unanimously decide guilt or innocence. Whatever the outcome, the jurors must feel that their decision is beyond a reasonable doubt. This was particularly difficult to achieve in this case. Going in, the public were already influenced by Simpson's likeability and star power as a pro football player and beloved celebrity. Changing that initial perception was going to be tough. While the abundance of evidence certainly provided more than enough to do so, the doubts cast by the sloppy police work were enough of a window. Additionally, some jurors have since admitted that the verdict was retribution for the acquittal of white police officers in the beating of Rodney King in 1992.

More information about the O.J. Simpson case can be found here.

Back to Crime Library

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Forensics at the OJ Simpson Trial - Crime Museum

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So...What Went Wrong? Evidence Collection From the beginning, there were issues involving evidence collection. An important bloody fingerprint located on the gateway at here. Back to Crime Library

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Good intro to A level chemistry terms

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Buy What is Chemistry? by Atkins, Peter (ISBN: 9780199683987) from Amazon's Book Store. Everyday low prices and free delivery on eligible orders.

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Page ID

3707

1. White light and Colors

2. The Origin of Color in Complex Ions containing transition metals

1. Octahedral Complexes

2. Tetrahedral Complexes

3. Factors Affecting the Color of Transition Metal complexes

1. The nature of the ligand

2. The oxidation state of the metal

3. The coordination of the Ion

4. Contributors and Attributions

This page is going to take a simple look at the origin of color in complex ions - in particular, why so many transition metal ions are colored.

White light and Colors

If you pass white light through a prism it splits into all the colors of the rainbow. Visible light is simply a small part of an electromagnetic spectrum most of which we cannot see - gamma rays, X-rays, infra-red, radio waves and so on. Each of these has a particular wavelength, ranging from 10⁻¹⁶ meters for gamma rays to several hundred metersfor radio waves. Visible light has wavelengths from about 400 to 750 nm. (1 nanometer = 10-9 meters.)

Figure: The diagram shows an approximation to the spectrum of visible light.

Example 1: Blue Color of Copper (II) Sulfate in Solution

If white light (ordinary sunlight, for example) passes through copper(II) sulfate solution, some wavelengths in the light are absorbed by the solution. Copper(II) ions in solution absorb light in the red region of the spectrum. The light which passes through the solution and out the other side will have all the colors in it except for the red. We see this mixture of wavelengths as pale blue (cyan). The diagram gives an impression of what happens if you pass white light through copper(II) sulfate solution.

Working out what color you will see is not easy if you try to do it by imagining "mixing up" the remaining colors. You wouldn't have thought that all the other colors apart from some red would look cyan, for example. Sometimes what you actually see is quite unexpected. Mixing different wavelengths of light doesn't give you the same result as mixing paints or other pigments. You can, however, sometimes get some estimate of the color you would see using the idea of complementary colors.

If you arrange some colors in a circle, you get a "color wheel". The diagram shows one possible version of this.

colors directly opposite each other on the color wheel are said to be complementary colors. Blue and yellow are complementary colors; red and cyan are complementary; and so are green and magenta. Mixing together two complementary colors of light will give you white light. What this all means is that if a particular color is absorbed from white light, what your eye detects by mixing up all the other wavelengths of light is its complementary color. Copper(II) sulfate solution is pale blue (cyan) because it absorbs light in the red region of the spectrum. Cyan is the complementary color of red.

We often casually talk about the transition metals as being those in the middle of the Periodic Table where d orbitals are being filled, but these should really be called d block elements rather than transition elements (or metals). This shortened version of the Periodic Table shows the first row of the d block, where the 3d orbitals are being filled.

The usual definition of a transition metal is one which forms one or more stable ions which have incompletely filled d orbitals. Zinc with the electronic structure [Ar] 3d Technically, the Sc ¹⁰4s ³⁺ ion does not count as a transition metal ion because its 3d level is empty. ² does not count as a transition metal whichever definition you use. In the metal, it has a full 3d level. When it forms an ion, the 4s electrons are lost - again leaving a completely full 3d level. At the other end of the row, scandium ( [Ar] 3d ¹4s ² ) does not really counts as a transition metal either. Although there is a partially filled d level in the metal, when it forms its ion, it loses all three outer electrons.

Example 2: Hexaaqua Metal Ions

The diagrams show the approximate colors of some typical hexaaqua metal ions, with the formula [ M(H ₂O) ₆ ] n+. The charge on these ions is typically 2+ or 3+.
Non-transition metal ions

These ions are all colorless. Transition metal ions

The corresponding transition metal ions are colored. Some, like the hexaaquamanganese(II) ion (not shown) and the hexaaquairon(II) ion, are quite faintly colored - but they are colored.

So, what causes transition metal ions to absorb wavelengths from visible light (causing color) whereas non-transition metal ions do not? And why does the color vary so much from ion to ion?

The Origin of Color in Complex Ions containing transition metals

Complex ions containing transition metals are usually colored, whereas the similar ions from non-transition metals are not. That suggests that the partly filled d orbitals must be involved in generating the color in some way. Remember that transition metals are defined as having partly filled d orbitals.

Octahedral Complexes

For simplicity we are going to look at the octahedral complexes which have six simple ligands arranged around the central metal ion. The argument is not really any different if you have multidentate ligands. When the ligands bond with the transition metal ion, there is repulsion between the electrons in the ligands and the electrons in the d orbitals of the metal ion. That raises the energy of the d orbitals. However, because of the way the d orbitals are arranged in space, it doesn't raise all their energies by the same amount. Instead, it splits them into two groups. The diagram shows the arrangement of the d electrons in a Cu ²⁺ ion before and after six water molecules bond with it.

Whenever 6 ligands are arranged around a transition metal ion, the d orbitals are always split into 2 groups in this way - 2 with a higher energy than the other 3. The size of the energy gap between them (shown by the blue arrows on the diagram) varies with the nature of the transition metal ion, its oxidation state (whether it is 3+ or 2+, for example), and the nature of the ligands. When white light is passed through a solution of this ion, some of the energy in the light is used to promote an electron from the lower set of orbitals into a space in the upper set.

Each wavelength of light has a particular energy associated with it. Red light has the lowest energy in the visible region. Violet light has the greatest energy. Suppose that the energy gap in the d orbitals of the complex ion corresponded to the energy of yellow light.

The yellow light would be absorbed because its energy would be used in promoting the electron. That leaves the other colors. Your eye would see the light passing through as a dark blue, because blue is the complementary color of yellow.

Tetrahedral Complexes

Simple tetrahedral complexes have four ligands arranged around the central metal ion. Again the ligands have an effect on the energy of the d electrons in the metal ion. This time, of course, the ligands are arranged differently in space relative to the shapes of the d orbitals. The net effect is that when the d orbitals split into two groups, three of them have a greater energy, and the other two a lesser energy (the opposite of the arrangement in an octahedral complex). Apart from this difference of detail, the explanation for the origin of color in terms of the absorption of particular wavelengths of light is exactly the same as for octahedral complexes.

Factors Affecting the Color of Transition Metal complexes

In each case we are going to choose a particular metal ion for the center of the complex, and change other factors. color changes in a fairly haphazard way from metal to metal across a transition series.

The nature of the ligand

Different ligands have different effects on the energies of the d orbitals of the central ion. Some ligands have strong electrical fields which cause a large energy gap when the d orbitals split into two groups. Others have much weaker fields producing much smaller gaps. Remember that the size of the gap determines what wavelength of light is going to get absorbed. The list shows some common ligands. Those at the top produce the smallest splitting; those at the bottom the largest splitting.

The greater the splitting, the more energy is needed to promote an electron from the lower group of orbitals to the higher ones. In terms of the color of the light absorbed, greater energy corresponds to shorter wavelengths. That means that as the splitting increases, the light absorbed will tend to shift away from the red end of the spectrum towards orange, yellow and so on.

There is a fairly clear-cut case in copper(II) chemistry. If you add an excess of ammonia solution to hexaaquacopper(II) ions in solution, the pale blue (cyan) color is replaced by a dark inky blue as some of the water molecules in the complex ion are replaced by ammonia.

The first complex must be absorbing red light in order to give the complementary color cyan. The second one must be absorbing in the yellow region in order to give the complementary color dark blue. Yellow light has a higher energy than red light. You need that higher energy because ammonia causes more splitting of the d orbitals than water does.

It is not often as simple to see as this, though! Trying to sort out what is being absorbed when you have murky colors not on the simple color wheel further up the page is much more of a problem. The diagrams show some approximate colors of some ions based on chromium(III).

It is obvious that changing the ligand is changing the color, but trying to explain the colors in terms of our simple theory is not easy.

The oxidation state of the metal

As the oxidation state of the metal increases, so also does the amount of splitting of the d orbitals. Changes of oxidation state therefore change the color of the light absorbed, and so the color of the light you see. Taking another example from chromium chemistry involving only a change of oxidation state (from +2 to +3):

The 2+ ion is almost the same color as the hexaaquacopper(II) ion, and the 3+ ion is the hard-to-describe violet-blue-grey color.

The coordination of the Ion

Splitting is greater if the ion is octahedral than if it is tetrahedral, and therefore the color will change with a change of co-ordination. The problem is that an ion will normally only change co-ordination if you change the ligand - and changing the ligand will change the color as well. Hence, you cannot isolate out the effect of the co-ordination change. For example, a commonly quoted case comes from cobalt(II) chemistry, with the ions [Co(H ₂O) ₆] ²⁺ and [CoCl ₄] ²⁻.

The difference in the colors is going to be a combination of the effect of the change of ligand, and the change of the number of ligands.

Contributors and Attributions

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Page ID 3707 White light and Colors The Origin of Color in Complex Ions containing transition metals Octahedral Complexes Tetrahedral Complexes Factors Affecting the Color of Transition Metal complexes The nature of the ligand The oxidation state of the metal The coordination of the Ion Contributors and Attributions This page is going to take a simple look at the origin of color in complex ions - in particular, why so many transition metal ions are colored.

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Discusses chemical reactions in a practical sense by a professor of chemistry from Oxford university.

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Peter Atkins captures the heart of chemistry in this book, through an innovative, closely integrated design of images and text, and his characteristically clear, precise, and economical exposition. Explaining the processes involved in chemical reactions, he begins by introducing a 'tool kit' of b...

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The link contains other articles on mass spec and analysis of drugs as well as several other articles.

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NMR Spectroscopy in Medicine: A Complete Guide

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Of all the technologies employed by medical professionals, NMR Spectroscopy is one of the most valuable. Officially known as Nuclear Magnetic Resonance Spectroscopy and sometimes referred to as M...

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Information on Ozone and data over time

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Throughout the 20th century, discoveries and observations trickled in that would allow scientists to understand how human-made chemicals like chlorofluorocarbons create a hole in the ozone layer over Antarctica each spring.

As early as 1912, Antarctic explorers recorded observations of unusual veil-type clouds in the polar stratosphere, although they could not have known at the time how significant those clouds would become. In 1956, the British Antarctic Survey set up the Halley Bay Observatory on Antarctica in preparation for the International Geophysical Year (IGY) of 1957. In that year, ozone measurements using a Dobson Spectrophotometer began.

Instruments on the ground (at Halley) and high above Antarctica (Total Ozone Mapping Spectrometer [TOMS], Ozone Monitoring Instrument [OMI], and Ozone Mapping and Profiler Suite [OMPS]) measured an acute drop in total atmospheric ozone during October in the early and middle 1980s. (Halley data supplied by J. D. Shanklin, British Antarctic Survey ).

These measurements gave the first clues that there was trouble in the ozone layer. In 1985, a group of scientists (J. C. Farman, B. G. Gardiner, and J. D. Shanklin) published in the journal Nature the first paper on observations of springtime losses of ozone over Antarctica. In 1986, NASA scientists used satellite data from the Total Ozone Mapping Spectrometer (TOMS) and the Solar Backscatter Ultraviolet (SBUV) instrument to demonstrate that the ozone hole is a regional-scale Antarctic phenomenon.

Between 1986 and 1987, several papers suggested possible mechanisms for the ozone hole, including chemical, dynamical (meteorological), and solar cycle influences. Among the key papers explaining the atmospheric chemistry of CFCs and ozone depletion was one by Susan Solomon and several colleagues. The paper also emphasized the need for polar stratospheric clouds to explain the reaction chemistry. Also in 1986, Michael B. McElroy and colleagues described a role for bromine in ozone-depleting reactions. Paul Crutzen and Frank Arnold proposed that the polar stratospheric clouds could be made of nitric acid trihydrate, which would explain the clouds' presence at an altitude and temperature that should not have been cold enough for the tiny amount of pure water vapor present in the stratosphere to condense.

Observational evidence of the role of chlorine in ozone loss continued to mount during that same period. For example, the National Ozone Expedition (NOZE) measured elevated levels of the chemical chlorine dioxide (OClO) during the springtime ozone hole from McMurdo Research Station. Then in 1987, the Antarctic Airborne Ozone Expedition flew the ER-2 and DC-8 research aircraft from Punta Arenas, Chile, into the Antarctic Vortex.

Aircraft measurements in the late 1980s confirmed the link between CFCs, chlorine, and ozone loss. Here a NASA ER-2 high-altitude research aircraft lifts off from Kiruna, Sweden on a mission to study Arctic ozone. (NASA Dryden Flight Research Center photo EC00-0037-22)

The aircraft observations produced the "smoking gun" linking CFC-derived chlorine to the ozone hole. The flight data showed a negative correlation between chlorine monoxide (ClO) and ozone: the higher the concentration of ClO, the lower the concentration of ozone. In 1988, the husband and wife team Mario and Luisa Molina described the chemical reactions through which ClO catalyzes the extremely rapid destruction of ozone.

NASA has been monitoring the status of the ozone layer through satellite observations since the 1970s, beginning with the TOMS sensors on the Nimbus satellites. The latest-generation ozone-monitoring technology, the Ozone Mapping and Profiler Suite (OMPS), is flying onboard the NASA/NOAA Suomi NPP satellite.

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Nasa Ozone Watch: Ozone hole history facts

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Throughout the 20th century, discoveries and observations trickled in that would allow scientists to understand how human-made chemicals like chlorofluorocarbons create a hole in the ozone layer over Antarctica each spring. As early as 1912, Antarctic explorers recorded observations of unusual veil-type clouds in the polar stratosphere, although they could not have known at the time how significant those clouds would become.

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Online practice at understanding a range of organic mechanisms

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To complete the investigations you will need to hone your investigative skills. Below are a series of examples, they focus on particular aspects of reaction mechanisms and will help you with the investigations section.

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Improve your maths skills with extra problems

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Shell UKs project to see if Hydrogen gas can replace methane in households being looked at for the first time.

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In Spring 2022, Cadent and British Gas wrote to every address in Whitby to confirm which properties are located within the proposed 'Village' boundary.
Cadent and British Gas have now started speaking in more detail with Whitby residents, landlords and businesses to answer questions, complete surveys and create a strong community programme.

As part of these surveys, every property in the proposed 'Hydrogen Village' area - around 2,000 homes in an area within Whitby - will receive a free in-home hydrogen assessment.

In 2023 Ofgem, the energy regulator for Great Britain, will confirm which location the full programme will go ahead in (the area within Whitby in Ellesmere Port, or another area, in the North East of England).

Properties in the confirmed 'Hydrogen Village' will be provided with a free 'hydrogen-ready' gas boiler upgrade, and hydrogen will be supplied from 2025 for the duration of the programme - currently expected to last around two years.

More details will be announced soon but if you'd like to speak to someone now, please get in touch.

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In Spring 2022, Cadent and British Gas wrote to every address in Whitby to confirm which properties are located within the proposed 'Village' boundary. Cadent and British Gas have now started speaking in more detail with Whitby residents, landlords and businesses to answer questions, complete surveys and create a strong community programme.

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The Nobel Prize in Chemistry 2022 was awarded jointly to Carolyn R. Bertozzi, Morten Meldal and K. Barry Sharpless “for the development of click chemistry and bioorthogonal chemistry”

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As a continuation of Cambridge Chemistry Challenge, the UK Chemistry Olympiad is a competition suitable for Year 13 students. Organised by the Royal Society of Chemistry, this written paper is held in schools around January / February each year and past papers are available through this link.

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If you have feedback on the Cambridge Chemistry Challenge email us using feedback@c3l6.com.
Supported by: St Catharine's College, Cambridge Chemistry, Oxford University Press and the Michael & Morven Heller Charitable Foundation. Privacy Policy.

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As a continuation of Cambridge Chemistry Challenge, the UK Chemistry Olympiad is a competition suitable for Year 13 students. Organised by the Royal Society of Chemistry, this written paper is held in schools around January / February each year and past papers are available through this link.

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Practise answering Olympiad-style questions with these past papers and mark schemes with answers

For students planning to take part in the Chemistry Olympiad, trying out past papers gives them the opportunity to hone key skills and boost their confidence ahead of the competition. This collection brings together past papers from 2003 onwards, each accompanied by a summary of the topics covered as well as a mark scheme with answers.

Some papers also include the original student answer booklet and the examiners’ report.

Past papers can be used flexibly by teachers and students, with varying degrees of independence. Browse the summary of papers and topics below to find a particular question, or select a paper to work through from beginning to end.

Chemistry Olympiad papers focus on problem-solving and reward resilience, so students should attempt as many parts of each question as they can, and try not to be put off if there are parts they can’t answer. Even a seemingly ‘low’ mark can earn a medal in the competition.

Questions often get more difficult as a paper progresses, but students studying A-level or equivalent qualifications should be able to attempt most parts.

Additional support and resources

Check out our Chemistry Olympiad support booklet for further guidance on the types of questions used in the competition, as well as example questions with commentary and analysis.

Build up to tackling a whole past paper using our series of worked answers, designed to help students less familiar with Olympiad-style questions develop the skills and confidence they need, one step at a time.

Browse and download resources by year

2022

Topics include E10 petrol; the chemistry of nitrous oxide; synthesis of the platonic solid cubane; calculations of equilibria in lateral flow tests; synthesis and structure of the world’s smallest Chinese knot; and calculations based on the ensilication of vaccine components to improve their stability.

Past paper | Answer booklet | Mark scheme | Examiners’ report

2021

Topics include reactions that produce phosphine; carbon capture by calcium looping; synthesis and reactions of levulinic acid; Newman projections; synthesis of hydroxychloroquine; and kinetics of the formation of xenon difluoride.

Past paper | Answer booklet | Mark scheme

2020

Topics include the reactions and structure of calcium carbide; the thermodynamics of hydrogen as a fuel; the structure of UV-absorbing chemicals; structures of silicon oxides; kinetics in colourful compounds; and synthesising [5]-ladderanoic acid.

Past paper | Answer booklet | Mark scheme

2019

Topics include carbon dioxide in fizzy drinks; electrolysis of precious metals and NMR spectra of platinum complexes; the kinetics of nerve agent treatments; synthesising pesticides; and calculations on biodegradable polymers.

Past paper | Answer booklet | Mark scheme

2018

Topics include reactions within lithium-ion batteries; sustainable methods of producing ammonia; isotopes and reactions of Uranium; the synthesis of dextromethorphan; and compounds of helium and sodium.

Past paper | Mark scheme | Examiners’ report

2017

Topics include the properties of sodium hypochlorite; reactions and kinetics of atmospheric gases; alloys of titanium; the structure and synthesis of twistane; and reactions of superbases.

Past paper | Mark scheme | Examiners’ report

2016

Topics include thermodynamics of the sulfur–iodine cycle; reactions of tungsten; NMR of unsaturated molecules; the synthesis of Flibanserin; and analysing ancient bones.

Past paper | Mark scheme | Examiners’ report

2015

Topics include calculations based on indium tin oxide; properties of 2-methylpropanenitrile; the synthesis of Ritalin; rates of metabolism of ethanol; and reactions and thermodynamics of ‘green’ jet fuel.

Past paper | Mark scheme | Examiners’ report

2014

Topics include the reactions of lanthanum carbonate; ionisation energies of sodium; the synthesis of tazarotene; analysis of chemicals that bombardier beetles use to defend themselves; and the structure and reactions of methane hydrates.

Past paper | Mark scheme | Examiners’ report

2013

Topics include reactions and thermodynamics of hydrazine; copper in Olympic medals; colours of compounds; synthesis of benzodiazepines; and the structure of creatine.

Past paper | Mark scheme | Examiners’ report

2012

Topics include cyclic allotropes of sulfur; reactions of amino acids in keratin; chemistry general knowledge; the synthesis of Liptor; and structure of the world’s smallest car.

Past paper | Mark scheme

2011

Topics include analysing ingredients of snack food; calcium carbide; using explosives in civil engineering; NMR of organic compounds; and the synthesis of phenobarbital.

Past paper | Mark scheme

2010

Topics include synthesising Ambrox; analysing a copper complex using titrations; thermodynamics of halogen fluorides; calculations on salty solutions; using osmium compounds in organic reactions; and the structure of gold.

Past paper | Mark scheme

2009

Topics include nitrogenated fuels; burning methane; reactions and structure of phosphorus sulfides; the synthesis of loperamide; and detecting arsenic.

Past paper | Mark scheme

2008

Topics include methods of producing pure silicon; the kinetics of vitamin D production in mushrooms; thermodynamics and structure of mercury fulminate; reactions and analysis of aluminium compounds; mass spectrometry of polypeptides; and the synthesis of fexofenadine.

Past paper | Mark scheme

2007

Topics include the reactions and thermodynamics of rocket fuels; structures of phosphorus allotropes; analysing phosphate levels in blood; spectroscopic analysis of flame retardants; the synthesis of Tamiflu; and reactions of chlorine dioxide.

Past paper | Mark scheme

2006

Topics include reactions of the ingredients in sherbet lemons; reactions in vehicle exhausts; structures of acyl chloride compounds; thermal decomposition of copper(II) sulfate; producing oxygen in emergencies; the synthesis of sildenafil; and mass spectrometry and NMR of haloalkanes.

Past paper | Mark scheme

2005

Topics include the properties of carbon oxides; reactions of diiodine pentoxide; calculations with methanoic acid; NMR spectra of NanoPutians; estimating blood alcohol levels; and the synthesis of rimonabant.

Past paper | Mark scheme

2004

Topics include redox reactions; reactions of pollutants that erode monuments; calculating dissolved oxygen in water; the structure of agent orange; the thermodynamics of white and grey tin; electronic transitions in hydrogen; and structures of sulfur-containing compounds.

Past paper | Mark scheme

2003

Topics include heating a can of coffee; the structure of Reinecke’s salt; calculating atom economy; writing redox equations; calculations based on historical experiments; hydroxylamine and its reaction with iron(III) ions; the synthesis of flunitrazepam; and the properties of polonium.

Past paper | Mark scheme

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Download past papers from the UK Chemistry Olympiad, including questions and mark schemes with answers from 2003 onwards.