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1911 - The Quantum Theory and Model & The Bohr Model

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1911 - The Quantum Theory and Model & The Bohr Model

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Neil Bohr - "Bohr Model":

In 1911, the same year in which the results of Rutherford's gold foil experiment were published, Niels Bohr went to go study with Rutherford. He was an interested Chemist and Physicist who was also interested in the mathematical model set forth by Max Planck and Albert Einstein to explain the behaviour of electromagnetic energy. He realized that the mathematical principles could be applied to Rutherford's atomic model.

Using his analysis of the gold foil experiment, and his calculations for proportions of alpha particles that went straight through, got deflected and the ones that bounced almost completely backward - He predicted the likely positions of electrons in an atom.

What is the Bohr Model?

A Planetary Model was the resulting model of these assumptions that Bohr made that represents electrons in orbits around a central nucleus.

Each orbit has a specified number of electrons, which correlate to energy levels and orbitals in the modern-day model.

This is very close to reality - But also very flawed

How was he close to reality but also flawed?

Due to the electrons!

Werner Heisenberg - Quantum Theory

German Theoretical Physicist Werner Heisenberg showed everyone how big and mind-blowing the electron problem truly was. He took all the information given and became the glue that bound everything together by discovering that it was possible to know the momentum of an electron or any subatomic particle and its exact position with accuracy.

If you can't measure the position or momentum of an electron, then you can't say with certainty that the electrons in an atom are aligned in circular orbits.

He thereby came up with the new theory called, "The Quantum theory" with the new wave of Chemists and Physicists.

What is the Quantum Theory?

It proposed that electrons weren't particles or waves, but they had properties of both and neither.

With this theory, the arrangement of electrons around a nucleus can only be described by probability through the key concept of Orbitals

Orbitals: Certain regions where electrons are most likely to be found

Orbitals go by names like s, d, p, f

They form sigma and pi bonds

Cloud Model of the Atom/Quantum Model of the Atom: Because atoms are based on probability - They are often drawn as clouds with large intensities of colour that represent the probability of finding an electron in any particular position as opposed to singular electrons.

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1909 - Rutherford's Gold Foil Experiment & The Discovery of Protons and The Nucleus 

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1909 - Rutherford's Gold Foil Experiment & The Discovery of Protons and The Nucleus 

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Ernest Rutherford was a Physicist from New Zealand who designed an experiment that involved using an extremely thin sheet of gold foil alongside a screen coated with Zinc Sulfide to help discover particle behaviour.

He bombarded the sheet with alpha particles (It's important to note that he didn't know what they were, just that they were produced by the decay of radium, were positively charged, and were really small)

He assumed that the particles would fly through the foil with no detection.

Although some did, a lot of them were deflected at large angles or sometimes almost straight backward

What did he find from this experiment?

That the entire positive charge of an atom (Charge that would repel an alpha particle) must be concentrated in a very small area that he referred to as a "Nucleus"

Because most of the atoms passed through the atom undeterred, he concluded that the atom was mostly empty space.

He also bombarded Nitrogen with alpha particles, creating a lot of Hydrogen ions. He assumed that these positively charged ions were fundamentally important, and thus, found Protons

This helped scientists have a fairly good idea of the structure of atoms. But now they just needed to find out what the electrons were doing.

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1886 - Electrons & The Plum Pudding Model

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1886 - Electrons & The Plum Pudding Model

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Eugen Goldstein - Lights emitted from Positive Electrode Discovery

In 1886, German Physicist, Eugen Goldstein discovered that the tubes also emitted light from the positive electrode due to a ray headed in the opposite direction. This indicated that there was a positive charge in the matter.

However, it was still not understood why this exactly occurred. As the negative charges seen in these tubes were also not truly understood either.

J.J. Thompson - Electrons & Plum Pudding Model

This research into the Discharge Tube was further ventured into by English Physicist, J.J. Thompson.

He estimated the mass of these rays by measuring the amount of heat the cathode rays generated and how much they could be bent by magnets and other elements.

He discovered that the mass was about 1000 lighter than Hydrogen, which was the smallest form of matter known at the time.

Conclusion: Cathode Rays weren't rays or waves, but were very small light and small negatively charged particles called "Electrons"

He called Electrons Corpuscles

Electrons were found to be negative and positive components in matter. But it was not known what shapes the electrons took or "How were they arranged in the atom?"

Thompson though did know that the atom itself had an overall neutral charge. So he assumed that the negatively charged electrons must be distributed randomly in a positively charged matrix. He visualized this as a plum pudding. Hence, "Plum Pudding Model". It was also assumed that the single electron motion was random, but the overall distribution was not.

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1870s - Investigation of Atomic Structure 

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1870s - Investigation of Atomic Structure 

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Due to the main question of why atoms behave the way they do - An investigation was embarked on Atomic Structure.

Scientists probed what stuff was made using discharge tubes.

Discharged Tubes: Gas-filled tubes with electrodes at each end that emit light when an electrical current passes through them.

This light is created using a negative electrode or cathode known as a "Cathode Ray" which has a negative charge

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Portrait of English Chemist, James Dalton

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1800s - Antoine Lavoisier & John Dalton 

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1800s - Antoine Lavoisier & John Dalton 

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In the 1800s, we saw much development in the theories originally proposed by Leucippus and Democritus. The main 2 is that of Antoine Lavoisier and John Dalton

Antoine Lavoisier was a French Chemist who proposed The Law of Conservation of Mass = Even if matter changes shape or form, the mass stays the same

James Dalton was an English Chemist who determined that elements exist as discrete packets of matter.

These discoveries came nearly 2300 years after Leucippus and Democritus' theory

But due to these discoveries, there was a better understanding of the general behaviour of atoms

However, 1 question remained: "Why do they behave the way they do?"

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Leucippus & Democritus - "The Theory of Atomism"

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Leucippus & Democritus - "The Theory of Atomism"

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Greek Philosopher and his pupil, Democritus were heavily credited for developing the theory of atomism.

Came up with the idea of matter being made of tiny particles

Their assumption was that if you cut a particle enough times, you would reach a point where the particle couldn't be cut anymore

These particles were known as, "Atomos"

ATOMOS = Uncuttable or Indivisible

They also attributed specific properties to each form of atoms, but this was very basic and ground-level.

Much of this theory was also based off a blind guess seeing that they did not have access to years worth of information from other scientists, microscopes, or cathode rays.

But regardless, they came up with the general idea of atoms.