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A force causes an object to undergo a specific change. Unbalanced forces cause changes in speed, shape or direction. Gravitational forces act between masses. Friction opposes motion

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thrust - increases the velocity of an object

air resistance - decreases the velocity of an object

torque - changes the rotation of an object

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Isaac Newton was an English physicist and mathematician. He determined that gravity was a force that acts between two masses. The larger the masses, the larger the gravitational force between them

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Newton discovered that objects will continue to do what they are doing until an unbalanced force acts on the object. From this we can determine that:

forces act on objects

forces cause changes

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Atoms consist of a nucleus containing protons and neutrons, surrounded by electrons in shells. The numbers of particles in an atom can be calculated from its atomic number and mass number.

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Atoms consist of a nucleus containing protons and neutrons, surrounded by electrons in shells. The numbers of particles in an atom can be calculated from its atomic number and mass number.

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The typical size of atoms and small molecules is around 1 × 10-10 m (0.1 nm). For example:

diameter of a hydrogen atom, H 1.0 × 10-10 m

width of a hydrogen molecule, H2 2.9 × 10-10 m

length of an H-H bond 0.74 × 10-10 m

Nucleus and shells

An atom consists of:

a positively charged nucleus

surrounded by negatively charged electrons

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The atomic number of an atom is the number of protons in its nucleus. Remember that:

all the atoms of a given element have the same atomic number

the atomic number of each element is unique - no two elements have the same atomic number

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An atom contains an equal number of protons and electrons. Since protons and electrons have equal and opposite charges, this means that atoms are neutral overall.

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All living things are made up of cells.

Most cells are so small that you can only see them with a microscope.

Cells have different components and each performs its own function within the cell.

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Cells are the basic building blocks of all animals and plants.

Inside cells are various structures that are specialised to carry out a particular function. Both animal and plant cells have these components:

Cell membrane – this surrounds the cell and allows nutrients to enter and waste to leave it.

Nucleus – this controls what happens in the cell. It contains DNA, the genetic information that cells need to grow and reproduce.

Cytoplasm – this is a jelly-like substance in which chemical reactions happen.

Mitochondria – these are the powerhouse of the cell. They are structures where respiration takes place.

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Plant cells have all the parts in the list above, plus a few extra structures:

Cell wall - this is an outer structure that surrounds the cell and gives it support.

Vacuole - this is a space within the cytoplasm of plant cells that contains sap.

Chloroplasts - these contain chlorophyll and are the site of photosynthesis

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Cells can vary greatly in size. You need a microscope to see most human cells.

Red blood cells are some of the smallest cells in the human body. These have a diameter of 0.008 mm, meaning a line of 125 red blood cells is only 1 mm long.

The ovum (or egg cell) is one of the largest cells in the human body. It has a diameter of roughly 0.1 mm, so you can see them without a microscope. A line of 10 egg cells is 1 mm long.

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Mendeleev made an early periodic table. In the modern periodic table, elements are in order of atomic number in periods and groups. Electronic structures model how electrons are arranged in atoms.

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Before discovering protons, neutrons and electrons, scientists tried to classify the elements by arranging them in order of their atomic weights. The atomic weight of an element is equivalent to what we now call its relative atomic mass.

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Early periodic tables were incomplete, since many elements were unknown. Also, some elements were placed in groups with elements that were not similar to them.

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Iodine has a lower atomic weight than tellurium. So iodine should be placed before tellurium in Mendeleev's periodic table. However, iodine has similar chemical properties to chlorine and bromine. To make iodine line up with chlorine and bromine in his table, Mendeleev swapped the positions of iodine and tellurium.

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https://m.youtube.com/watch?v=GuY0n7-zfds