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In 1803, the theory became the first full attempt at describing matter. First, all matter is made up of atoms. They are indivisible and indestructible. Second, elements are made up of identical atoms. The atoms of different elements contain different masses. Third, atoms of elements combine in a whole number ratio to form a compound.

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J. J. Thomson placed two plates on opposite sides around the cathode ray. The ray was drawn onto the positively charged plate, this is the same with two magnets around it. The cathode ray must be made out of negatively-charged particles. These particles have the same mass in each element: Around 1/2000 the mass of an H atom. These negatively-charged particles are called "electrons" and are present in all elements.

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Ernest Rutherford shot a thin gold foil with a thin beam of alpha particles. Alpha particles are He nuclei and are given off in various processes of radioactive decay. When the beam hit the gold foil, some parts of the beam scattered instead of passing straight through it. In this model, atoms are mostly made up of empty space. Positively-charged particles are bundled up in a tiny volume of the atom which contains lots of mass.

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Niels Bohr's theory explained that electrons move in a fixed orbit. Each orbit (shell) has a fixed energy level with the furthest orbit being the valence shell. This is similar to the rings of Saturn, or more accurately the planets orbiting around the Sun. This model doesn't work with systems with more than one electron.

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Erwin Schrodinger's theory treats electrons as matter waves. Matter behaves strangely at the subatomic level like when an electron behaves like a particle and a wave. Electrons have an intrinsic property called "spin", there are two possible "spins" which are "spin-up" and "spin-down". In an orbit, two electrons must have spins that are opposite to each other.