Atoms

Atoms are the smallest unit of matter. After that, they are divided into subatomic particles, like protons, neutrons, and electrons. Protons and neutrons are made up of even smaller particles called quarks.

Atoms make up all matter.

History of the Atomic Model
Around 300 BC, Democritus, an ancient Greek scholar, proposed that matter was composed of really tiny particles, which he called "atomos", meaning "not cut", implying that atoms could not be divided into smaller pieces. His contemporaries scorned and mocked him, believing in Aristotle's theory that matter was uniform and could not be divided into smaller pieces. If Aristotle's theory were true, trees shouldn't have been able to be cut, and things couldn't break. Aristotle was not a smart man.

In 1803, John Dalton proved that atoms exist. His model was a simple sphere.

In 1897, JJ Thomson discovered electrons. He used a cathode ray tube to discover the electrons. His model was a "plum pudding" model, meaning the nucleus was a giant ball of positive charge, and electrons were sticking to the outside of it, like plums in the pudding. A more modern visual would be chocolate chips in a cookie dough ball.

In 1911, Ernest Rutherford, a student of JJ Thomson, changed the model of the atom yet again. Rutherford experimented with his gold foil experiment using a machine. The results of his gold foil experiment were that most of the atoms passed through the foil, but some of the atoms were deflected to random places that were farther away from the rest of them. His model proved that electrons orbited the nucleus like planets revolve around stars.

In 1913, Niels Bohr, a student of Ernest Rutherford, proved that electrons orbit the nucleus in fixed energy levels.

In 1926, James Chadwick theorized that the electrons don't necessarily orbit the nucleus in fixed energy levels, but instead, there was an electron "cloud", and electrons had orbital levels. The cloud model states the electrons jump from level to level, depending on how much energy they have.