History of the Atom timeline.

Democritus believed that all matter was composed of tiny particles called atoms that were unable to be divided.

Heraclitus believed that all matter was composed of fire.

Aristotle believed that all matter was composed of the four elements: water, fire, air, and earth. This theory was most popular and most influential to science for the next several hundred years.

Thomson used a Cathode Ray (A beam that carries a ray of electrons from the cathode to the anode) and magnets to determine that some particles carried a negative charge and some carried a positive charge. This was important because it was the first time people acknowledged that there were positive and negative subatomic particles. To represent this, Thomson used a "plum pudding" to demonstrate that Atoms were made of electrons in a soup of protons.

Dalton had five postulates that compiled his theory about matter.
1. "All matter is composed of extremely small particles called atoms."
2." All atoms of a given element are identical in properties; all atoms of different elements differ in properties."
3. "Atoms can't be subdivided, created, or destroyed."
4. "Atoms of different elements combine in simple whole-numbers ratios to form chemical compounds."
5. "In chemical reactions, atoms are combined, separated, or rearranged."

Rutherford conducted an experiment using alpha particles shot towards a piece of gold foil. When the alpha particles deflected off of the gold foil, Rutherford concluded two things; Atoms contain a small, dense body called a nucleus, and the nucleus has an overall positive charge. This experiment was important because it was the first time people became aware of the nucleus of an atom.

Bohr discovered that electrons have designated positions in the atom. Using the Light Emission Spectrum, Bohr discovered that atoms cannot gain or lose just any amount of energy. The amount of energy an atom can gain when going from the grounded state to the excited state is determined by the type of element. With this, Bohr concluded that electrons has assigned positions and orbited around the nucleus. Bohr's model was called the solar system model.

Schrodinger discovered that electrons actually didn’t orbit and that we don’t actually know where in the “cloud” the electrons are. Schrodinger theorized that we can’t determine exactly where the electrons are, but by knowing the amount of energy the electrons carry, we can figure out the electrons’ positions relative to the nucleus, known as the cloud. Schrodinger’s model was called the Electron cloud model.

Chadwick suspected that there had to be a neutral subatomic particle that accounted for the extra mass unexplained by the protons and electrons but that didn’t affect the charge. Using different metals and alpha particles, Chadwick discovered what he was looking for: the neutron. His model was called the Electron cloud model 2.0 and is the most up to date model of an atom so far.