Democritus

In 400 BC, famous Greek philosopher Democritus proposed to his colleagues that if you kept cutting something, such as gold, in half, eventually it would get too small to be cut further. He called these tiniest particles “atomos,” coming from the prefixes -tom (to cut) and -a (no, or not), meaning “uncuttable.”

Dalton Model

In 1803, Dalton experimented with gases and through his findings proposed that all matter is composed of the smallest particles, called atoms, which have many properties. Atoms can’t be created or destroyed: in chemical reactions, atoms are combined, separated, and rearranged, but never changed. Atoms of one element are the same, atoms of different elements have different weights and different chemical properties. Also, atoms of different elements combine in simple ways. Some of his theory was influential for today’s, as he was correct that all things are made of atoms, and that atoms of different elements are in fact different.

Goldstein

In 1886 Eugen Goldstein noticed that perforated cathode-ray tubes had “canal rays” as he called them traveled in the opposite direction as cathode rays. Goldstein believed that not only did electrons, or cathode rays, travel from the negative to the positive, or the cathode to the anode, but there was another ray, the anode ray, also known as canal rays, that traveled from the positive anode towards the negative cathode.

Nagaoka

Hantaro Nagaoka’s theory was electrons could not be located in the positively charged atom; but that instead, they orbited around it. in 1903 he created a new model for the atom, of which he called The Saturnian Model, after the planet Saturn because of the shape of his atom model.

Thomson Model

In 1904, The Thomson model proposed that there were particles called electrons, which were pulled together by a sort of positively charged soup which pulled them in close to each other and kept them there. This was because the nucleus had not been discovered. However, this model is important because of the discovery of electrons.

Millikan

In 1909, Millikan conducted an oil drop experiment which showed that there is a charge on a single electron, and also that there is a smallest charge on particles, which he called “quantized.” This was important because it revealed that all particles have charges. He won the Nobel Prize in Physics in 1923.

Rutherford Model

In 1911, Rutherford discovered the nucleus through his famous gold foil experiment, in which they took a piece of gold foil and shot a laser at it. They were able to see the way the laser was reflected, or went through, and thought there must be something other than what Thomson’s model showed. He theorized that the smaller electrons orbited the nucleus, made up of protons and neutrons, in a shell.

Bohr Model

In 1913 Niels Bohr changed the way we looked at electrons. Rather than swirling randomly around the nucleus, he theorized that they orbited the nucleus in energy levels that resemble orbits of planets. The energy levels were significant because it explained how atoms could gain and lose energy. This theory explains how a lot of glow in the dark objects you use work.

De Brogile

In his 1924 thesis, de Broglie suggested that all matter have wave properties. He had discovered the wave nature of electrons, where the wavelength is inversely proportional to the force of the moving particle. His discovery was immensely important to the development of quantum mechanics, as well as being true for the atom and molecule as well, and he won the Nobel Prize for Physics in 1929.

Schrodinger

In 1926, Schrodinger developed a new model based on the Bohr atom model, which showed the probability of where an electron might be in orbit around the nucleus. This model is commonly known as the Cloud Model, because the electrons are in a “cloud,” and the denser the area of the cloud is, the more likely an electron will be there.

Chadwick

James Chadwick’s theory was that besides the positive protons and negative electrons there were particles within an atom that had no charge. Chadwick tried to prove his theory, unsuccessfully, in the 1920s. In 1932 another scientist in his field had used the energy from gamma rays to expel protons from a wax substance. Chadwick questioned the credibility of the gamma rays alone to eject these protons out of the wax. This is when he came to the conclusion that the protons were actually colliding with other particles of a similar mass, thus finally discovering the neutron. This lead to neutrons being added to the atomic model and Chadwick’s earning of the Nobel Prize for Physics in 1935.