Unit 2 final


Democritus (discontinuous theory of matter)

400 bce

long with his colleague Leucippus, created the discontinuous theory of matter. However, this theory was rejected for many years due to a flawed scientific system based on debates and Aristotle’s continuous theory of matter. He believed that everything was made of atoms which were indestructible and constantly in motion. He also believed that atoms were constantly in motion and that they could not be broken down any further.

John Dalton (atomic theory)


Dalton thought that all matter is composed of small particles called atoms which cannot be further divided, created, or destroyed. He also believed that all atoms of the same element were identical and that in chemical reactions atoms were rearranged to created different reactions. These thoughts are known as The atomic Theory created in 1804. Dalton also stated the law of multiple proportions which states that matter is combine in small whole numbered ratios. Finally, Dalton believed that atoms were like billiard balls in that they were round but he did not believe there was anything inside of them.

J. J. Thomson (Plum pudding model and electrons)

1897 - 1900

Using cathode rays and Crook’s findings, Thomson found that there were negative particles floating around inside of positive fields in the atoms. This led him to the creation of his plum pudding model and the finding of what we now know of as electrons.

Robert Millikan (quantity of the charge of electrons)


sed the falling-drop method (dropping oil into an electrically charged field) to prove the quantity of the charge of an electron. He stated that this quantity was constant among all electrons. This finding allowed us to better understand the makeup of atoms and understand the atomic structure of electricity.

Ernest Rutherford (Nucleus and protons)


By sending Alpha particles through a piece of gold foil leading him to discover that there was a small spot in the center of the foil that would reflect the particles. This discovery led him to the discovery of the positively charged nucleus in the atom.

Niels Bohr (planetary model)


Bohr based his model on the idea that energy was quantized and that electrons orbit at fixed distances from the nucleus. He calculated energy levels and formed the base of the quantum theory, however, was later found to have flawed.

Erwin Schrodinger (Electron clouds)


Schrodinger realized that electrons behaved with a wave-particle duality meaning that they displayed attributes of both. Using this discovery, he found that he was able to predict with 90% certainty where electrons were likely to be found in an atom. The dense area where it was likely to find an electron is known as an electron cloud.

James Chadwick (neutrons)


Chadwick used alpha particles, Paraffin wax, and beryllium atoms to find the missing mass of atoms. He forced alpha particles into beryllium atoms and realized there was strange radiation created that had mass but no charge. He called those particles neutrons.