Atomic+Theory+Timeline

Democritus (-400 BC) Democritus was nicknamed “the laughing philosopher” due to his cheery attitude and from what folklore states, he blinded himself in order to not be distracted in his speculations. Democritus’ name is known for being related to the first account of the atomic theory of matter. In his studies, he states that matter is made of indivisible atoms which are all exactly alike. He believed that atoms and empty space are all that formed the universe and everything else is just thought to exist. Also, he thought that atoms, empty space, and the motion in which atoms move in that space are eternal. Furthermore, he believed that atoms were unlimited in both size and number and that they make up everything in the universe. Although he studied heavily upon the nature of matter, only parts of his work remain today.

Dalton (1804) Dalton was an English chemist and a physicist, as well as a meteorologist. He was known for many discoveries: one being his discovery and experiments with blindness/color blindness. He stated that all atoms of the same element are identical, that atoms of different elements had different masses and that atoms from each element is different from that of another. Dalton also stated that all matter is indivisible as well as that atoms only combine in small, whole numbers such as—1:1, 1:2, 2:3



Goldstein (1886) Goldstein’s contribution to the atomic theor﻿y would be the discovery of protons in his anode ray experiments. His theory states that since atoms contain negatively charged particles, they must contain positively charged particles, known as protons, to be electrically neutral.

Thomson (1897) Thomson is famously known as the first person to discover electrons. He made this discovery by performing an experiment to prove that cathode rays coming from the cathode were negatively charged streams of particles, known as electrons. A cathode ray is when “…a gas sample is introduced into the region between two charged plates, a current flow can be observed, suggesting that the atoms have been broken down into charged constituents. The source of these charged particles is a heated cathode that, in fact, causes the atoms of the sample to ionize.” After performing his experiment, he was correct in his hypothesis and credited to the discovery of electrons. Furthering his study of the atom and it’s electrons, he created what is commonly known as “the plum pudding model”. This model states that electrons were embedded into the atom and looks similar to a plum pudding or even a blueberry muffin.

Rutherford (1909-19011) Rutherford discovered that electrons are located on the outside of the atom and that they travel in orbits around the nucleus. He’s model is considered to be the first “planetary model”. He also used gold foil and a gamma ray to project to test out his theory. It didn’t go as planned, but it helped to create his other theory. Ernest Rutherford, famous for the “gold-foil experiment,” discovered that the core of an atom is very small relative to the size of the whole atom. He did this by shooting small positively charged particles at a thin gold foil sheet. He discovered 99% of the particles that traveled through the gold foil missed the nucleus. Rutherford built his theory on the few particles that did not go through the gold foil sheet. He hypothesized that the particles which did not go through the nucleus were hitting a small, hard, dense object in the foil. He calculated the size of the nucleus to be at least 10,000 times smaller than the size of the whole atom. After measuring the different angles the particles were deflected, he was able to better determine the shape and size of the nucleus.



Schrodinger (1926) Schrodinger won the Nobel Prize for Physics in 1933, for his 1926 introduction of Schrödinger's wave, the mathematical equation of wave mechanics that is still the most widely used piece of mathematics in modern quantum theory. He also was a physicist and a catholic by faith. As far as education goes, he attended Cambridge and Austria's University of Graz. He attended Dublin Institute for Advanced Studies, where he worked for seventeen years after relocating during WWII when the Nazis invaded Austria.

Chadwick (1931) Chadwick helped with the discovery of nuclear fission when uranium is bombarded with neutrons and the eventual development of nuclear weapons and nuclear power production. He also proved the existence of neutrons, the elementary particle with a neutral charge and a fundamental building block of the atom's nucleus. His discovery of the neutron was a big contribution to the atomic theory, since the job of the neutron in an atom is the mass of the atom. Chadwick also happened to be a student of Ernest Rutherford’s and of Hans Geiger. At the outbreak of WWI he worked in Germany, where he was imprisoned for the length of the war. During WWII, he cam to America where he contributed to the Manhattan Project that developed the world’s first atomic weapon.



Bohr (1940)  Bohr created what is known as the second planetary model, next to the first planetary model that was created by Rutherford. It was dubbed this name due to it’s appearance with the nucleus appearing as the sun with electrons rotating on energy levels around it, just as the planets in our solar system do. He was also the first to conclude that the nucleus contains protons and neutrons. Along with this, Bohr said that the number of electrons in the outer orbit determine the chemical properties of that element.