Part 2 - Atomic Theory Timeline

Atomic Theories Discovered By Scientist's


400 BCE - 370 BC

Democritus was an ancient Greek who created the first atomic model; it was a simple round sphere with no electrons, protons, or neutrons. His atom helped other scientists understand and further look into the science of the basic structure of an atom.
Democritus believed that all matters consist of invisible particles called atoms. The atoms are indestructible, solid but invisible and are homogenous. All atoms differ in size, shape, mass, position, and arrangement. Democritus has proven that solids are made of small, pointy atoms. Liquids are made of large, round atoms and Oils are made of very fine, small atoms that can easily slip past each other.
The description of his model; is unbreakable particles, with a different particle for each different substance.

John Dalton

1803 A.D. - 1808 A.D.

John Dalton was an English scientist that believed that all matter is made up of atoms which are indivisible and indestructible. A rearrangement of atoms is a chemical reaction and a combination of two or more different kinds of atoms are formed by compounds. Dalton proved that each element has different atoms with different weights and properties.
The description of his model; was atoms were unbreakable spheres.

Joseph Thomson

1879 A.D. - 1904 A.D.

Joseph Thomson was an English scientist who discovered the electron by studying the Cathode Rays. He proved that electrons were in every atom. He also stated “If every atom contains negative particles, there must be a positively charge material in every atom to balance the electrical charge”.
Thomson invented the plum-pudding model in 1904. The description of his model was that each atom was a solid ball of positively-charged material with electrons embedded in it.

Ernest Rutherford

1911 A.D. - 1918 A.D.

Ernest Rutherford was an English scientist born in New Zealand who used radio-activity to test the Plum-Pudding model and discovered the alpha and beta rays which describe the two different types of radiation emitted by uranium and thorium. He set forth the laws of radioactive decay, and identified alpha particles as helium nuclei.
The Rutherford’s model is a simplified version of the Plum-pudding. The description of his model was that atoms were mostly empty space with a tiny, dense positively-charged nucleus in the middle. The electrons must orbit around the nucleus.

Niels Bhor

1913 A.D.

Niels Bohr was a Danish scientist who worked out mathematically that the electrons must be in a precise layered orbit. Bohr’s model shows the atom as a small, positively charged nucleus surrounded by orbiting electrons. A maximum number of electrons can fit in each shell which determines the properties of an element.

James Chadwick

1932 A.D.

James Chadwick is an English scientist who discovered the neutron. The neutron was found to have the same mass as protons which accounted for more of the mass of the atom and allowed the masses to match. Chadwick’s model shows electrons orbit around the nucleus using the “Bohr” energy levels which contains protons and newtons in the nucleus.