Atomic Theory Timeline



430 B.C.E

He proposed first idea of the atom: substances made up of tiny indestructible, invisible particles called "atoms".


384 B.C.E. - 332 B.C.E.

Aristotle thought differently than Democritus; he thought that the world was made up of air, water, fire, and earth, instead of tiny particles. Aristotle's ideas were used by the Catholic church in the middle ages, and the people who followed Democritus, were shunned or killed.

Joseph Priestley


Discovered oxygen, carbon monoxide, and nitrogen dioxide



Law of Conservation of Mass: matter cannot be created or destroyed, only rearranged.



Law of Definite Proportions: a pure substance will always contain the same element, combined in the same proportion by mass.

John Dalton

1810 - 1844

Law of Multiple Proportions:
same two elements can combine in different compounds; ratio of the first element is fixed, while the second element varies.

He proposed first atomic theory
1-all matter is made up of tiny invisible atoms
2-atoms of the same element are the same
3-atoms of different elements are different
4-atoms combine in whole number ratios to form compounds

JJ Thompson

1894 - 1940

Used cathode rays to discover the electron.

It measured the bending of the rays to determine the charge to mass ratio of the electron.

Modified cathode ray tubes were used to discover the positively charged proton.

Plum pudding model. Thomson thought that the atom looked like raisin pudding, a ball with negative electrons clumped on it.

Ernest Rutherford

1904 - 1937

He studied substances, especially alpha particles.

He introduced the idea of an element radioactively decaying into a different element. Also introduced a model of an atom. Also discovered the proton. H

Conducted gold foil experiment: he bombarded gold foil with alpha particles. Most particles pass through the foil but somewhere deflected. He concluded that the atom is an empty space, with a very small positive nucleus core.

Albert Einstein


In 1905 Albert Einstein published his paper on the Theory of Relativity. For his explanation of the photoelectric effect, Einstein won the Nobel Prize for Physics in 1921.

Robert Millikan

1911 - 1953

Robert Millikan
Perform the oil drop experiment, which determines the charge of electrons as negative.

Niels Bohr


Improved upon the atomic model by trying to answer the question: why aren't electrons pulled into the nucleus?

Electrons travel in definite energy levels without radiating energy
Electrons in each orbit have a certain amount of energy
Energy increases as distance from nucleus increases
Electrons lose energy by dropping energy levels

Marie Curie


Discovered Radioactivity which is used for x-rays after working in a lab. Her work is used worldwide.

Louis DeBrogile


His hypothesis: if waves can behave like particles, then particles can behave like waves

Erwin Schrodinger


Schrodinger's wave equation:
Treat the electron like a wave
Developed an the equation used to determine the probability of finding the electron in any given place around the nucleus
Solutions to the equation are quantum numbers
Quantum Model

James Chadwick


Discovered the neutron

Chadwick discovered that neutrons help reduce the repulsion between protons to stabilize the atom's nucleus. Neutrons always reside in the nucleus of atoms, and they are about the same size as protons. However, neutrons have no charge (a charge of 0).