Atomic Theory Timeline



500 B.C.

From Ancient Greece - created the theory of atomism which states that indivisible particles combine in different arrangements to compose all matter
Leucippus Link:

The Model of Democritus

400 B.C.

The Model of Democritus was simply a round sphere that represents the smallest part of matter - It was the first atomic model and it opened the doors to further atomic research
The Model of Democritus Link:


Approx. 400 B.C.

Expanded Atomic Theory of Leucippus - matter is made up of small invisible particles that are so small they cant be divided
Democritus Link:

The Billiard Ball Model


The Billiard Ball model showed that atoms are the smallest particle of an element, that atoms of different elements have different masses, and that they are solid, indestructible units, much like a billiard ball. Model by Dalton
The Billiard Ball Model Link:

John Dalton

Approx. 1803

Pioneer in the development of the modern atomic theory - he expands upon Democritus and Leucippus by stating in his theory that all atoms are indivisible - he also states all atoms of a given element are identical, atoms from different elements are different, compounds are formed by more than one different atoms and can be rearranged
John Dalton Link:

Eugen Goldstein


During his experiments with Cathode Rays he discovered the Canal Ray (Protons) - with this he determined that protons are electrons attract each other and that there is for the charge of the atom to be neutral the number of protons and electrons has to be the same
Eugen Goldstein Link:

J.J. Thomson


Found out there were negative charges (electrons) in atoms in his experiments with his invention of the Cathode-Ray Tube (CRT) - His theory is explained in the Plum Pudding Model
J.J. Thomson Link:

Plum Pudding Model


The negative electrons represented the raisins in the pudding and the dough contained the positive charge - the model was wrong because it didn't have individual protons
Plum Pudding Model Link:

Robert Millikan


Determined the charge of an electron is his oil drop experiment which allowed the mass of the electron to be calculated
Robert Millikan Link:

Ernest Rutherford


Proved Thomson's atomic theory and model wrong in his Gold Foil Experiment - the deflections observed in the experiment proved to Ernest that there must be a concentrated nucleus in the center of the atom
Ernest Rutherford Link:

The Planetary Model


By Ernest Rutherford - States that electrons orbit around appositively charged nucleus which contains most of the atoms mass
The Planetary Model Link:

The Bohr Model


Bohr showed that the atom consists of positively charged nucleus of protons and neutrons surrounded by negatively charged electrons traveling in different orbits at fixed distance from the nucleus. When electron moves from one orbit to another, the energy difference between two states can increase and decrease (photons)
The Bohr Model Link:

Niels Bohr


Improved upon atomic model and structure, showed that electrons have certain orbits, they don't orbit randomly - he found out that different orbits have different energy levels - he created the Bohr Model with his findings
Niels Bohr Link:

Erwin Shrodinger


Used mathematical equations to build upon the Bohr Model to find the locations of electrons - shown in the Cloud Model (Quantum Mechanical Model)
Erwin Schrodinger Link:

The Cloud Model


Made by Schrodinger - determined the location of electrons, electrons have a higher probability of being found in the denser regions on the model
The Cloud Model Link:

Werner Heisenberg


Developed the uncertainty principle which states "It is impossible to measure simultaneously the exact position and exact velocity or momentum of a sub-atomic particle like electron and neutron"
Werner Heisenberg Link: