Atomic Model Timeline

Democritus (460–370 B.C.)

Proposed that matter is made of tiny indivisible particles called "atomos"; had no experiments or data—purely philosophical idea.

(a) atoms were indivisible and indestructible.

-did not explain chemical behavior 

-were not backed by experimentation

John Dalton (1808)

Formed the first modern atomic theory: atoms are indivisible

(a)  All elements are made up of tiny, indivisible particles called atoms

(b) Atoms of the same element are identical, and different from atoms of other elements

(c) Atoms of different elements can mix or chemically combine in simple whole-number ratios to form compounds.

(d) Atoms join, separate, or rearrange in chemical reactions, but never change into other types of atoms through chemical reactions

J.J. Thomson (1897)

Discovered the electron using the cathode-ray tube; found that rays were made of negatively charged particles; proposed the “plum pudding” model (electrons embedded in positive mass).

Robert Millikan (1909)

Measured the charge and mass of an electron with the oil-drop experiment; found the electron’s mass ≈ 1/1840 of a hydrogen atom.

Eugen Goldstein (1886)

Observed canal rays traveling opposite to cathode rays; discovered positively charged particles (later known as protons).

Ernest Rutherford (1911 – 1919)

Used the gold-foil experiment to discover the nucleus—tiny

Niels Bohr (1913)

Proposed electrons orbit the nucleus in fixed energy levels; developed the planetary model explaining atomic spectra and quantized energy jumps.

James Chadwick (1932)

Discovered the neutron by bombarding beryllium with alpha particles; showed neutrons have no charge and similar mass to protons.

Erwin Schrödinger (1926)

Created the quantum mechanical model; treated electrons as waves and described orbitals as probability clouds where electrons are likely to be found.