History of Atomic Theory

Atomic Theory Timeline

Democritus: 460-370 B.C.

• 1. There are basic elements from which all matter is made

• 2. Everything is made of small atoms moving in a void

• 3. Some atoms are round, pointy, oily, have hooks, etc. to account for their properties

• 4. Ideas rejected by leading philosophers

John Dalton: 1766-1844

• 1. Each element is composed of extremely small indivisible particles called atoms

• 2. All the atoms of a given element are identical, but different from those of any other element

• 3. Atoms are neither created nor destroyed in any chemical reaction

• 4. A given compound always has the same relative numbers and kinds of Atoms. (Ex water is always H2O)

John Dalton: 1766-1844

• Where Dalton was wrong– 1. We can divide the atom• Subatomic particles include protons, neutrons,

& electrons• E = mc2 Atomic Bomb – 2. Not all atoms of a given element are

identical• Isotopes

J.J. Thomson: 1856-1940

• 1.Discovered electron in 1897 – Cathode Ray Tube Experiment – Electron has (-) charge: “e-” – Mass of e- = 9.11 x 10-28g• 1/1840 the mass of a proton– Protons discovered in 1886 by Goldstein• Proton has (+) charge: “p+” • Mass of p+ = 1.67 x 10-24g

J.J. Thomson: 1856-1940

• Cathode Ray Tube Experiment

• The beam Is attracted to (+) plate and repelled by (-) plate so it must be made of (-) charged particles!

J.J. Thomson: 1856-1940

• Thomson knew the atom as a whole was neutral so there must be (+) charged particles also.

J.J. Thomson: 1856-1940

• 2. Developed Plum Pudding model in 1904 – Atom is a sphere of (+) charge with (-)electrons

randomly scattered throughout – Modern version: “Choc. Chip Cookie” ?

J.J. Thomson: 1856-1940

• 3. Discovered isotopes in 1913– Different “versions” of the same atom. Atoms

of the same element, but with different masses.

• Nucleus and neutrons had not been discovered yet so he didn’t fully understand why isotopes existed

Ernest Rutherford: 1871-1937

• 1. Discovered Nucleus w/“Gold Foil Experiment” in 1909

– Bombarded a piece of gold foil (gold atoms) w/large, (+) charged alpha particles.

– Most passed right through but a few bounced back or were deflected at an angle

Ernest Rutherford: 1871-1937

• 2. Developed Nuclear Atom Theory in 1910– 1. The atom is mostly empty space– 2. In the center of the atom is a tiny nucleus,

which contains most of the atom’s mass– 3. The nucleus has a (+) charge – 4. Electrons are in afixed orbit in the empty

space around the nucleus

Ernest Rutherford: 1871-1937

James Chadwick: 1891-1974

• Discovered the neutron in 1932 – Neutron has no charge: “n0” – Mass of n0: 1.67 x 10-

24g

• Almost the same mass as a proton (both are much larger than the e-)

• Described the nucleus and explained isotopes discovered by JJ Thomson in 1913

– Isotopes of the same element have different masses (same # protons but different #s of neutrons)

Niels Bohr: 1885-1962

• Developed Planetary Model in 1913 • Nucleus is surrounded by electrons that are

orbiting in definite paths “like planets orbit the sun”

Niels Bohr: 1885-1962

• 2. Electrons exist in discrete “quantized” energy levels.

• • Quantized = the opposite of continuous, goes from one level to the next with nothing in between