The History of the Atom
Democritus: ~380 B.C.
 matter made up of tiny indivisible particles, “atom”
John Dalton: 1805
 Dalton’s Atomic Theory, lasted approx. 100 years
 Limitations: Could not explain why atoms combined in certain ratios
The atom is not the smallest particle
J.J. Thomson: 1897
 Using Cathode ray tube, discovers negatively charged particle with
mass of 1/2000 that of a hydrogen atom, “electron”
 Raisin Bun model: negative charge embedded in positive atom
Ernest Rutherford: 1911
 studied nuclear radiation at McGill University from 1898 to 1907
 Gold foil experiment: shot beam of alpha radiation (positively
charged) at thin piece of gold foil, according to Thomson, all particles
should go through the foil with slight deflections. Not true. Some
particles deflected at great angles and some bounce directly back at
the source.
 Conclusion: the atom is made up of mostly empty space, with small,
massive region of concentrated charge at the centre
 Eventually would prove that centre was positively charged, “proton”
 Would become known as the planetary model of the atom, electrons
orbiting nucleus, giving of continuous energy
 Frederick Soddy, 1913: Also used mass spectrometry to discover
isotopes
James Chadwick: 1932
 Alpha particle bombardment, explains why mass of nucleus does not
equal mass of protons present, “neutrons”
Quantum Theory
 Limitations of Rutherford model: energy emitted should be observed
as visible light, orbits should shrink as electrons give off continuous
energy and electrons should slow and spiral into nucleus
 This does not happen!