2.2 - The Development of the Atomic Model

Subatomic Particles

An atom contains protons, neutrons and electrons.

Particle	Relative charge	Relative mass
Proton, $p$	$+1$	$1$
Neutron, $n$	$0$	$1$
Electron, $e$	$-1$	$\frac{1}{1840}$

Historical Development of the Atomic Structure Model

• Democritus, about 500 BC: introduced an early theory about atoms.
• John Dalton (1766–1844):
  • matter is made of particles called atoms;
  • an atom is the smallest spherical body and cannot be created, destroyed or divided further;
  • atoms of the same element are identical.
• J. J. Thomson (1856–1940):
  • discovered negatively charged electrons;
  • proposed an atom as a positively charged sphere containing electrons.
• Ernest Rutherford (1871–1937):
  • established the nucleus as the centre of the atom;
  • discovered positively charged protons in the nucleus;
  • concluded that almost all atomic mass is concentrated in the nucleus and electrons move outside it.
• Niels Bohr (1885–1962): proposed that electrons move in shells around the nucleus.
• James Chadwick (1891–1974):
  • discovered neutral particles called neutrons in the nucleus;
  • established that neutrons contribute almost half the mass of an atom.

The sequence of increasingly complete models is Dalton → Thomson → Rutherford → Bohr → Chadwick.