The History of the Atom

• when Democritus first conceived of atomic theory, around 500 BC, he described atoms like:

  • Small spheres
  • The smallest possible unit of matter 
  • Separated from each other by empty space
• In the 1800's, John Dalton described atoms as solid spheres , and suggested that different types of spheres make up the different elements
• In 1897 J. J. Thomson theorised that an atom consisted of a ball of positive charge, with negative electrons mixed throughout it called the plum pudding model
• How Rutherford developed the nuclear model:

  • In Rutherford's experiments, alpha particles were fired at a thin sheet of gold foil.
  • Most particles passed through, but some were deflected off course. 
  • This caused him to hypothesise that there was a dense region of positive charge at the centre of the atom that repelled the alpha particles.
  • As a result he developed the nuclear model of the atom, in which there was a central positive nucleus, surround by negative electrons. 
• in 1913 Bohr suggested that electrons orbit the nucleus in shells, which prevents the atom from collapsing
• In the 20th century, Chadwick discovered neutral particles in the atomic nucleus called neutrons