John Dalton’s Model (1803)

• Dalton imagined that all matter was made of tiny solid particles called atoms
• Dalton’s model proposed:
  • Atoms are the smallest constituents of matter and cannot be broken down any further
  • Atoms of a given element are?identical?to each other and atoms of different elements are?different?from one another
  • When chemical reactions occur, the atoms rearrange to make different substances

   

J.J. Thomson’s Model (1897)

• Thomson discovered the electron
• He then went on to propose the ‘plum pudding’ model of the atom
• In this model:
  • The atom consists of positive and negative charges in equal amounts so that it is neutral overall
  • They were modelled as spheres of positive charge with uniformly distributed charge and density. The negatively charged electrons were thought to be stuck to the sphere like currants in a plum pudding

   

Rutherford’s Gold Foil Experiment (1909 – 1911)

• Hans Geiger and Ernest Marsden set out to test the plum pudding model
• They aimed beams of positively charged particles (alpha particles) at very thin gold foil
• According to the plum pudding model, these particles should have passed straight through, However, many of them were backscattered
• Ernest Rutherford explained these results in his ‘planetary model of atom’ which states:
  • Atoms have a central, positively charged nucleus containing the majority of the mass
  • Electrons orbit the nucleus, like planets around a star

   

Neils Bohr’s Model (1913)

• Bohr improved upon Rutherford’s planetary model
• Using mathematical ideas, he showed that electrons occupy?shells?or?energy levels?around the nucleus
  • These are at particular distances from the nucleus

   

Quantum Mechanical Model (1926)

• Erwin Schr?dinger took Bohr's model further and used equations to calculate the likelihood of finding an electron in a certain position
• This model can be portrayed as a nucleus surrounded by an electron cloud. Where the cloud is most dense, the probability of finding the electron is greatest and vice versa
• The atom was thought to only have a positively charged nucleus surrounded by negatively charged electrons. James Chadwick then discovered the neutron in 1932, which completes the model of the atom we know today

Timeline of the changing models of the nucleus