Changing Models of the Nucleus (AQA A Level Physics)

John Dalton’s Model (1803)

• Dalton imagined that all matter was made of tiny solid particles called atoms
• Dalton’s model proposed that:
  • 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 which proposed that:
  • the atom consists of positive and negative charges in equal amounts so that it is neutral overall
  • atoms can be 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)

• Ernest Rutherford, 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 deflected, or even backscattered
• The Rutherford model was developed as a result of this experiment, which states:
  • atoms are mostly empty space
  • atoms have a central, positively charged nucleus containing the majority of the mass

Neils Bohr’s Model (1913)

• Neils Bohr improved upon Rutherford’s model
• Bohr used mathematical ideas to develop the ‘planetary model of the atom’ which states:
  
  • electrons orbit the nucleus, like planets around a star
  • electrons occupy well-defined shells or energy levels around the nucleus

Quantum Mechanical Model (1926)

• Erwin Schrödinger 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 completed the model of the atom we know today

Timeline of the changing models of the nucleus