The Plum Pudding Model (AQA GCSE Physics: Combined Science)

• Scientists understanding of what atoms are has changed through time
• Different models have been developed, and then replaced as new evidence from experiments is discovered
  • A model is a way of describing something in order to explain the way it behaves

Early Models of the Atom

• Greek and Indian philosophers were the first to try and describe the idea of everything being made up of smaller parts
• The Greek philosopher, Democritus, thought that although objects could be cut into smaller pieces, the smallest possible piece would be indivisible (it could not be cut any further)
  • The Greek word for ‘indivisible’ is atomos

Democritus thought about cutting objects into smaller and smaller pieces until they were indivisible

• Therefore, atoms were initially thought to be tiny spheres that could not be divided before the discovery of the electron
  • Later models described the atom as small solid spheres

What is the Plum Pudding Model of the Atom?

• At the end of the 19th Century, Physicist Joseph Jon Thomson discovered the existence of electrons
• This new evidence meant a better model of the atom was required
• Thompson proposed the Plum Pudding model
  • The atom was thought to consist of negatively charged electrons (the ‘plums’) in a positively charged ‘dough’ or ‘pudding’

J J Thomson thought of the atom as being a positively charged mass embedded with small negatively charged electrons – a bit like a plum pudding

• It was known that electrons were much smaller than atoms, so it made sense that they should be embedded within the larger atom
• Since electrons have a negative charge, it was reasoned that the rest of the atom would be positive, making the atom neutral overall

Why was the Plum Pudding Model Replaced?

• Further experiments in 1909 produced results that could not be explained by the Plum Pudding model, so it was replaced by the nuclear model