Scientific Models Of The Atom (AQA GCSE Combined Science: Synergy: Life & Environmental Sciences): Revision Note

Development of atomic models

• Science is always changing as we discover new evidence. 

• Early scientists had simple ideas about atoms, but experiments helped us improve those ideas.

• At first (1804), atoms were thought to be solid spheres that couldn’t be divided

• As scientists made new discoveries (like the electron), they had to update the model of the atom

The evolution of models of atomic structure

The plum pudding model

• In 1897, J.J. Thomson discovered the electron, a tiny particle with a negative charge.

  • This showed that atoms weren’t just solid spheres

  • It suggested that they have smaller parts inside them

• Thomson suggested a new model called the plum pudding model:

Diagram showing the plum pudding model of the atom

Rutherford Scattering

• In 1911, Rutherford fired tiny, positively charged particles (called alpha particles) at a very thin sheet of gold foil

• He expected:

  • The alpha particles would pass straight through — based on the plum pudding model

• What really happened:

  • Most particles went straight through

  • Some were deflected a little

  • A few bounced straight back

• What he concluded:

  • Atoms are mostly empty space

  • There is a tiny, dense, positively charged nucleus at the centre of the atom

  • Electrons move around the outside of the nucleus

• Later, scientists realised that the nucleus contains smaller particles called protons, each with a positive charge.

Nuclear model of the atom

Diagram showing Rutherford's nuclear model of the atom

Electrons in fixed shells

• Rutherford's nuclear model was later improved

• It was proposed that electrons orbit the nucleus in fixed shells (or energy levels)

• These shells are at specific distances from the nucleus

• This explained why electrons don’t spiral into the nucleus

• This model matched real experiments, so it was accepted by scientists

Diagram showing the electron shell model of the atom

The discovery of the neutron

• In 1932, another particle called the neutron was discovered in the nucleus

• Neutrons have no charge (they are neutral)

• They helped explain the mass of atoms, which couldn’t be explained by protons and electrons alone