History of the Atom (AQA GCSE Chemistry): Revision Note

History of the atom timeline

• Atoms & Scientific 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, 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

What is 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

The plum pudding model

Diagram showing the plum pudding model of the atom

Rutherford Scattering

What did Rutherford's scattering experiment show?

• 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 Ernest Rutherford's nuclear model of the atom

The Bohr model

What is the Bohr model?

• Niels Bohr improved Rutherford’s nuclear model

• He said 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

• Bohr’s model matched real experiments, so it was accepted by scientists

The Bohr model

Diagram showing Niels Bohr’s model of the atom

The discovery of the neutron

When was the neutron discovered?

• In 1932, James Chadwick discovered the neutron, another particle found 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