History of the atom

Start of 19th century, John Dalton described atoms as solid spheres, different spheres made up different elements

Plum Pudding

• In 1897 JJ Thomson concluded that they weren’t solid spheres

• His measurements of charge and mass showed they had to contain even smaller negatively charged particles - electrons.

• This was called the plum pudding model.

  

Nuclear model

• In 1909 Rutherford and his student Marsden conducted an alpha particle scattering experiment.

• Fired positively charged alpha particles at an extremely thin sheet of gold.

• More particles deflected than expected and lots in the middle, suggested there was a positively charged nucleus and electrons orbiting nucleus

• Called nuclear model

  

Bohrs Nuclear model

• Electrons in a cloud around nucleus would be attracted to positive charge and atom would collapse, proved Rutherford wrong.

• 1914 Bohr suggested that electrons orbiting nucleus in fixed shells at specific distances from nucleus

  

James Chadwick

• Discovered neutron