Atoms are spheres and each element is made up of different spheres.
1897 JJ thompson
PLUM PUDDING : discovered electrons. Atom wasn’t solid and made up of other particles. Positive “pudding“ with negative electrons
1909 Ernest Rutherford
Discovered nucleus. - very small and positively charged. Concluded atom wasn’t solid mainly empty space : negative cloud.
Ernest Rutherford Gold leaf experiment
Positive alpha particles fired at thin gold leaf. Most went through, mainly empty space. Small number deflected back: hit a small positive nucleus
1913 Niels Bohr
Problem with Rutherford; cloud would collapse into positive nucleus.
Proposed electrons fixed into fixed energy shells.
Niels Bohr proof
When electromagnetic radiation is absorbed electrons move between shells. They emit this radiation when electrons move back down to lower energy shells.
Model today
Electrons don’t have to be in the same energy shells; subshells. This explains ionisation trends.
