Atomic Structure - the Plum Pudding model & the Nuclear model

1)

1840, John Dalton agreed with Democritus that all matter is made up of tiny spheres (atoms) that cannot be broken up, but each element is made up of different types of atoms.

2)

100 years later, J.J. Thompson discovered particles called electrons that could be removed from atoms ( Daltons theory was incorrect - atoms could be broken up). Thompson suggests that atoms were spheres of positive charge with tiny negative electrons stuck in them like the fruit in a plum pudding - the plum pudding model.

3)

The plum pudding theory didn't last, in 1990, scientists in Rutherford's lab tried firing a beam of alpha particles at thin gold foil - the alpha scattering experiment. From the plum pudding, they expected the particles to pass straight through the gold sheet, or only be slightly deflected.

4)

But although most of the particles did go straight through the sheet, some were deflected back the way they had come - something the plum pudding model couldn't explain.

5)

Because a few alpha particles were deflected back, the scientists realised that most of the mass of the atom was concentrated at the centre in a tiny nucleus. This nucleus must also have a positive charge, since it repelled the positive alpha particles.

6)

They alos realised that because nearly  all  the alpha particles passed straight through, most of an atom is just empty space. This was the first nuclear model of the atom.