Backscattering

In the Rutherford experiment alpha particles were observed to be 'backscattered'. That does not mean they were 'reflected' by the foil. Reflection is when the angle of incidence = the angle of reflection - we use that term primarily when looking at light.

Scattering is different from reflection in that there is no fixed mathematical relationship between the angle before interaction and the angle after interaction. Scattering occurs within a wide range of angles. A particle can be scattered by a tiny amount - just changing its direction slightly, or by a large amount, an amount greater than 90^o, meaning that it reverses its general direction and is 'back scattered'.

Do not ever say that alpha particles are 'reflected' by the gold foil - or that they 'bounce back' - that will get you no marks!
The electrostatic repulsion between the nucleus of a gold atom and the alpha particle (because they both have a positive charge) causes the alpha particle to change its path direction - we say 'it is scattered' by the interaction. It does not 'bounce off' the foil!
Alpha particles can be 'back scattered' by the gold foil - actually reversing their general direction.

You need to show the examiner that you understand:

- it is the positive alpha being repelled by the positive gold nucleus that causes the scattering

- that the small number of alphas (1/8000) back scattered shows how 'widely spaced out' the gold nuclei are (most of the atom being empty space) and

- that the charge on the gold nucleii is very large compared to the alpha particle's charge.