Question

Why did Rutherford study radioactivety and how did the gold foil experiment really come about?

Answer 1

Rutherford's experiment:

A stream of α-particle (much like a stream of tiny bullets) was directed at a thin foil of gold atoms and a detector arranged to surround the sample completely except for a small hole for entry of the particles. The foil was several thousands of atoms thick.

What was expected?

Keeping the prevailing model in mind (Thomson's 'plum pudding') the cloud of positive electricity should offer little resistance to the passage of an α-particle even though the latter is charged. The reasoning is that α-particles are moving at high speed and have a great amount of forward momentum [(mass) x (velocity)] = momentum]. Even though there would be some repulsion due to like charges, the momentum would overcome this and the particles ought to be deflected slightly or go straight through.

What was observed?

• Most of the α-particles pass straight through the gold foil without any deflection from their original path.

• A few α-particles are deflected through small angles and few are deflected through large angles.
• A very few a-particles (‘1 in 1,00,000’) on hitting the gold foil bounced back, that is, were deflected by nearly 180°.

Explanation:

• Since most of the α-particles pass straight through the gold foil without any deflection it shows there is a lot of empty space in an atom.

• Some of the α-particles are deflected through small and large angles, which show that there is a 'centre of positive charge' in an atom, which repels the positively charged α particles and deflects them from the original path.

• Very few α-particles rebound on hitting the gold foil, which shows the nucleus is very dense and hard which does not allow α-particles to pass through it. The whole mass of the atom is centered at its nucleus.