Question

What was the source of particles used by Rutherford in the experiment that showed the existence of the nucleus?

Answer 1

Alpha particles which are also called alpha rays are composed of two protons and two neutrons which are bound together into a particle which is equivalent to helium-4 nucleus. Due to the presence of protons, these are considered as positively charged species and are relatively slow as compared to other forms of nuclear radiations.

Complete answer:
In 1905, Rutherford did an experiment in order to test the plum pudding model. He directed the alpha particles at a very thin gold foil which was suspended in vacuum. Since the gold foil was very thin, it was expected that the alpha particles could pass straight away or may destroy the foil but the observations lead to a different conclusion.
On the basis of Rutherford alpha scattering experiment, following observations were made:
1. Most of the alpha particles elapsed straight through the foil.
2. A small number of alpha particles were deflected by a large angle $\left( { > {4^o}} \right)$ as they were passed through the foil.
3. A very small number of alpha particles were deflected back to an angle of ${180^o}$ i.e., the particles came straight back off the gold foil.
Therefore, Rutherford gave following conclusions on the basis of the alpha scattering experiment:
1. Most of the space of the atom is empty.
2. A small number of alpha particles were deflected at a large angle suggesting that there is a concentration of positive charge in the atom. As like repels like, so the positive alpha particles experienced repulsion by the positive charge.
3. The very small number of alpha particles returned back at an angle of ${180^o}$ suggested that the mass and the positive charge of atoms are concentrated in a tiny volume which was termed as nucleus.
Thus, Rutherford’s gold foil experiment proved the existence of a nuclear atom in which a small positively charged nucleus surrounded by empty space and a layer of electrons from outside the atom.
Hence, we can conclude that the source of particles used by Rutherford in the experiment that showed the existence of the nucleus was alpha particles.

Note:
It is important to note that the vacuum in the experiment was necessary because any deflection of alpha particles would only be because of collision with gold foil and not because of collisions with air particles and gold metal was used because it is the only metal which can be converted into thin sheets without cracking.