Question

A lead block used to keep the source in $ \alpha - ray $ scattering experiment. What is its purpose?

Answer 1

Hint: $ \alpha - ray $ scattering experiment was done by Rutherford who was a British scientist. He conducted this experiment, noted down to observations and based on these observations he gave the atomic model which is also known as Rutherford atomic model. It led to the discovery of electrons in an atom.

Complete answer:
Rutherford, a British scientist, conducted the $ \alpha - ray $ scattering experiment in which he bombarded a thin sheet of gold with the $ \alpha - particles $ and then studied the path taken by the $ \alpha - particles $ after bombardment.
So, now we will discuss the observations. He observed that the majority of the particles passed the foil without being deflected and he concluded that most of the space in an atom is empty. He observed that some $ \alpha - particles $ were deflected by a very small angle and concluded that positive charge is concentrated or present at the centre of the atom. He also noticed that very few particles deflected back at $ {180^o} $ angle and hence, it was concluded that volume of the positively charged particles is very less as compared to total volume of the atom.
He discovered that electrons are negatively charged particles which are surrounding the concentrated positive charge in an atom.
In this experiment, a lead block was used to keep the source of $ \alpha - ray $ which is a very dense material and $ \alpha - particles $ cannot pass through it (or penetrate it) and bombard on the targeted material.

Note:
There are many limitations of the Rutherford atomic model such as it could not explain the stability of the electron revolving in an atom. It could not explain the arrangement of electrons inside the atom. He also failed to explain the experimental results of the Heisenberg uncertainty principle and quantum mechanics.