An atom of an element has full $K$ and $L$ shells and has $5$ electrons in its $M$ shell. What is its atomic number?

$(a)$ Why did Rutherford choose gold and not any other lighter element (such as aluminium) for his experiment? Explain your answer giving two reasons.
$(b)$ Write the name and symbol of the particle chosen by Rutherford for bombardment against the gold foil in his experiment.

Atomic models have been improved over the years. Arrange the following atomic models in their chronological order:
$(i)$ Rutherford's atomic model
$(ii)$ Thomson's atomic model
$(iii)$ Bohr's atomic model

$J. Chadwick$ discovered a subatomic particle which has no charge and has a mass nearly equal to that of a proton. Name the particle and give its location in the atom.

State whether the following statement is true or false:
In a chemical substance,the elements are always present in varying proportions by mass.

List applications of any three isotopes in various fields.