The first model of an atom was given by

$(a)$ 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.

Dalton's atomic theory successfully explained which of the following laws?
$(i)$ Law of conservation of mass
$(ii)$ Law of constant composition
$(iii)$ Law of radioactivity
$(iv)$ Law of multiple proportion

Give reasons for the following:
$(a)$ In potassium and calcium,the filling of electrons in the $N$-shell starts after the $M$-shell has acquired $8$ electrons,although the $M$-shell can accommodate up to $18$ electrons. Why?
$(b)$ There is a difference between mass number and atomic mass.

$(a)$ Write two drawbacks of Rutherford's model of an atom.
$(b)$ State Bohr's postulates to overcome the drawbacks of Rutherford's model.

Rutherford's alpha $(\alpha)$ particles scattering experiment resulted in the discovery of: