Write the conclusions drawn by Rutherford for the following observations during his $\alpha$-scattering experiment:
$(a)$ Most of the alpha particles passed straight through the gold foil.
$(b)$ Some alpha particles got deflected from their path.
$(c)$ $A$ very small fraction of alpha particles got deflected by $180^{\circ}$.

Write two differences between isobars and isotopes.

In the Thomson's model of atom,which of the following statements are correct?
$(i)$ The mass of the atom is assumed to be uniformly distributed over the atom.
$(ii)$ The positive charge is assumed to be uniformly distributed over the atom.
$(iii)$ The electrons are uniformly distributed in the positively charged sphere.
$(iv)$ The electrons attract each other to stabilize the atom.

Write any two observations which support the fact that atoms are divisible.

The atomic numbers of calcium and argon are $20$ and $18$ respectively,but the mass number of both these elements is $40$. What is the name given to such a pair of elements?

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