Calculate the number of neutrons present in the nucleus of an element $X$ which is represented as $^{31}_{15}X$.

The electron distribution in an aluminium atom is

$A$ helium atom has two electrons in its valence shell,but its valency is not two. Explain.

Match the names of the scientists given in column $A$ with their contributions towards the understanding of the atomic structure as given in column $B$.

Column $A$	Column $B$
$(a)$ Ernest Rutherford	$(i)$ Indivisibility of atoms
$(b)$ $J$.$J$. Thomson	$(ii)$ Stationary orbits
$(c)$ Dalton	$(iii)$ Concept of nucleus
$(d)$ Neils Bohr	$(iv)$ Discovery of electrons
$(e)$ James Chadwick	$(v)$ Atomic number
$(f)$ $E$. Goldstein	$(vi)$ Neutron
$(g)$ Mosley	$(vii)$ Canal rays

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

Find out the valency of the atoms represented by the Fig. $(a)$ and $(b)$.