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

Find the number of electrons,protons,and neutrons possessed by the alpha particles $\left({ }_{2}^{4} He ^{2+}\right)$ used in the gold leaf experiment.

Given that a natural sample of iron $(Fe)$ has isotopes $_{26}^{54}Fe$,$_{26}^{56}Fe$,and $_{26}^{57}Fe$ in the ratios of $5\%$,$90\%$,and $5\%$ respectively. What will be the average atomic mass of iron $(Fe)$ (in $u$)?

Study the table given below and answer the questions that follow:

Particle	Electrons	Protons	Neutrons
$M$	$2$	$3$	$4$
$N$	$10$	$9$	$10$
$O$	$8$	$8$	$8$
$P$	$8$	$8$	$10$

$(a)$ Write the mass numbers of particles $M, N, O$ and $P$.
$(b)$ Select a cation and an anion from the above table.

Explain with examples $(i)$ Isotopes and $(ii)$ Isobars. Give any two uses of isotopes.

$(a)$ What are isobars?
$(b)$ Atomic number of an element $Y$ is $17$.
$(i)$ Write its electronic configuration.
$(ii)$ What is the number of valence electrons in $Y$?
$(iii)$ How many electrons are needed to complete the octet of $Y$?
$(iv)$ Is it a metal or non-metal?
$(c)$ The valency of $Na$ is $1$ and not $7$. Give reason.