Discuss the variations in atomic and ionic radii of elements,the trends in ionisation enthalpies and electronegativity of group-$15$ elements.

(N/A) $1$. Atomic and Ionic Radii: The atomic and ionic radii of group-$15$ elements are smaller than those of group-$14$ elements due to increased effective nuclear charge. Down the group,radii increase due to the addition of new principal shells. The increase is significant from $N$ to $P$,but smaller from $As$ to $Bi$ due to the poor shielding effect of completely filled $d$ and $f$-orbitals.
$2$. Ionisation Enthalpy: Ionisation enthalpy decreases down the group due to the increase in atomic size and shielding effect. Group-$15$ elements have higher ionisation enthalpies than group-$14$ elements because of their extra stable half-filled $p$-orbital electronic configuration $(ns^2 np^3)$.
$3$. Electronegativity: Electronegativity decreases down the group with increasing atomic size. Nitrogen is the most electronegative element in this group.