Explain variations in atomic radii and ionisation enthalpies in group-$16$.

(N/A) $(i)$ Atomic radii: The atomic radii of group-$16$ elements are smaller than the corresponding group-$15$ elements. This is due to an increase in effective nuclear charge.
In group-$16$,moving down the group,the atomic radii and ionic radii increase due to the addition of new electron shells at each successive element.
$(ii)$ Ionisation enthalpy: The first ionisation enthalpies of group-$16$ elements are lower than the corresponding group-$15$ elements. This is due to the fact that group-$15$ elements have extra stable half-filled $p$-orbitals.
The ionisation enthalpy of group-$16$ elements is quite high due to their small size and high nuclear charge. However,down the group,the ionisation enthalpy decreases with the increase in atomic size.