Classification

On moving down in a group atomic radii increases due to successive addition of extra shell hence

Further As is in group 15 having one less electron in its orbital hence have higher atomic radii than group 16 elements.
i.e.,

Electron gain enthalpy of Na+ is reverse of (IE) Because reaction is reverse so

If elements are arranged in order of their increasing atomic numbers, element coming at intervals of and will have similar physical and chemical properties and thus grouped in one particular group.

For isoelectronic species ionic radii decreases as the charge on ion decreases. Further on moving down in a group ionic radii increases. Hence the correct order is

Isoelectronic species. If number of protons are more size will be less.

VI A group

group, (5th period)

Group 16 elements have less IP than Group 15 elements due to half filled configurations of Group 15 elements.

$S_1

In representative ( - and -block) elements, there is a general decrease of atomic radius in a period because the effective nuclear charge increases across a row. When a negative ion is formed, one or more electrons are added to an atom and the effective nuclear charge is reduced. The electron cloud expands and the size of negative ion becomes more. Hence, the correct order of increasing ionic radii is .

In case of 'Be' electron remove from '2s' orbital while in case of 'B' electron remove from '2p' orbital. '2s' orbital have greater penetration effect then '2p' orbitals. So 'Be' having more I.E. then 'B'

Second electron gain enthalpy is always positive for every element.