Explain the periodic trend of ionisation potential.

Why the first ionisation enthalpy of sodium is lower than that of magnesium while its second ionisation enthalpy is higher than that of magnesium.

By using paulings method calculate the ionic radii of K⁺ and CI^- ions in the potassium chloride crystal. Given that dk⁺-cl^-=3.14 Å. 

Explain the following, give appropriate reasons.
(i) Ionisation potential of N is greater than that of O.
(ii) First ionisation potential of C-atom is greater than that of B atom, where as the reverse is true is for second ionisation potential.
(iii) The electron affinity values of Be, Mg and noble gases are zero and those of N (0.02 eV) and P (0.80 eV) are very low.
(iv) The formation of F-(g) from F(g) is exothermic while that of O^2-(g) from O (g) is endothermic.

State the trends in the variation of electronegativity in group and periods.