Among the second period elements the actual ionisation enthalpies are in the order `LiltBltBeltCltOltNltFltNe`.
Explain why (a) Be has higher `Delta_(i)H` than `B` and (b) `O` has lower `Delta_(i)H` than `N` and `F`?

Explanation

(i) The ionization enthalpy among other things depends upon the type of electron to be removed from the same principal shell. In case of Be `(1 s^(2) 2 s^(2))` the outermost are more strongly attracted by the nucleus than 2p- electron, therefore lesser amount of energy is required to knock out a 2p- electron than a 2s- electron . Consequently `Delta_(i) H` of Be is higher than that `Delta_(i)H` of B.
(ii) The electronic configuration of `N (1 s^(2) 2s^(2) 2p_(x)^(1) 2p_(y)^(1) 2p_(z)^(1))` in which 2p-orbitals are exactly half- filled is more stable than the electronic configuration of `O(1 s^(2) 2s^(2) 2p_(x)^(2) 2p_(y)^(1) 2p_(z)^(1))` in which a the 2p- orbitals are neither exactly half - filled nor completely filled. Therefore it is difficult to remove an electron from N than from O. As a result `Delta_(i)H` of N is higher than that of O. Further the electronic configuration of F is `1 s^(2) 2s^(2) 2p_(x)^(2) 2p_(y)^(2) 2p_(z)^(1)`. Because of higher nuclear charge (+9) the first ionization of stability of F is higher than that of O. Further , the effect of increased nuclear charge outweights effect of stability due to exactly half -filled orbitals therefore the `Delta_(i) H` of N and O are lower than that of F.