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

The atomic size decreases as we move from left to right in a period. This is because when we move along a period the nuclear charge increases and therefore, the attraction of the nucleus for the outer electrons increases and hence the atomic size decreases. Within a group, the atomic size increases down the group. This is because of addition of a new energy shell at each succeeding element while the number of valence electrons remain the same. For detail, refer Text.