The reactivity of an element is very much related to its inoization enthalpy. Observe the following graph in wich the first ionozation enthalpies `(delta_1H)` of elements of the sercond period are ploptted agiains their atomic number (Z):

Explanation

From the graph it is clear that, Be has higher inoization enthalpy than B. nitrogen has higher ionisation enthalpy than O. this is because Be has complitlely filled `(1s^2 2s^2)` 2sorbital.