Ionization energy

 Energy required to remove an electron from an isolated atom is called ionization energy. 

Revolving electron and nucleus in center have an attraction force. To remove an electron from outermost electronic shell required breaking down of this attraction force.  Hence,  it required external energy. 

Electron that is more distance from nucleus have less attraction to nucleus. To remove it required less energy. Hence, less ionizaion energy. Simply,  to say more atomic raddi means more distance of electron from nucleus. Hence,  more atomic raddi indicates less attraction force and required less energy. 

In a group from top to bottom atomic raddi increases hence IE gradually decreases

In a perid from left to right atomic raddi decreases hence gradually in most case IE increases.  This happens due to gradual increasing of electron number in same electronic state shows more attraction to nucleus. Hence,  atomic raddi decreases. As a result IE jncreases. 

But yet difference is found in terms of B and Be

In electronic configeration of B and Be we find that outermost electron in  places in P orbital in B. 

But in Be outermost electron in s orbital. Due to shape of p orbital electron in p orbital more distance from nucleus. Moreover,  electron in s orbital of  also create sheilding effect to reduce the nucleus and outermost electron attraction.  

Again outermost electron places in same electronic state and same orbital in N and O. But yet in O,  any one of the p orbital is filled by 2 electron while every p orbital of N filled by single electron. Hence,  loosely bounded electron get more chance to be distance from nucleus due to electron electron repulsion and sheilding effect between nucleus and loosely bounded electron.  Sheilding effect is restriction of attraction between nucleus and outermost electron by other electron.