For the ionization energy, the trend is that it increases as we move from left to right across a period. However, when we go from N to O, the IP is lower b/c all 3 orbitals are being occupied and the electron of O hs to be put in a orbital which already contains a electron. Therefore electron-electron repulsion occurs and O has a ower IP than N. This is true for P and As.
But when we get to Sb and Bi, the IP of the next elements INCREASES and doesn't follow the above rule. How come?