The drop in the ionization energy in going from nitrogen (1s22s22p3) to oxygen (1s22s22p4) is usually explained asIn nitrogen each 2p electron is in a separate orbital. When the extra electron for oxygen is added, one 2p orbital becomes doubly occupied. The electron repulsions between the electrons in the doubly occupied orbital make either of these electrons easier to remove.from oxygen (1s22s22p4) to fluorine (1s22s22p5) to neon (1s22s22p6), the ionization energy increases because in each case the electron being considered is in a doubly occupied orbital.