The 1st ionisation energies generally increase, moving across the period 2 of the periodic table, due to the following reasons: There is an increased nuclear charge on valence electrons, moving across the period. The distance between the nucleus and electron almost remains constant as shell number remains the same. The shield effect due to intervalence electrons remains roughly the same. So 1st Ionization Energy increases from Lithium to Neon. However, there are few anomalies in between: Ionization Energy of Boron is less than Beryllium: This is because the valence electron of beryllium is in 2s orbital, while of boron in 2p orbital. The 2p orbital is greater greater distance from the nucleus so lesser Ionization Energy. Ionization Energy of Oxygen is less than Nitrogen: This is because the electron removed from oxygen is paired and experiences repulsion from electron within the same orbital called spin-pair repulsion.