First ionisation energy (or first ionization energy) refers to the energy required to remove an electron from a gaseous atom. Patterns of first ionisation energies in the Periodic Table. The lower the ionisation energy, the more easily this change happens: You can explain the increase in reactivity of the Group 1 metals (Li, Na, K, Rb, Cs) as you go down the group in terms of the fall in ionisation energy. Apart from zinc at the end, the other ionisation energies are all much the same. Which element among the following has the highest ionisation energy: fluorine, oxygen, neon. In general successive ionization energies increase in magnitude IE1