Question

why fulfilled electron configuration has more ionisation energy?

Answer 1

Ionisation energy is the amount of energy required to remove an electron from the valence shell of the element.

For Be the electronic configuration is 1s2 2s2.

This one is stable form where it has filled valence shell… Thus to remove the 1st electron we need more energy.

Whereas in comparison Li has configuration 1s2 2s1. Here last shell has one electron for stability lithium must want to release the electron to get into fulfilled valence shell criteria. Thus it has less I.E.