Ionization energy tells us about how easy or difficult to remove electron.
It is relatively easy to remove valence electrons i.e. require low ionization energy
It is more difficult to remove core (inner) electrons i.e. require much higher ionization energy
The first ionization energy is the key factor in an element’s chemical reactivity.
Atoms with a low Ei1 tend to form cationsduring reactions, whereas those with a high Ei1 often form anions (except the noble gas).