The ionization energy (IE) is qualitatively defined as the amount of energy required to remove the most loosely bound electron, the valence electron, of an isolated gaseous atom to form a cation.
It is quantitatively expressed in symbols as X + energy → X+ + e−
(a) Mg → Mg2+ + 2e−
(b) Ba → Ba2+ +2e-
(c) I + e−→ I-
(d) Al → Al3+ + 3e−