Ionization energy is those energy which is required to remove a electron from the outermost shell of an element.
Ionization energy (IE):It isdefined 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.H(g)→H+(g)+e−This energy is usually expressed in kJ/mol, or the amount of energy it takes for all the atoms in a mole to lose one electron each. When considering an initially neutral atom, expelling the first electron will require less energy than expelling the second, the second will require less energy than the third, and so on.