Successive Ionisation Energies

The energy required to remove an electron from a neutral gaseous atom is called First Ionisation Energy and is represented by I.E₁,

M (g)   +    I.E₁  → M⁺ (g)    +    e ^–

Once the first electron has been removed from the gaseous atom, it is possible to remove second electron from unipositive ion and then third electron from dipositive ion and so on. The energies required to remove first electron, second electron and third electron from neutral gaseous atom. Unipositive ion and dipositive ion are known as First Ionisation Energy (I.E₁), Second Ionisation Energy (I.E₂) and Third Ionisation energy (I.E₃)  respectively.

M ⁺   +   I.E₂   →  M²⁺    +    e ^–

M²⁺ +    I.E₃   →  M³⁺       +   e ^–

The second, third Ionisation energies and so on are collectively known as Successive Ionisation Energies

It may be noted that;

I.E₃   >    I.E₂   >    I.E₁

The order can be explained as follows;

After the removal of first electron, atom changes into Unipositive ion. In this ion the number of electrons decreases by one, but nuclear charge remains same as in neutral atom. As a result the force of attraction per electron increases and hence more energy is required to remove an electron. Hence value of I.E₂ is higher than I.E_1. Similarly removal of third electron requires more energy because the force of attraction per electron has further increased; hence I.E₃ is more than I.E_2.