Question

Among the following, third ionization enthalpy is highest for

• A. Mg
• B. B
• C. Be
• D. Al

Answer 1

The correct option is C Be

Let's first look at the electronic configurations.

(Magnesium) Mg - 1s²2s²2p⁶3s²

(Boron) B - 1s²2s²2p¹

(Beryllium) Be - 1s²2s²

(Aluminium) Al - 1s²2s²2p⁶3s²3p¹

So, it is easy to deduce that both Be and Mg would have a significant increase in ionization enthalpy after losing 2 electrons as they are losing an entire shell.
But the 3rd ionization enthalpy of Be would be much higher since the shielding from the inner electrons is much lesser as compared to that of Mg.

So, Beryllium is the answer.