For Mg the ionisation enthalpies are Delta_iH_1 = 738, Delta_iH_2 = 1451 and Delta_iH_3 = 7733 kJ mol^-1. The very large third value arises because

the third electron must be taken from the compact, tightly bound 2p^6 core of Mg^2+ with a large effective nuclear charge

Mg^2+ has a half-filled 3p subshell

Mg^2+ has a larger radius than Mg+

the third electron must be taken from the compact, tightly bound 2p^6 core of Mg^2+ with a large effective nuclear charge

the third electron is shielded by the 3s electrons

JEE Advanced levelSingle correct optionDifficulty 3 of 3pt150

For $Mg$ the ionisation enthalpies are $Δ_{i} H_{1} = 738$ , $Δ_{i} H_{2} = 1451$ and $Δ_{i} H_{3} = 7733$ $kJ mo l^{- 1}$ . The very large third value arises because

A $Mg X^{2 +}$ has a half-filled $3 p$ subshell
B $Mg X^{2 +}$ has a larger radius than $Mg X^{+}$
Cthe third electron must be taken from the compact, tightly bound $2 p^{6}$ core of $Mg X^{2 +}$ with a large effective nuclear charge
Dthe third electron is shielded by the $3 s$ electrons

After two electrons are lost,

Mg X^{2 +}

has the neon configuration. The next electron comes from the

n = 2

shell, closer to the nucleus and not shielded by any outer electrons, so the energy jumps by a factor of about 5 (7733/1451 = 5.3).