Q.Magnesium loses electrons successively to form Mg⁺, Mg²⁺ and Mg³⁺ ions. Which step will have the highest ionisation energy and why?

Ionisation enthalpy (ionisation energy) is the minimum energy needed to remove the most loosely bound electron from an isolated, neutral, gaseous atom in its ground state: $M_{(g)} + IE_1 \rightarrow M^+_{(g)} + e^-$. Removing further electrons from the resulting cations defines successive ionisation energies ($IE_2, IE_3,\ldots$), which always increase, $IE_1 < IE_2 < IE_3 < \ldots$, since each successive electron is removed from a progressively more positively charged (and so more tightly binding) species.

Across a period, ionisation enthalpy generally increases left to right, since the effective nuclear charge on the valence shell grows steadily (same mechanism as the atomic-radius trend). Two well-known exceptions occur where losing an electron takes a stable filled or half-filled subshell into an unstable partially-filled one, or vice versa: Be ($2s^2$, filled) has a higher IE1 than B ($2s^2 2p^1$), and N ($2p^3$, half-filled) has a higher IE1 than O ($2p^4$). …