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The first ionisation enthalpies of Na, Mg, Al and Si are in the order:

(i) Na < Mg > Al < Si

 (ii) Na > Mg > Al > Si

(iii) Na < Mg < Al < Si

(iv) Na > Mg > Al < Si

Answers (1)

The answer is the option (i) Na < Mg > Al < Si

Explanation: While moving across the periods, the increasing nuclear charge overpowers the shielding effect. Consequently, the valence electrons are held more and more tightly and thus; the ionisation enthalpy increases across the period. In case of Mg, the valence electron is in the s-orbital which is a stable gas configuration, and penetration effect of s-subshell is more as compared to aluminium (Al) in which the outermost electron is in p-subshell.

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