A) Na < Mg > Al < Si
B) Na > Mg > Al > Si
C) Na < Mg < Al > Si
D) Na > Mg > Al < Si
Correct Answer: A
Solution :
First ionisation energy increases from left to right across a period, but Mg has extra stability than Al, due to full-filled 3s-orbitals. \[N{{a}_{11}}=1{{s}^{2}},2{{s}^{2}},2{{p}^{6}},3{{s}^{1}}\] \[M{{g}_{12}}=1{{s}^{2}},2{{s}^{2}},2{{p}^{6}},3{{s}^{2}}\] \[A{{l}_{13}}=1{{s}^{2}},2{{s}^{2}},2{{p}^{6}},3{{s}^{2}},3{{p}^{1}}\] \[S{{i}_{14}}=1{{s}^{2}},2{{s}^{2}},2{{p}^{6}},3{{s}^{2}},3{{p}^{2}}\] The correct order of first ionisation potential is Na < Mg > Al < Si
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