The first ionization energy (in kJ/mol) of Na, Mg, Al and Si, respectively

a. 496, 737, 577, 786

b. 496, 577, 737, 786

c. 496, 577, 786, 737

d. 786, 737, 577, 496

Answer:

The expected order is Na < Mg < Al < Si.

But the actual/experimental order turns out to be Na < Al < Mg < Si, because of the fully filled s- sub shell of magnesium and the s²p¹ configuration of Al which makes it relatively easy for Al to lose its outermost electron.