Question

Which one of the following is correct order of second ionisation potential of $Na , Ne , Mg$ and $Al$ ?

• A. Al < Na < Mg < Ne
• B. Ne < Al < Na < Mg
• C. Mg < Al < Ne < Na
• D. Na < Mg < Ne < Al

Answer 1

Answer: (C)

The electronic configuration of $Na , Ne , Mg$ and $Al$ after the removal of an electron is as

$Na ^{+}: 1 s^{2}, 2 s^{2}, 2 p^{6}$

$Ne ^{+}: 1 s^{2}, 2 s^{2}, 2 p^{5}$

$Mg ^{+}: 1 s^{2}, 2 s^{2}, 2 p^{6}, 3 s^{1}$

$Al ^{+}: 1 s^{2}, 2 s^{2}, 2 p^{6}, 3 s^{2}$

The ionisation energy decreases as the size of the atom/ion increases. Further ionisation energy increases with increase in nuclear charge. Since, $Na ^{+}$ gives a stable electronic configuration hence its $IE _{2}$ is highest among the given species and therefore the correct order of their $IE _{2}$ is

$Mg < Al < Ne < Na$