Question

The correct orders of ionization enthalpies of the given elements are \[ \begin{aligned} \text{I. }& \mathrm{Li}>\mathrm{B}>\mathrm{Be}>\mathrm{Mg} \text{II. }& \mathrm{P}>\mathrm{Mg}>\mathrm{Al}>\mathrm{Na} \text{III. }& \mathrm{O}>\mathrm{F}>\mathrm{N}>\mathrm{C} \end{aligned} \] The correct answer is

• A. I, II, III
• B. II, III only
• C. I, III only
• D. I, II only

Hint:

Remember the important exceptions in ionization enthalpy: \[ \boxed{\mathrm{Be}>\mathrm{B}} \] and \[ \boxed{\mathrm{N}>\mathrm{O}} \] due to the stability of filled and half-filled subshells.

Answer 1

The Correct Option is C

Step 1: Check statement I. The first ionization enthalpy decreases down a group and generally increases across a period. The correct order is \[ \boxed{\mathrm{Li}>\mathrm{Be}>\mathrm{B}>\mathrm{Mg}.} \] Hence, statement I is correct.

Step 2: Check statement II. The correct order among the given elements is \[ \boxed{\mathrm{P}>\mathrm{Mg}>\mathrm{Al}>\mathrm{Na}.} \] Hence, statement II is correct.

Step 3: Check statement III. Because of the extra stability of half-filled \(2p^3\) configuration, \[ \mathrm{N}>\mathrm{O}. \] Also, \[ \mathrm{F}>\mathrm{N}>\mathrm{O}>\mathrm{C}. \] Thus, statement III is incorrect. Therefore, only statements I and II are correct. Hence, \[ \boxed{\text{Correct answer: I and II only}.} \] Therefore, the correct option is \(\boxed{(D)}\).