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Question #44707

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The Question

Consider four elements $W$ , $X$ , $Y$ , and $Z$ belonging to the second period of the periodic table. Their unipositive cations ( $W^+$ , $X^+$ , $Y^+$ , $Z^+$ ) are isoelectronic with four neutral atoms $A_1$ , $A_2$ , $A_3$ , and $A_4$ respectively.

The valence electrons of atoms $A_1$ to $A_4$ reside in a subshell with the quantum number constraint $(n+l)=3$ . The atoms are characterized as follows: \begin{itemize} \item $A_1$ : Possesses exactly two unpaired electrons and experiences the lowest effective nuclear charge ( $Z_{\text{eff}}$ ) among the four. \item $A_2$ : Exhibits the maximum possible exchange energy for this subshell. \item $A_3$ : Possesses exactly two unpaired electrons and a higher $Z_{\text{eff}}$ than $A_1$ . \item $A_4$ : Possesses exactly one unpaired electron and the highest $Z_{\text{eff}}$ among the four. \end{itemize}

Which of the following represents the correct increasing order of the second ionization enthalpies ( $IE_2$ ) of the elements $W$ , $X$ , $Y$ , and $Z$ ?

Options

A.
$W < X < Y < Z$
B.
$W < Y < X < Z$
C.
$X < W < Y < Z$
D.
$W < X < Z < Y$

Check Your Answer

Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Numerical_Precision

Improve calculation stamina and avoid rounding errors in high-stakes competitive examinations.

Concepts Behind This Question

Quantum Number Constraints|Isoelectronic Energy Ordering|Ionization Enthalpy (IE) Anomalies

Why This Question Is Tricky

Common Mistakes

The question tests the student's ability to map quantum number constraints to specific electronic configurations and subsequently apply periodic trends with their quantum mechanical exceptions.

Students first identify the neutral atoms $A_1-A_4$ as C, N, O, and F based on the $(n+l)=3$ ( $2p$ subshell) constraint and unpaired electron counts.

This identifies the elements $W, X, Y, Z$ as N, O, F, Ne.

The core trap lies in evaluating the second ionization enthalpy ( $IE_2$ ).

A common textbook error is to assume $IE$ strictly increases with $Z_{\text{eff}}$ across a period ( $W < X < Y < Z$ ).

However, $IE_2$ involves removing an electron from the unipositive cations. $X^+$ ( $O^+$ ) has a half-filled $2p^3$ configuration, granting it anomalous stability due to high exchange energy (Hund's Rule).

Consequently, removing an electron from $O^+$ requires more energy than from $F^+$ ( $Y^+$ ), which has a $2p^4$ configuration and gains stability upon losing an electron to become $2p^3$ .

Thus, the correct order is $W < Y < X < Z$ .

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Structure of Atom - Class 11 Part 1
Concept	Quantum Number Constraints
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free