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Question #44303

Difficulty: 8/10 | 🟢 Free

The Question

Consider two pairs of elements $(X, Y)$ and $(Z, W)$ that exhibit diagonal relationships in the periodic table. $X$ is an alkali metal that forms predominantly a normal oxide ( $\ce{X2O}$ ) upon heating in excess air. $Z$ is an alkaline earth metal whose oxide is amphoteric. Which of the following represents the correct sequence of standard first electron gain enthalpies ( $\Delta_{eg}H^\ominus$ in $\text{kJ mol}^{-1}$ ) for the isolated gaseous atoms of $X, Y, Z,$ and $W$ respectively at $298\ \text{K}$ ?

Options

A.
$-60,\ +67,\ +66,\ -43$
B.
$-60,\ -60,\ +66,\ +66$
C.
$-60,\ -43,\ +66,\ +67$
D.
$+66,\ +67,\ -60,\ -43$

Check Your Answer

Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Mechanism_Trap

Don't get tricked by the process! Learn the mechanical traps used in competitive exam questions.

Concepts Behind This Question

Electron Gain Enthalpy (EGE)|Diagonal Relationship|Screening Effect in Inner Shells

Why This Question Is Tricky

Common Mistakes

Students often over-extrapolate the 'Diagonal Relationship' heuristic.

While diagonally related pairs like Li/Mg and Be/Al share similar atomic/ionic radii, polarizing power, and electronegativity due to comparable charge-to-size ratios, their Electron Gain Enthalpies ( $\Delta_{eg}H^\ominus$ ) do not align.

The hidden variable is the strict dependence of $\Delta_{eg}H^\ominus$ on the exact valence electronic configuration (n+l rule).

Mg and Be have fully filled stable s-subshells ( $ns^2$ ), forcing the incoming electron into a higher energy p-orbital, resulting in a positive (endothermic) $\Delta_{eg}H^\ominus$ .

Conversely, Li ( $ns^1$ ) and Al ( $ns^2 np^1$ ) can accommodate electrons in their current subshells, yielding negative (exothermic) values.

The trap penalizes the assumption that diagonal similarity extends to orbital-dependent thermodynamic properties.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Classification of Elements and Periodicity in Properties - Class 11 Part 1
Concept	Electron Gain Enthalpy (EGE)
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free