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Question #44695

Difficulty: 8/10 | 🟢 Free

The Question

Consider the following statements regarding the quantum mechanical vs. classical descriptions of Group 15 hydrides (\ce{NH3} and \ce{PH3}) at $298\text{ K}$ :

(I) The Bohr model's postulate of fixed trajectories with quantized angular momentum ( $mvr = \frac{nh}{2\pi}$ ) violates the Heisenberg Uncertainty Principle, rendering it incapable of explaining the directional probability densities required for orbital overlap and hybridization.

(II) The \ce{H-P-H} bond angle in \ce{PH3} ( $93.5^\circ$ ) implies that its bonding electrons reside in almost pure p-orbitals, leaving the lone pair in an orbital with high s-character ( $\approx 83\%$ ), a 3D spatial distribution impossible to rationalize using Bohr's 2D planar orbits.

(III) Due to the lower electronegativity of Phosphorus compared to Nitrogen, Bent's rule dictates that the \ce{P-H} bonding orbitals in \ce{PH3} must possess a higher fractional s-character than the \ce{N-H} bonding orbitals in \ce{NH3}.

(IV) In the quantum mechanical model, the lone pair in \ce{NH3} occupies an orbital with significantly less s-character than the lone pair in \ce{PH3}, primarily because the larger size of Phosphorus reduces steric repulsion between the hydrogen atoms, making hybridization energetically unnecessary.

Which of the above statements are CORRECT?

Options

A.
I, II, and IV
B.
I, II, and III
C.
II, III, and IV
D.
I, III, and IV

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

Bohr’s Model Limitations|Hybridization in Non-Standard Cases|Bent's Rule

Why This Question Is Tricky

Common Mistakes

The question tests the fundamental limitations of the Bohr model when applied to molecular geometry, forcing students to bridge classical atomic theory with quantum mechanical hybridization.

The 'Chemical Exception' tested is Drago's Rule and the misapplication of Bent's Rule.

Students often incorrectly assume that lower electronegativity of the central atom increases the s-character of its bonds, failing to recognize that relative electronegativity of the substituent (H) dictates s-character preference.

Additionally, it traps students who memorize that period 3 elements don't hybridize due to a 'larger ns-np energy gap' (a common misconception; the gap is actually smaller), correctly attributing it to reduced steric hindrance and weak bond energies.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Structure of Atom - Class 11 Part 1
Concept	Bohr’s Model Limitations
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free