Q.5 Among CH4, H2O, NH3 and PH3, the molecule having the smallest percent s character for the covalent bond (X—H) between the central element (X = C, O, N or P) and hydrogen is
(A) CH4
(B) H2
(C) NH3
(D) PH3
PH3 has the smallest percent s-character in its X-H bonds among the given options. This stems from its minimal hybridization compared to the sp³ hybridization in CH4, H2O, and NH3.
Option Analysis
CH4: Carbon undergoes sp³ hybridization, mixing one 2s and three 2p orbitals to form four equivalent sp³ orbitals. Each C-H bond uses an sp³ orbital with 25% s-character (1/4 s).
H2O: Oxygen also shows sp³ hybridization despite two lone pairs. The O-H bonds form from sp³ orbitals, maintaining 25% s-character, though bond angles compress to 104.5° due to lone pair repulsion.
NH3: Nitrogen exhibits sp³ hybridization with one lone pair. N-H bonds involve sp³ orbitals with 25% s-character, and the bond angle is 107° from bond pair-lone pair repulsion.
PH3: Phosphorus barely hybridizes due to the inert pair effect—the large 3s-3p energy gap keeps the 3s orbital mostly unhybridized with the lone pair. P-H bonds form primarily from pure 3p orbitals (nearly 0% s-character), yielding a ~93.5° bond angle.
Introduction to Percent S-Character in X-H Bonds
Percent s-character determines bond strength and length in molecules like CH4, H2O, NH3, and PH3. For CSIR NET aspirants studying hybridization, understanding why PH3 deviates from sp³ norms is crucial. This analysis covers each molecule’s central atom (X = C, O, N, P) bonding with hydrogen.
Hybridization Breakdown
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CH4 (sp³, 25% s): Perfect tetrahedral geometry, 109.5° angles, all bonds equivalent.
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H2O (sp³, 25% s): Bent shape, two lone pairs compress angles to 104.5°.
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NH3 (sp³, 25% s): Trigonal pyramidal, one lone pair gives 107° angles.
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PH3 (minimal hybridization, ~0% s): Bonds use pure p orbitals; inert pair keeps s electrons non-bonding.
| Molecule | Hybridization | % s-Character (X-H) | Bond Angle |
|---|---|---|---|
| CH4 | sp³ | 25% | 109.5° |
| H2O | sp³ | 25% | 104.5° |
| NH3 | sp³ | 25% | 107° |
| PH3 | ~pure p | ~0% | 93.5° |
Why PH3 Has Smallest S-Character
The inert pair effect in phosphorus stabilizes the 3s² electrons, preventing effective s-p mixing. Unlike nitrogen’s small 2s-2p gap in NH3, PH3 bonds rely on 3p orbitals overlapping with H 1s, minimizing s-contribution.
Answer: (D) PH3
