15. A molecular cage allows the nitrogen of ammonia to become sp2 hybridized
(instead of the usual sp3 hybridization). What do you expect the structure of this
ammonia molecule to be, and how many orbitals originating from the 2nd shell do
you expect it to have?
a. Flat and 4
b. Flat and 2
c. Pyramidal and 3
d. Pyramidal and 2
Molecular Cage Ammonia sp2 Hybridization: Structure and Orbitals Explained
A molecular cage forces ammonia’s nitrogen to adopt sp² hybridization, altering its typical structure for CSIR NET Life Sciences exam topics on molecular geometry and hybridization.
Normal Ammonia Hybridization
Ammonia (NH₃) normally features sp³ hybridization on nitrogen, using one 2s and three 2p orbitals from the second shell (n=2) to form four equivalent sp³ hybrid orbitals. Three orbitals bond to hydrogen atoms, while the fourth holds a lone pair, yielding a trigonal pyramidal shape with ~107° bond angles due to lone pair repulsion. This involves all four n=2 valence orbitals (2s, 2pₓ, 2pᵧ, 2p_z).
Effect of Molecular Cage
A molecular cage sterically constrains ammonia, preventing pyramidal inversion and enforcing planarity, which promotes sp² hybridization on nitrogen instead of sp³. In sp² hybridization, nitrogen mixes one 2s and two 2p orbitals (e.g., 2pₓ, 2pᵧ) to create three sp² hybrid orbitals in a trigonal planar arrangement (~120° angles), leaving one pure 2p orbital (e.g., 2p_z) for the lone pair perpendicular to the plane.
Orbital Count from 2nd Shell
Nitrogen’s valence shell (n=2) provides four orbitals: 2s, 2pₓ, 2pᵧ, 2p_z. Sp² hybridization uses three (2s + two 2p), producing three sp² hybrids plus one unhybridized 2p orbital, totaling four orbitals originating from the 2nd shell.
Option Analysis
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a. Flat and 4: Correct. The cage induces a flat (trigonal planar) structure via sp² hybridization, using three n=2 orbitals for hybrids but retaining four total (three sp² + one p).
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b. Flat and 2: Incorrect. Sp² requires one s + two p (three orbitals), not two; total n=2 orbitals remain four.
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c. Pyramidal and 3: Incorrect. Normal sp³ is pyramidal with four orbitals; cage forces flat sp², not pyramidal.
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d. Pyramidal and 2: Incorrect. Pyramidal matches normal sp³ (four orbitals), and two orbitals do not fit any standard hybridization.
