Why Nf3 Smaller Bond Angle Than Nh3
NF3 than in NH3. In NF3 the lone pair therefore exerts greater repulsion toward the bonded pairs than in NH3. In addition, the longer N—F bond length makes the bp-bp distance greater in NF3 than in NH3, so that the bp/bp repulsion in NF3 is less than that in NH3. The net effect is that the bond angles are reduced more in NF3. We can represent this situation as:
- 107°
- Lone pair/bonded pair repulsions are stronger than bonded pair/bonded pair repulsions
Bonded pair/bonded pair repulsions are weaker in NF3 than in NH3 due to the longer N—F bond
We might expect the larger F atoms (r = 0.72 A) to repel each other more strongly than the H atoms (r = 0.37 A), leading to larger bond angles in NF3 than in NH3. This is not the case, however, because the N—F bond is longer than the N—H bond. The N—F bond density is farther from the N than the N—H bond density.
With the same kind of reasoning, VSEPR theory predicts that sulfite ion, SO32-, has tetrahedral electronic geometry. One of these tetrahedral locations is occupied by the sulfur lone pair, and oxygen atoms are at the other three locations. The molecular geometry of this ion is trigonal pyramidal, the same as for other AB3U species.
C. Valence Bond Theory
Experimental results suggest four nearly equivalent orbitals (three involved in bonding, a fourth to accommodate the lone pair), so we again need four sp3 hybrid orbitals.
A model of sulfite ion, SO32 .
hybridize sp3
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