The bond angle in a molecule of methane, ammonia and water decreases in the order 109.5, 107 and 104.5 degrees because
Correct answer: B. the number of lone pairs on the central atom increases from zero to one to two
- A. the central atoms become larger in that order
- B. the number of lone pairs on the central atom increases from zero to one to two
- C. the bonds become more ionic
- D. the molecules become heavier
Explanation
Each additional lone pair exerts stronger repulsion than a bond pair, squeezing the remaining bonds closer together, so the angle falls by roughly two and a half degrees for each one added. The underlying arrangement of four electron regions is tetrahedral in all three cases. This trio is the classic demonstration that lone pairs must be counted but are not seen in the shape.
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About VSEPR Theory
Valence shell electron pair repulsion theory predicts molecular shape by arranging bonding pairs and lone pairs around a central atom to minimize repulsion. Questions cover linear, trigonal planar, tetrahedral, trigonal bipyramidal and octahedral electron geometries, bond angles and distortions caused by lone pairs. Electron geometry is not always the same as molecular shape.
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