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According to my teacher, these three Lewis bases generally decrease in strength in the following order:

  1. Ammonia
  2. Water
  3. Hydroxide ion

I agree. However, he argues that their ability to act as Lewis bases stems from Coulombic repulsions. For example, nitrogen has only one lone pair. Water however has two lone pairs, and when water uses one of its lone pairs in a Lewis base fashion, the other lone pair weakens the resulting Lewis adduct through exerting a repulsive force on the lone pair used in forming the Lewis adduct. Same with hydroxide, except that the repulsions are even greater in hydroxide since it has three lone pairs.

I say that in addition to this effect, there is also the effect of the Lewis base being able to stabilize its valence electron density. Nitrogen is less electronegative than oxygen. Thus, any valence electron density on nitrogen will be more reactive than that on oxygen. On the other hand, oxygen in hydroxide ion has a greater valence electron density than that of oxygen in water, and hydroxide ion is a poorer Lewis base than hydroxide ion.

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    $\begingroup$ Why did you think that hydroxide would be weaker base? Also last words don't make sense. $\endgroup$ – Mithoron Jun 23 '15 at 22:14
  • $\begingroup$ As Mithoron rightly states: hydroxide ion is a poorer Lewis base than hydroxide ion is obviously a typo. What did you actually mean to say? $\endgroup$ – Greenonline May 24 '18 at 0:31

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