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Question
Explain why NH3 is basic while BiH3 is only feebly basic.
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Solution 1
Nitrogen has a small size, due to which the lone pair of electrons is concentrated in a small region. This means that the charge density per unit volume is high. On moving down a group, the size of the central atom increases, and the charge gets distributed over a large area, decreasing the electron density. Hence, the electron-donating capacity of group 15 element hydrides decreases on moving down the group.
Solution 2
- NH3 is more basic than BiH3, in spite of the fact that both N in NH3 and Bi in BiH3 contain a single pair of electrons, should operate as Lewis bases, and should have similar basic strengths.
- The reason for this is that the N atom is smaller (70 pm) than the Bi atom (148 pm).
- The N atom in NH3 has a significantly greater electron density than the Bi atom in BiH3 because of its smaller size.
- Because of this, NH3 is more basic and more effective at donating a single pair of electrons.
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