Question

Why basicity decreases down the group for group 15 elements?

Answer 1

The idea of giving H cations and OH anions in aqueous solution belong to Arrhenius, the donating and accepting protons to B & L and the donating and accepting of lone electron pairs to Lewis. Reducing agents do not "donate" electrons in a red-ox reaction; they lose electrons to oxidizing agents. Bases can get their electrons back i.e. the donation is recoverable but in case of reducing agents, there is no donation in the first place. Hydrogen is a reducing agent. It is more freely available (more weakly bound) in BiH3 than in ammonia, and therefore BiH3 is the stronger reducing agent. Ammonia is the smaller molecule, and the lone pair electrons are a more prominent feature of the molecule, and have no significant possibilities of being stabilized within the molecule by exchange (or jumping) around the additional d, and f orbitals that are at much lower energies in the heavier members of the family. They, therefore, can share ("donate") more strongly/easily with acidic species. For group 15 elements the order of basicity given is NH3 > PH3 > AsH3 > SbH3 > BiH3 And order of reducing strength is BiH3 > sbH3 > AsH3 > PH3 > NH3 .