Explanation:
In general, a compound with formula \({\rm{A}}{{\rm{B}}_{\rm{3}}}\) is \({\rm{s}}{{\rm{p}}^{\rm{2}}}\) hybridised with triangular planar geometry. However, the given compound has pyramidal structure which is possible only when it has a lone pair of electrons. Thus \({\rm{A}}{{\rm{B}}_{\rm{3}}}\) has 3 bond pairs and one lone pair.
\({\rm{Eg}}\,{\rm{:}}\,{\rm{N}}{{\rm{H}}_{\rm{3}}}{\rm{,}}\,{\rm{SnCl}}_{\rm{3}}^{\rm{ - }}{\rm{,}}\,{\rm{P}}{{\rm{H}}_{\rm{3}}}{\rm{,}}\,{\rm{PC}}{{\rm{l}}_{\rm{3}}}{\rm{,}}\,{\rm{NC}}{{\rm{l}}_{\rm{3}}}\)
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