What is the structure of phosphorus(iii)bromide and phosphorus(iii)chloride and why?

Both phosphorus (III) bromide and phosphorus (III) chloride have structures similar to ammonia (NH3). Phosphorus forms four hybrid valence orbitals, three of which contain only one electron, while the fourth one contains two (lone pair). The three incomplete orbitals are involved in forming bonds with three bromine or chlorine atoms. Each bromine and chlorine atom forms four hybrid orbitals, with only one containing only one electron. That incomplete orbital on bromine or chlorine atom forms a bond with a similar incomplete orbital on the phosphorus atom. Consequently, the resulting molecule PBr3 or PCl3 has a triangular pyramid shape. It is also a dipole depending on the relative electronegativity of the atoms involved. Since phosphorus is less electronegative than either bromine and chlorine, the phosphorus side of the molecule is more positively charged than the other (bromine or chlorine) side of the molecule that is more negatively charge.