Trigonal pyramidal geometry is also shown by molecules which have four atoms or ligands. The molecular geometry of lone pairs of electrons give rise to an sp 3 hybridization and form tetrahedral geometry. The name of PBr 3 is phosphorus tribromide or tribromo phosphine.įor PBr 3, the central atom is phosphorus, three bromine atoms form bonds with it and it has one modified shape to trigonal pyramidal geometry. The above image shows the bond angle between phosphorus and bromine atoms which is 101 degrees in phosphorus and each bromine atom. The Lewis structure of this molecule is given below. But it has a trigonal pyramidal shape or geometry due to the presence of lone pairs on the central atom. So, there are four regions of electron density around the central atom.

Hence, the electron geometry will be tetrahedral because the phosphorus atom has one lone pair and three bonded pairs with bromine atoms. The phosphorus has one lone pair of electrons and bromine atoms have three lone pairs of electrons. The phosphorus is at the centre and it is surrounded by bromine atoms. The Lewis structure of PBr 3 contains three sigma bonds between phosphorus and three bromine atoms. I n the Lewis structure of PBr 3, the formal charge on the terminal bromine atom is zero. PBr 3 has 26 valence electrons of which bromine atoms can consume eight each in bonding pairs and two electrons in lone pairs on phosphorus. Since the geometry is asymmetrical, the molecule will be polar.

Hence, the hybridization of PBr 3 is sp 3 and the geometry will be trigonal pyramidal due to the presence of a lone pair of electrons on the central atom.

In PBr 3, phosphorus has five valence electrons in its valence shell, out of these five electrons, three form sigma bonds with three bromine atoms and two electrons remain on the phosphorus atom as a lone pair of electrons.

Electron domain geometry chart