# Using the VSEPR theory, what would you predict the shape of the PCI3 molecule to be?

$\text{Trigonal pyramidal...............}$
There are 4 regions of electron density around the central phosphorus atom: $3 \times P - C l$ bonds, and one phosphorus-centred lone pair. These adopt a tetrahedral geometry as the shape which minimizes electron-pair interactions. But because we describe molecular geometry on the basis of bonding pairs of electrons, symmetry descends to trigonal pyramidal.
Because $\ddot{P}$ has a lone pair, which is close to the phosphorus atom, $\angle C l - P - C l$ should be somewhat compressed from ${109.5}^{\circ}$.