PCl3 is a covalently-bonded molecule which is officially named phosphorus trichloride. It has three chlorine atoms around a single phosphorus atom, and that phosphorus also has a lone pair of electron which complete its octet.
Its lewis structure is shown below:
The phosphorus atom in the centre needs three single bonds and needs room for one lone pair of electrons. None of these require pi-bonding, which would result in needing unhybridized 3p orbitals.
Thus, the 3s orbital and all three 3p orbitals hybridize together to form four degenerate sp3-hybridized orbitals. “Degenerate” means they are all the same energy as each other. They are higher in energy than the 3s but lower in energy than the three 3p orbitals that were ‘used’ to make them.
sp3 hybridization usually leads to a tetrahedral shape, and if you are asked for Electron Pair Geometry, your teacher wants to hear tetrahedral. But because one of those four hybridized orbitals is a lone pair, there isn’t really anything there – just space where the electron ‘probably’ is.
The molecular geometry of PCl3 is trigonal pyramidal and its VSEPR notation is AX3E. The “A” represents the central atom (the phosphorus), each X represents a chlorine atom, and the E represents the lone pair.
You can watch me draw the Lewis Dot Diagram for PCl3 here:
Previously on this blog, I showcased why CH3F is tetrahedral and has AX4 VSEPR notation.