We gots PCl_3PCl3, and around the central phosphorus atom there are 3xx"bonding electron pairs"3×bonding electron pairs, the P-ClP−Cl bonds, and a "phosphorus-centred"phosphorus-centred lone pair.
To a first approximation, these electron pairs arrange themselves in a tetrahedron........with ideal bond angles of 109.5^@109.5∘. But we describe molecular geometry on the basis of the geometry of the ATOMS, not on the electron pairs. The symmetry descends from tetrahedral to trigonal pyramidal with /_Cl-P-Cl∠Cl−P−Cl angles of approx. 100-103^@100−103∘ due to the disproportionate influence of the phosphorus lone pair.
