The geometry of a molecule is a result of interactions between electron pairs within the molecule. Each of these pairs has identical charge sign and magnitude. The electron pairs therefore repel each other and assume positions in the molecule as far apart from each other as possible. This theory describing the positions of electron pairs is the valence shell electron pair repulsion theory (VSEPR theory). The VSEPR theory predicts the shapes of the molecules. Four orbitals form a tetrahedral shape, three orbitals form a trigonal planar shape, and two sp orbitals are linear. Additional hybridization may occur with the use of one or more d subshells. For example, if a central atom of phosphorous is bonded to five chlorine atoms, one s orbital, three p orbitals, and one d orbital will hybridize to form five orbitals. Five hybridized orbitals assume a trigonal bipyramidal configuration. Six hybridized orbitals assume an octahedral configuration. The number of orbitals containing unshared electrons plus the number of sigma bonds gives the number of hybridized orbitals in a molecule. This number is the steric number for that particular molecule and corresponds to the given geometric shape.
