▁bond ▁angle ▁is ▁the ▁angle ▁formed ▁between ▁three ▁atoms ▁across ▁at ▁least ▁two ▁b onds . ▁For ▁four ▁atoms ▁bond ed ▁together ▁in ▁a ▁chain , ▁the ▁t ors ional ▁angle ▁is ▁the ▁angle ▁between ▁the ▁plane ▁formed ▁by ▁the ▁first ▁three ▁atoms ▁and ▁the ▁plane ▁formed ▁by ▁the ▁last ▁three ▁atoms . ▁ ▁There ▁exists ▁a ▁mathematical ▁relationship ▁among ▁the ▁bond ▁angles ▁for ▁one ▁central ▁atom ▁and ▁four ▁peri pher al ▁atoms ▁( l abeled ▁ 1 ▁through ▁ 4 ) ▁expressed ▁by ▁the ▁following ▁determin ant . ▁ ▁This ▁constraint ▁removes ▁one ▁degree ▁of ▁freedom ▁from ▁the ▁choices ▁of ▁( origin ally ) ▁six ▁free ▁bond ▁angles ▁to ▁leave ▁only ▁five ▁choices ▁of ▁bond ▁angles . ▁ ▁( Note ▁that ▁the ▁angles ▁ θ 1 1 , ▁ θ 2 2 , ▁ θ 3 3 , ▁and ▁ θ 4 4 ▁are ▁always ▁zero ▁and ▁that ▁this ▁relationship ▁can ▁be ▁modified ▁for ▁a ▁different ▁number ▁of ▁peri pher al ▁atoms ▁by ▁expand ing / cont ract ing ▁the ▁square ▁matrix .) ▁ ▁M ole cular ▁geometry ▁is ▁determined ▁by ▁the ▁quantum ▁mechanical ▁behavior ▁of ▁the ▁electrons . ▁Using ▁the ▁val ence ▁bond ▁approximation ▁this ▁can ▁be ▁understood ▁by ▁the ▁type ▁of ▁b onds ▁between ▁the ▁atoms ▁that ▁make ▁up ▁the ▁mole c ule . ▁When ▁atoms ▁interact ▁to ▁form ▁a ▁chemical ▁bond , ▁the ▁atomic ▁orbit als ▁of ▁each ▁atom ▁are ▁said ▁to ▁combine ▁in ▁a ▁process ▁called ▁orb ital ▁hy brid isation . ▁ ▁The ▁two ▁most ▁common ▁types