In chemistry, Bent's rule describes and explains the relationship between the orbital hybridization and the electronegativities of substituents. The rule was stated by Henry A. Bent as follows: 
Valence bond theory gives a good approximation of molecular structure. Bents rule addresses disparities between the observed and idealized geometries. According to Bents rule, a central atom bonded to multiple groups will rehybridize so that orbitals with more s character are directed towards electropositive groups, and orbitals with more p character will be directed towards groups that are more electronegative.  By removing the assumption that all hybrid orbitals are equivalent, Bents rule leads to improved predictions of molecular geometry and bond strengths.  Bents rule can be justified through the relative energy levels of s and p orbitals. Bent's rule represents a modification of VSEPR theory for molecules of lower than ideal symmetry. For bonds with the larger atoms from the lower periods, trends in orbital hybridization depend strongly on both electronegativity and orbital size.