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The steric number of a molecule is the number of atoms bonded to the central atom of a molecule plus the number of lone pairs on the central atom. It is often used in VSEPR theory (valence shell electron-pair repulsion theory) in order to determine the particular shape, or molecular geometry, that will be formed.
Steric number in VSEPR
Calculating the steric number of a molecule's central atom is a vital step in figuring out its geometry. On the molecule SF4, for example, the central sulfur atom has four ligands about it. In addition to the four ligands, sulfur also has one remaining lone pair (expanded octet does not apply in this compound). Thus, the steric number is 5. Comparing a central atom's steric number to the number of ligands (not counting lone pairs) allows one to figure out the geometry of that central atom.