steric number

[Dahriel Aron 3A]

A tutor taught me how to calculate the steric number to easily determine the geometry of a structure based on its chemical formula, but I forgot how to calculate this number. Does anyone know how to get the steric number? I only remember that oxygens are not counted or included in this number.

[Chem_Mod]

The steric number is the same as the number of regions, or directions, of electron density. Any bond whether it be single double or triple counts as one region. Lone pairs also count as one region. So CH4, NH3, H2O all have steric numbers of 4. While BF3 has a steric number of 3.

[Alondra Loera 1A]

To determine the steric number of an atom, first count the number of bonds to that element; secondly count the number of lone pairs of electrons. Add the two numbers, and the result is the steric number.

For ex: the O in water H2O has a steric number of 4: 2-H atoms and 2-lone pairs.NH3 also has a steric number of 4: 3-H and 1-lone pair.

Also, with different # of bonds to them might have the same steric number, depending on the number of attached lone pairs.

Hope that helps!