# What is the logic behind the steric number formula?

My teacher taught us two methods two calculate steric number for determining hybridisation.

The first method was to count the total number of sigma bonds and add the lone pairs of the central atom. I have no problem with this method.

But we were also given a formula:

Steric No. = 0.5( V + SA (+/-) G)

where

V= no. of valence electrons in central atom;

SA= no. of monovalent surrounding atoms;

G= charge (if charge is positive we subtract it and if negative, we add it)

When I calculate the steric number by this formula it always comes out to be equal to the number calculated by the first method. But I am unable to understand why. What is the logic behind this formula?

EDIT: A similar question has been asked: The logic for hybridisation formula

But it doesn't answer my question as the answer in the link suggested assumes that I already understand why the formula works for monovalent atoms and is explaining why the formula works if there are polyvalent atoms too. In my case, I don't even know why the formula works for monovalent atoms.

• Does this answer your question? The logic for hybridisation formula Commented Oct 5, 2022 at 17:07
• @Mithoron the answer in the link suggested assumes that I already understand why the formula works for monovalent atoms and is explaining why the formula works if there are polyvalent atoms too. In my case, I don't even know why the formula works for monovalent atoms. Commented Oct 5, 2022 at 17:32