What's the "Charge" in Steric Number Formula

Question

I was reading about the Steric Number Formula here.

There, I came to know that the

Steric Number $N=\frac{V+M \pm I}{2}$ where $V = n(\ce{e-})$, the number of valence electrons of central atom, which is equal to the group number according to the old IUPAC system, $M = n(\text{atom})$, the number of monovalent atoms directly bonded to it, and $I$ is the number of positive or negative charges present (subtract it if the charge is positive, and add it if the charge is negative).

I was trying to apply the formula to $\ce{HNO_3}$ with lewis structure:

Image from here

From this: $N = \frac{5}{2}$ since there is no net charge on the compound and there is no monovalent atom bonded with $\ce{N}$. $N = \frac{5}{2}$ is not possible.

Maybe, I think the charge means, the charge on the central atom, but then again $\ce{N = \frac{4}{2} = 2}$ which is not right. (It's actually $3$)

Also, my friend says that the steric number is the half of

1. original valence electrons in central atom +
2. number of monovalent atoms in the entire compound
3. minus net charge on the entire compound

in which case, the formula would hold right as $V = 5, M = 1$ (as we now consider $\ce{H}$) and $\ce{I = 0}$ which would be $3$ and correct.

I would be glad if some explained what the "charge" represents. If the "charge" represents the net charge on the central atom, then we'd still need to make Lewis Structure and get the charge, then this technique would be not so easy again?

Thanks!

Answer 1

You are indirectly using the formula already stated by TRC though I am trying to make your formula accurate and unambiguous (especially because of the efforts you have put into the question).

$$N=\frac{V+M-T-I}{2}$$

N=Steric Number of atom X (say).

V=original (ie when an atom is isolated (not experimentally possible for all atoms)) valence electrons on the central atom.

M=Number of atoms directly bonded to X through a single bond.

T=Number of atoms directly bonded to X through a triple bond (use the example of $\ce{HCN}$ to check this).

I=Number of positive charges (formal charge) present on X atom (take I negative if the charge is negative).

Don't consider/take into account 'D=Number of atoms directly bonded to X through a double bond.'

Relating it to $\sigma +$lone pair formula:

$$\sigma=M+T+D$$ $$LP _{\text{(Remaining on X after bonding)}} =\frac{V-M-2D-3T-I}{2}$$

As per Buraian's suggestion, you may apply this formula for all the examples available here. Scroll down and find AXE method in that see the second table last row.