Question

What are the formal charges in `"O"_3` (ozone)?

Answer 1

`-1,0,1`

Explanation:

A formal charge is equal to the number of valence electrons of an atom MINUS the number of electrons assigned to an atom.

Consider the resonance structures for `"O"_3`.

Oxygen has `6` valence electrons. Look at the top left oxygen atom. It has two lone pairs (`4` electrons) and a double bond (`2` electrons).

Even though a double bond contains `4` electrons total and is counted as such when seeing that oxygen's octet is filled, `2` electrons belong to each oxygen and they are shared among the two.

Let's examine the top resonance structure:

Left: `6` valence `-6` assigned `=color(blue)(0` formal charge
Center `6` valence `-5` assigned `=color(blue)(1` formal charge
Right: `6` valence `-7` assigned `=color(blue)(-1)` formal charge

Notice that even though the atoms have varying formal charges, the overall charge of `"O"_3` is the sum of the formal charges in the molecule: `0+1+(-1)=0`.

Ions' formal charge sums are `!=0`.