Question

How do you calculate the formal charge of `CO_2`?

Answer 1

Do you mean `CO_2` or `CO_3^(2-)`?

Explanation:

None of the atoms in our representation of the `O=C=O` molecule bear a formal charge. For carbon and oxygen, `Z=6` and `Z=8` respectively, i.e. there are 6 and 8 formal positive charges in the carbon and oxygen nuclei.

For neutrality, the carbon and oxygen atoms must also be each associated with 6 and 8 formal negative charges. Around oxygen there are 2 lone pairs (i.e. 4 electrons), and the oxygen has a half share in the four electrons that constitute the `O=C` bond. That's `4+2+2" (inner core)"` electrons; 8 electrons in total, and thus `O` is electrically neutral. Carbon shares 4 electrons from the double bonds, and with its 2 inner core electrons, has the required 6 electrons, 6 formal negative charges to balance the 6 positive nuclear charges.