Question

What is the formal charge on each atom in ${\mathrm{CO}}_2$?

Answer 1

Formal charge

• The formal charge is the charge assigned to an atom in a molecule based on the assumption that electrons in all chemical bonds are evenly shared across atoms, independent of relative electronegativity.
• It is the difference between the number of valence electrons an atom has in its neutral free state and the number assigned to it in a Lewis structure.
• The formula for formal charge:

$FC=V-N-\frac{B}{2}$

• Here, $FC$ is the formal charge, $V$ is number of valence electrons, $N$ is number of nonbonding valence electrons and $B$ is total number of electrons shared in bonds.

Formal charge on Carbon atom in ${\mathbf{CO}}_{\mathbf{2}}$

• In carbon dioxide $\left({\mathrm{CO}}_2\right)$, carbon double bonded to both oxygen atoms.
• The valence electron of carbon is four.
• The number of nonbonding valence electron is zero.
• The total number of electrons shared in bonds is eight.
• The formal charge on carbon atom in ${\mathrm{CO}}_2$:

$\begin{array}{rcl}FC& =& V-N-\frac{B}{2}\\ & =& 4-0-\frac{8}{2}\\ & =& 4-0-4\\ & =& 0\end{array}$

• The formal charge on carbon atom in ${\mathrm{CO}}_2$ is zero:

Formal charge on oxygen atom in ${\mathbf{CO}}_{\mathbf{2}}$

• The valence electron of oxygen is six.
• The number of nonbonding valence electron is four.
• The total number of electrons shared in bonds is four.
• The formal charge on oxygen atom in ${\mathrm{CO}}_2$:

$\begin{array}{rcl}FC& =& V-N-\frac{B}{2}\\ & =& 6-4-\frac{4}{2}\\ & =& 2-2\\ & =& 0\end{array}$

• The formal charge on oxygen atom in ${\mathrm{CO}}_2$ is zero: