Question

What is the charge of S atom in ${\mathrm{SO}}_3$?

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 the number of valence electrons, $N$ is the number of nonbonding valence electrons and $B$ is total number of electrons shared in bonds.

Charge of Sulfur atom in Sulfur trioxide

• The valence electron of the sulfur atom is six.
• Structure of Sulfur trioxide $\left({\mathrm{SO}}_3\right)$:

• The number of nonbonding valence electron is zero.
• The total number of electrons shared in bonds are twelve.
• Formal charge of S atom is as follows:

$\begin{array}{rcl}FC& =& V-N-\frac{B}{2}\\ & =& 6-0-\frac{12}{2}\\ & =& 6-0-6\\ & =& 0\end{array}$

• Therefore, the charge of sulfur in ${\mathrm{SO}}_3$ is zero.