Question

What is the formal charge on the indicated atom in each of the following species? (a) sulfur in \(\mathrm{SO}_{2}\) (b) nitrogen in \(\mathrm{N}_{2} \mathrm{H}_{4}\)

Short answer

Answer: (a) The formal charge of sulfur in SO₂ is +2. (b) The formal charge of each nitrogen atom in N₂H₄ is +2.

Step-by-step solution

Valence electrons

Determine the number of valence electrons for each atom in the molecule. Sulfur has 6 valence electrons and oxygen has 6 valence electrons.

Divide bonding electrons

In \(\mathrm{SO}_{2}\), sulfur is double-bonded to two oxygen atoms. Each double bond consists of 4 electrons, which should be divided equally between the sulfur atom and the two oxygen atoms. Therefore, 2 electrons are assigned to sulfur from each bond, making a total of 4 assigned electrons for sulfur.

Calculate the formal charge

To calculate the formal charge of sulfur, subtract the assigned electrons from the valence electrons: Formal charge of S = Valence electrons - Assigned electrons = 6 - 4 = +2. So, the formal charge of sulfur in \(\mathrm{SO}_{2}\) is +2. (b) Finding the formal charge of nitrogen in \(\mathrm{N}_{2} \mathrm{H}_{4}\)

Valence electrons

Determine the number of valence electrons for nitrogen and hydrogen in the molecule. Nitrogen has 5 valence electrons, hydrogen has 1 valence electron, and there are two nitrogen atoms present in the molecule.

Divide bonding electrons

In \(\mathrm{N}_{2} \mathrm{H}_{4}\), each nitrogen atom is forming three single bonds (one with another nitrogen and two with hydrogen atoms). Each single bond has 2 bonding electrons, so 1 electron will be assigned to nitrogen from each bond, making a total of 3 assigned electrons for each nitrogen.

Calculate the formal charge

To calculate the formal charge of nitrogen, subtract the assigned electrons from the valence electrons: Formal charge of N = Valence electrons - Assigned electrons = 5 - 3 = +2. So, the formal charge of each nitrogen atom in \(\mathrm{N}_{2} \mathrm{H}_{4}\) is +2.