Determine the formal charge on the chlorine atom in the hypochlorite ion ClO- and the perchlorate ion, ClO4-, if the Cl atom has an octet. b. What are the oxidation numbers of chlorine in ClO- and ClO4-? c. What are the essential differences in the definitions of formal charge and oxidation number that lead to the differences in your answers to part (a) and (b)?
The Correct Answer and Explanation is:
a. Formal Charge Calculation
To determine the formal charge (FC) of chlorine in each ion, we use the formula:
Formal charge = (Valence electrons) – (Nonbonding electrons) – ½(Bonding electrons)
For ClO⁻ (hypochlorite ion):
Assume the Lewis structure has:
- Cl single bonded to O
- One lone pair on Cl
- 3 lone pairs on O
- One extra electron due to the negative charge
Chlorine (Cl) has 7 valence electrons.
Cl has:
- 2 nonbonding electrons
- 2 bonding electrons with O
FC = 7 – 2 – ½(2) = 7 – 2 – 1 = +4
But this is not realistic. For ClO⁻, a more reasonable Lewis structure shows Cl with 6 electrons around it (3 lone pairs) and one single bond to O.
So FC = 7 – 6 – ½(2) = 7 – 6 – 1 = 0
For ClO₄⁻ (perchlorate ion):
Assume Cl is in the center with four double bonds to four oxygens. Each O has two lone pairs. Total electrons: 7 (from Cl) + 4×6 (from O) + 1 (extra for negative charge) = 32 electrons.
Each double bond = 4 bonding electrons
FC = 7 – 0 – ½(8) = 7 – 0 – 4 = +3
b. Oxidation Numbers
In ClO⁻:
Oxygen = –2
Overall charge = –1
So Cl + (–2) = –1 → Cl = +1
In ClO₄⁻:
O = –2 × 4 = –8
Cl + (–8) = –1 → Cl = +7
c. Explanation of Differences
The difference between formal charge and oxidation number lies in how electrons are assigned:
- Formal charge assumes equal sharing of bonding electrons, so shared electrons are divided equally between the two atoms.
- Oxidation number assumes ionic sharing, where bonding electrons are given entirely to the more electronegative atom.
Because of this, formal charges often show less extreme values, while oxidation numbers reflect complete electron transfer and may appear larger or smaller in magnitude. This explains why chlorine has a formal charge of 0 or +3, but oxidation numbers of +1 and +7 in these ions.
