What is the oxidation number of chlorine in Cl-?

• A. 0
• B. -1
• C. +1
• D. +2

Answer: (B)

Oxidation numbers follow the rule that for a single-atom ion, the oxidation state equals the ion’s charge. The chloride ion consists of one chlorine atom with an overall -1 charge, so the oxidation number of chlorine in Cl− is -1. That fits all by itself, since there are no other atoms to balance a different charge. For contrast, 0 would be the state of elemental chlorine (Cl2), and +1 or +2 occur in chlorine-containing species where chlorine is more oxidized, which isn’t the case here. Therefore, -1 is the correct oxidation number.