In hydrogen chloride (HCl), what are the oxidation numbers of hydrogen and chlorine?

• A. H -1, Cl +1
• B. H 0, Cl 0
• C. H +2, Cl -2
• D. H +1, Cl -1

Answer: (D)

Oxidation numbers are a bookkeeping way to reflect electron control in a bond: the more electronegative atom tends to take a negative assignment, and the total must balance to the charge of the species. In HCl, chlorine is more electronegative than hydrogen, so chlorine gets -1. To balance that and make the molecule neutral, hydrogen must be +1. So the oxidation numbers are hydrogen +1 and chlorine -1.

The other options don’t fit this pattern: hydrogen isn’t more electronegative than chlorine, so it shouldn’t be -1; the molecule isn’t without charge separation, so zero for both doesn’t apply; and hydrogen +2 with chlorine -2 would contradict the typical oxidation states for hydrogen in binary compounds with nonmetals and isn’t required to balance the molecule.