Section: Chemistry: Section I

21) In which compound does hydrogen have an oxidation number of -1?

1. NH₃
2. KH
3. HCl
4. H₂O

Hydrogen usually has a +1 oxidation number. However, it can have a -1 oxidation number when it behaves as the nonmetal. This occurs only when it joins with Group 1 metals, which are less electronegative than hydrogen.