When bonded to a metal, what is the oxidation number of hydrogen?

When hydrogen is bonded to a metal, its oxidation number is typically -1. This occurs primarily in metal hydrides, which are compounds formed between hydrogen and more electropositive metals, such as alkali or alkaline earth metals. In these compounds, the hydrogen acts as a hydride ion (H⁻), and because metals tend to lose electrons and take on positive oxidation numbers, hydrogen's electron configuration shifts to reflect a -1 oxidation state.

For example, in sodium hydride (NaH), sodium has an oxidation number of +1, and consequently, hydrogen must hold a -1 oxidation state to balance out the charge of the compound, resulting in an electrically neutral molecule.

When analyzing the other oxidation state options, hydrogen generally exhibits a +1 oxidation state when combined with nonmetals (such as in water, H₂O). The 0 oxidation state is seen in diatomic hydrogen (H₂) in its elemental form. Meanwhile, the -2 oxidation state is not typically associated with hydrogen; hydride ions are the most common representation of hydrogen in a negative oxidation state.

Therefore, the correct representation of hydrogen's oxidation number when bonded to a metal is -1.