What is the oxidation number of hydrogen when bonded to metals?

What is the oxidation number of hydrogen when bonded to metals?

• A. 0
• B. +1
• C. -1
• D. +2

Answer: (B)

When assigning oxidation numbers, think about which element tends to attract electrons in the bond. Metals are electropositive and donate electron density to hydrogen in metal hydrides, so hydrogen ends up gaining electrons and behaving as the hydride ion H−. This is seen in examples like sodium hydride, where Na is +1 and H is −1, and calcium hydride, where Ca is +2 and each H is −1 to balance the charge. Hydrogen does have +1 in many compounds with nonmetals (where it behaves as a positive contributor to the bond), but in metal hydrides the situation is different: hydrogen takes the negative oxidation state. So the oxidation number of hydrogen when bonded to metals is −1.

When assigning oxidation numbers, think about which element tends to attract electrons in the bond. Metals are electropositive and donate electron density to hydrogen in metal hydrides, so hydrogen ends up gaining electrons and behaving as the hydride ion H−. This is seen in examples like sodium hydride, where Na is +1 and H is −1, and calcium hydride, where Ca is +2 and each H is −1 to balance the charge.

Hydrogen does have +1 in many compounds with nonmetals (where it behaves as a positive contributor to the bond), but in metal hydrides the situation is different: hydrogen takes the negative oxidation state. So the oxidation number of hydrogen when bonded to metals is −1.