What is the oxidation number of an element in its uncombined, elemental form?

• A. 0
• B. +1
• C. -1
• D. +2

Answer: (A)

Oxidation numbers are a way to keep track of electron ownership in a system. In its uncombined, elemental form, an element has not gained or lost any electrons relative to its neutral, standard state, so its oxidation number is defined as zero. This applies to any pure element, whether it’s a metal solid like iron or a diatomic molecule like nitrogen gas. The idea is that there’s no net charge associated with the atom in the element itself, so its oxidation state is zero.

The other numbers would imply the atom has gained or lost electrons in forming ions or compounds, which isn’t the case for the element when it’s by itself.