In which of the following species does nitrogen have an oxidation number of zero?

• A. N2
• B. HNO3
• C. NO2
• D. NH3

Answer: (A)

Verified step by step guidance

1. Recall that the oxidation number of an element in its elemental form (uncombined with other different elements) is always zero. This is a fundamental rule in assigning oxidation states.
2. Examine each species given: N2, HNO3, NO2, and NH3. Identify whether nitrogen is in its elemental form or combined with other elements.
3. For N2, nitrogen is bonded to another nitrogen atom, so it is in its elemental form. Therefore, the oxidation number of nitrogen in N2 is zero.
4. For HNO3, NO2, and NH3, nitrogen is bonded to other elements (oxygen and hydrogen), so its oxidation number will not be zero. You would assign oxidation numbers based on the known oxidation states of hydrogen (+1) and oxygen (-2) to find nitrogen's oxidation state in these compounds.
5. Conclude that only in N2 does nitrogen have an oxidation number of zero, because it is in its elemental diatomic form.