Question

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

Answer 1

The most common species of nitrogen in which its oxidation state is zero is the diatomic nitrogen ($N_{2}$). This is because the oxidation number of pure elements (whether alone as an atom or combined with other atoms of the same element such as the diatomic nitrogen) is always zero. 

In addition to diatomic nitrogen, there are diazonium ($R-N_{2}^{+}$) compounds where nitrogen has an oxidation number of zero.