Final answer:
The oxidation number of oxygen in the nitrate ion (NO3−) is -2.
Step-by-step explanation:
The oxidation number of oxygen in the nitrate ion (NO3−) is -2. This is determined using several guidelines for assigning oxidation numbers. According to guideline 3, oxygen is normally assigned an oxidation number of -2. Furthermore, using guideline 4, the sum of the oxidation numbers for all atoms in a polyatomic ion must equal the charge on the ion.
For the nitrate ion, NO3−, with one nitrogen and three oxygen atoms, and knowing the oxidation number for nitrogen is +5, the oxidation numbers for the oxygen atoms must sum to -6, because the ion has a -1 charge overall. Since there are three oxygen atoms, the oxidation number for each oxygen atom is also -2, indicating that the correct answer is c. -2.