Video Transcript
Determine the oxidation number of nitrogen in Ca3N2.
An oxidation number shows the degree of oxidation of an atom in isolation or in a compound in terms of counting electrons. Oxidation numbers are often used as an accounting system to keep track of the number of electrons lost or gained during oxidation or reduction. There are a number of rules that govern the oxidation number an atom has and how to calculate an oxidation number.
In this question, we want to determine the oxidation number of nitrogen in the compound calcium nitride. Calcium nitride is an ionic compound. And despite the fact that it consists of positively charged cations and negatively charged anions, the compound is neutral overall. This information leads us to our first rule for calculating an oxidation number. The sum of the oxidation numbers in a neutral compound is zero. Since we want to determine the oxidation number of nitrogen and the sum of the oxidation numbers in the compound is zero, we must know the oxidation number of calcium.
Calcium is found in group two of the periodic table. This group is also called the alkali earth metals. According to the rules for calculating oxidation number, the oxidation number of an alkali earth metal in a compound is plus two. Now that we know the oxidation number of calcium, we can calculate the oxidation number of nitrogen. We know that the sum of the oxidation numbers from calcium and nitrogen must equal zero. We know the oxidation number of calcium is plus two. Since we don’t know the oxidation number of nitrogen, let’s define this value as the variable 𝑛.
Before we can add these oxidation numbers together, we need to take into account the number of each ion in a formula unit of calcium nitride. In one formula unit of calcium nitride, there are three calcium ions and two nitride ions. Now, we can perform the calculation and rearrange the equation in order to solve for 𝑛, the oxidation number of nitrogen. We have determined that the oxidation number of nitrogen is minus three. But just to be sure, we can substitute this value back into the equation for 𝑛 and verify that the oxidation numbers from calcium and from nitrogen do in fact add up to zero.
Using the rules for calculating oxidation number, we have determined that the oxidation number of nitrogen and calcium nitride is minus three.
