There are nine atoms in a compound of calcium nitrate. A compound of nitrate, Ca(NO3)2, is composed of 1 atom of calcium and 2 molecules of nitrate. Each molecule of nitrate has 4 atoms - 1 from nitrogen and 3 from oxygen.
The compound calcium nitrate has an overall charge of -2. For this reason, 2 molecules of nitrate are required to neutralize this negative charge. This is because nitrate has an overall charge of +1. Calcium nitrate has a multiplicity of uses as it's employed in waste water treatment, as a fertilizer and also in the storage of energy in power plants.