Final answer:
In NaCl, the oxidation number of chlorine is -1 because it must balance the +1 charge of sodium to make the compound neutral. Chlorine typically has a negative oxidation state in halide compounds like NaCl, but its oxidation state can vary in other compounds.
Step-by-step explanation:
In the ionic compound NaCl, the oxidation number of Chlorine is -1 because halogens typically have negative oxidation states when they form halide compounds. In the case of NaCl, sodium (Na) has an oxidation number of +1, and since the compound is neutral overall, chlorine (Cl) must balance this with an oxidation number of -1. The oxidation number signifies the electric charge an atom would have if the compound was composed of ions.
However, the oxidation state of chlorine can vary when combined with other elements. For example, in NaClO3 (sodium chlorate), chlorine has an oxidation number of +5 while oxygen has an oxidation number of -2. This is because in a polyatomic ion, the sum of the oxidation numbers must equal the charge of the ion, and oxygen almost always has an oxidation number of -2.