An element that gains electrons has a negative oxidation number. This is because when an element gains electrons, it becomes more negatively charged. The oxidation number of an element is a measure of its relative charge compared to other elements in a compound. The more electrons an element gains, the more negative its oxidation number becomes.

For example, when chlorine gas (Cl2) reacts with sodium metal (Na), the chlorine atoms gain electrons from the sodium atoms. This causes the chlorine atoms to become negatively charged, and they take on an oxidation number of -1. The sodium atoms, on the other hand, lose electrons and become positively charged, taking on an oxidation number of +1.

The oxidation number of an element can be determined by looking at the number of electrons it has gained or lost in a reaction. If an element has gained electrons, it will have a negative oxidation number. If an element has lost electrons, it will have a positive oxidation number.