The oxidation number of an atom is shown with a negative number when it is bonded to a more electronegative element. This means that the atom has lost electrons to the more electronegative element. For example, in the compound NaCl, the oxidation number of sodium is +1, and the oxidation number of chlorine is -1. This is because the chlorine atom has taken one electron from the sodium atom.

The oxidation number of an atom can also be zero. This means that the atom has not gained or lost any electrons. For example, in the compound CO, the oxidation number of carbon is +2, and the oxidation number of oxygen is -2. This is because the carbon atom has lost two electrons to the oxygen atom.

The oxidation number of an atom is important because it can help us determine the chemical properties of a compound. For example, a compound with a negative oxidation number is likely to be an oxidizing agent, which means that it can accept electrons from another compound. A compound with a positive oxidation number is likely to be a reducing agent, which means that it can donate electrons to another compound.