The oxidation number is a concept used in chemistry to keep track of the distribution of electrons in a compound or molecule. It is a measure of the charge that an atom would have if all the shared electrons were assigned to the more electronegative atom in a bond.

In the case of oxygen, its most common oxidation number is -2. However, in certain compounds, such as superoxides and peroxides, the oxidation number of oxygen deviates from -2.

Oxidation number of 1/2 is assigned to the oxygen atom in superoxides. Superoxides are a class of compounds that contain the superoxide ion, O2-. In this ion, each oxygen atom has an oxidation number of -1/2. This is because the oxygen-oxygen bond in the superoxide ion is a single bond with a bond order of 1/2. Therefore, each oxygen atom is assigned an oxidation number of -1/2 to account for the distribution of electrons in the bond.

In the case of peroxides, such as hydrogen peroxide (H2O2), the oxidation number of oxygen is -1. In peroxides, the oxygen-oxygen bond is a single bond with a bond order of 1. Each oxygen atom is assigned an oxidation number of -1 to account for the distribution of electrons in the bond.

When oxygen is bonded to fluorine, the oxidation number of oxygen is -1. Fluorine is the most electronegative element, and therefore, it attracts the shared electrons in the bond more strongly than oxygen. As a result, oxygen is assigned an oxidation number of -1 to account for the unequal distribution of electrons in the bond.

When oxygen is bonded to metals, the oxidation number of oxygen is typically -2. However, there are some exceptions to this rule, such as in certain metal peroxides or superoxides, where the oxidation number of oxygen deviates from -2.

In summary, the oxidation number of 1/2 is assigned to the oxygen atom in superoxides, where the oxygen-oxygen bond is a single bond with a bond order of 1/2. In other compounds, such as peroxides, fluorides, and most metal oxides, the oxidation number of oxygen is typically -1 or -2.