* It exists in its elemental form: This means the element is not bonded to any other atoms. There's no sharing or transfer of electrons within the element itself.

* No net charge: Since there's no bonding, there's no difference in electronegativity between atoms of the same element. This means there's no gain or loss of electrons, resulting in a neutral charge.

* Oxidation numbers are assigned based on electron sharing: Oxidation numbers represent the hypothetical charge an atom would have if all the bonds were purely ionic (complete transfer of electrons). In a pure element, there's no electron transfer, so the oxidation number is assigned as 0.

Here are some examples:

* Oxygen (O2): Each oxygen atom has an oxidation number of 0 in the diatomic oxygen molecule.

* Sodium (Na): A single sodium atom has an oxidation number of 0.

* Copper (Cu): A piece of pure copper metal has an oxidation number of 0 for all the copper atoms.

In summary: The oxidation number of a pure element is 0 because it is not bonded to any other atoms, and there is no net charge due to the absence of electron transfer.