Final answer:
The oxidation number of any element in its free state is zero. This rule applies whether the element is monatomic, diatomic, or polyatomic, such as H₂, O₂, or P₄. Monatomic ions have oxidation numbers equal to their ionic charge.
Step-by-step explanation:
The oxidation number of any element in its free state (uncombined with other elements) is zero.
For example, elements like hydrogen (H₂), bromine (Br₂), sodium (Na), and oxygen (O₂), all exhibit an oxidation number of 0 because they are in their native or elemental form. This rule applies to all free elements, whether they are monatomic, like sodium (Na), or polyatomic, like phosphorus (P₄).
It is important to remember that monatomic ions have oxidation numbers equal to their charge. For instance, the alkali metal lithium in its ionic form (Li⁺) has an oxidation number of +1. The oxidation number is a way to keep track of electrons during chemical reactions, especially in oxidation-reduction (redox) reactions.