Oxidation Numbers Basics

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The oxidation number of any pure element is zero.

The oxidation number of a monatomic ion equals its charge.

The oxidation number of oxygen in most compounds is -2.

Hydrogen's oxidation number is +1 when bonded to non-metals, and -1 when bonded to metals.

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The sum of the oxidation numbers of all atoms in a neutral compound is zero.

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Oxidation number (or oxidation state) represents degree of oxidation or reduction
It indicates number of electrons gained, lost, or shared during bond formation
Can be positive, negative, or zero
Different rules for assigning oxidation states

Pure elements: Oxidation number is zero (e.g., O₂, H₂, N₂)
Monatomic ions: Oxidation number equals ion charge (e.g., Na⁺ = +1, Cl⁻ = -1)
Oxygen: Usually -2, but -1 in peroxides (e.g., H₂O₂)
Hydrogen: +1 when bonded to non-metals, -1 when bonded to metals
Alkali metals (Group 1): +1 in compounds
Alkaline earth metals (Group 2): +2 in compounds
Fluorine: Always -1 in compounds
Sum of oxidation numbers: Zero in neutral compounds; equals charge in polyatomic ions