Oxidation Number Assignment Rules and Worked Examples Cheat Sheet

Key Facts

• The oxidation number of any free element in its standard or uncombined form is $0$, such as $\mathrm{Na}$, $\mathrm{O_2}$, $\mathrm{Cl_2}$, and $\mathrm{S_8}$.
• The oxidation number of a monatomic ion equals its charge, so $\mathrm{Na^+}$ is $+1$, $\mathrm{Ca^{2+}}$ is $+2$, and $\mathrm{Cl^-}$ is $-1$.
• The sum of all oxidation numbers in a neutral compound is $0$, so in $\mathrm{H_2O}$ the total is $2(+1) + x = 0$.
• The sum of all oxidation numbers in a polyatomic ion equals the ion charge, so in $\mathrm{SO_4^{2-}}$ the total is $x + 4(-2) = -2$.
• Oxygen is usually $-2$, but it is $-1$ in peroxides such as $\mathrm{H_2O_2}$ and positive when bonded to fluorine.
• Hydrogen is usually $+1$ when bonded to nonmetals, but it is $-1$ in metal hydrides such as $\mathrm{NaH}$.
• Fluorine is always $-1$ in compounds, and other halogens are usually $-1$ unless bonded to oxygen or a more electronegative halogen.
• Oxidation means an oxidation number increases, and reduction means an oxidation number decreases.

Vocabulary

Oxidation number

A number assigned to an atom that represents its apparent charge if electrons in bonds are assigned by electronegativity.

Redox reaction

A chemical reaction in which oxidation numbers change because electrons are transferred between substances.

Oxidation

The process in which an atom or ion loses electrons and its oxidation number increases.

Reduction

The process in which an atom or ion gains electrons and its oxidation number decreases.

Polyatomic ion

A charged group of covalently bonded atoms that acts as a single ion, such as $\mathrm{NO_3^-}$ or $\mathrm{SO_4^{2-}}$.

Monatomic ion

A single atom with a charge, such as $\mathrm{Mg^{2+}}$ or $\mathrm{Br^-}$.