Monday

Rules for Finding Oxidation Number | chemca

Redox Reactions

Rules for Finding Oxidation Number

A systematic guide to determining the hypothetical charge on an atom in a molecule.

By chemca Team • Updated Jan 2026

The Oxidation Number (O.N.) represents the electrical charge an atom would have if electrons were transferred completely to the more electronegative atoms. Calculating it correctly is essential for balancing redox reactions.

1. Fundamental Rules

Rule 1: Free Elements

The oxidation number of an element in its free or uncombined state is always Zero.

Examples: $H_2, O_2, N_2, P_4, S_8, Fe, Na \rightarrow \text{O.N. is 0}$

Rule 2: Monoatomic Ions

For simple ions composed of only one atom, the oxidation number is equal to the charge on the ion.

Examples: $Na^+ (+1), Mg^{2+} (+2), Cl^- (-1), Al^{3+} (+3)$

Rule 3: Hydrogen

Generally, the O.N. of Hydrogen is +1.

Exception: In metal hydrides (ionic hydrides) where H is bonded to a metal, O.N. is -1.
Examples: $NaH, CaH_2$

Rule 4: Oxygen

Generally, the O.N. of Oxygen is -2 (Oxide form).

Exceptions:

Peroxides ($O_2^{2-}$): O.N. is -1 (e.g., $H_2O_2, Na_2O_2$).
Superoxides ($O_2^{-}$): O.N. is -1/2 (e.g., $KO_2$).
With Fluorine: Fluorine is more electronegative, so O becomes positive.
$OF_2 \rightarrow +2$, $O_2F_2 \rightarrow +1$.

Rule 5: Halogens

Fluorine (F) is always -1 in all its compounds.

Chlorine (Cl), Bromine (Br), and Iodine (I) are usually -1, except when bonded to Oxygen or Fluorine (where they can be +1, +3, +5, +7).

2. The Sum Rule (Calculation Method)

To calculate the unknown oxidation number of an atom, we use the algebraic sum rule:

Neutral Molecule: Sum of all oxidation numbers = 0.
Polyatomic Ion: Sum of all oxidation numbers = Charge on the ion.

Examples:

1. Calculate O.N. of Mn in $KMnO_4$.
Let O.N. of Mn be $x$.
$K (+1) + Mn (x) + 4 \times O (-2) = 0$
$1 + x - 8 = 0 \Rightarrow \mathbf{x = +7}$

2. Calculate O.N. of Cr in Dichromate ion ($Cr_2O_7^{2-}$).
Let O.N. of Cr be $x$.
$2 \times x + 7 \times O (-2) = -2$
$2x - 14 = -2 \Rightarrow 2x = 12 \Rightarrow \mathbf{x = +6}$

3. Structure-Based Exceptions

Sometimes the formula gives a fractional or impossible value. In these cases, drawing the structure is necessary to find the true oxidation state (usually based on peroxide linkages).

$CrO_5$ (Chromium Pentoxide): Formula gives +10 (impossible). Structure is a "Butterfly" with 4 peroxide oxygens (-1) and 1 double-bonded oxygen (-2).
Result: Cr is +6.
$H_2SO_5$ (Caro's Acid): Contains one peroxy linkage ($O-O$). Average calculation gives +8 (wrong, max for S is +6). Structure reveals correct state.
Result: S is +6.

Knowledge Check

Test your ability to calculate Oxidation Numbers

Search This Blog

Monday