Oxidation numbers

As an element is oxidised, its oxidation number:

• increases as electrons are gained.

• increases as electrons are lost.

• decreases as electrons are gained.

• decreases as electrons are lost.

Answer explanation

The oxidation number (or oxidation state) of a given atom tells you how many electrons that atom gained or lost.

As an atom loses electrons (is oxidised), its oxidation number increases (becomes more positive).

As an atom gains electrons (is reduced), its oxidation number decreases (becomes more negative).

What is the oxidation state of a calcium ion?

• -2

• -1

• +1

• +2

Answer explanation

For simple monatomic ions, the oxidation state of the atom is equal to its charge.

A calcium ion (Ca²⁺) has a charge of 2+, meaning it has lost two electrons relative to the neutral atom. As such, its oxidation state is +2.

[Note that the +/- sign is behind the number for charge, but in front of it for oxidation number]

What is the oxidation state for the hydrogen atoms in water (H₂O)?

• -1

• 0

• +1

• +2

Answer explanation

Two useful rules to remember are on display here:

1. Hydrogen almost always has an oxidation state of +1, and oxygen almost always has an oxidation state of -2.

2. The total oxidation state of the atoms in a molecule is equal to the charge of the molecule.

Water has a charge of 0, so the two H atoms (2 × +1 = +2) and the O atom (-2) must sum to 0.

What is the oxidation state of the nitrogen atom in ammonia (NH₃)?

• -3

• -1

• +1

• +3

Answer explanation

As with the last example:

1. Hydrogen = +1

2. Molecular charge = 0, therefore total oxidation state = 0.

There are 3 H (3 × +1 = +3), so to equal 0, the N must be -3.

What is the oxidation state of the nitrogen atom in nitrogen dioxide (NO₂)?

• -3

• -2

• +2

• +4

Answer explanation

Oxygen = -2

Two O = 2 × -2 = -4

NO₂ = 0

Therefore, N = 0 - -4 = +4

To balance out the two oxygen atoms, the nitrogen has an oxidation number of +4. Two atoms of the same element in different molecules can have very different oxidation numbers depending on what they are bonded to, due to relative electronegativity.

What is the oxidation state of the sulfur atom in a sulfate ion (SO₄^2-)?

• +2

• +4

• +6

• +8

Answer explanation

Oxygen = -2

4 O = 4 × -2 = -8

Sulfate total = -2

Therefore, S = -2 - -8 = +6

Don't forget that the total of the oxidation numbers should equal the charge of the molecule (in this case, -2).

What is the oxidation state of the manganese atom in potassium permanganate (KMnO₄)?

[Hint: potassium permanganate is an ionic compound, not a single molecule.]

• +2

• +6

• +7

• +8

Answer explanation

KMnO₄ = K⁺ and MnO₄^-

O = -2

4 O = 4 × -2 = -8

MnO₄^- = -1

Therefore, Mn = -1 - -8 = +7

The very high oxidation state of the manganese atom causes permanganate to be a powerful oxidising agent (i.e. it wants to be reduced, taking electrons from other chemicals and oxidising them).

Fe₂O₃ + 3CO → 2Fe + 3CO₂

The oxidation state of iron (Fe) changes from ___ to ___.

• 0; +3

• +3; 0

• 0; +2

• +2; 0

Answer explanation

Left side:

Fe₂O₃ = 0

O₃ = 3 × -2 = -6

Fe₂ = 0 - -6 = +6

Fe = +6/2 = +3

Right side:

Elemental Fe = 0

Fe goes from +3 to 0; each iron(III) ion has gained 3 electrons and been reduced to elemental iron metal.