IMAT Chemistry · Periodic Table

Transition Metals and Their Oxidation States

The d-block metals are dense, high-melting and often coloured; they show several oxidation states, which you find by making the oxidation states in a formula add up to its charge.

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Why this matters

The 2012 paper asked for the false statement about iron, copper and zinc, testing their oxides, densities, ions, colours and reactivity. Oxidation states of metals also sit behind the formula questions on the first page.

Concept 1 of 2: Properties of the transition metals

In the d-block the new electrons go into an inner d subshell, so neighbouring metals have very similar outer shells and similar properties. The 4s and 3d electrons are close in energy, so these metals can lose different numbers of them, giving several oxidation states. Partly filled d subshells absorb some visible light, which is why their compounds are coloured.

Definition

Typical metals: hard, dense (much denser than group 1), high melting points (mercury is the exception), good conductors.
Variable oxidation states: iron +2 and +3, copper +1 and +2, manganese from +2 to +7, chromium +3 and +6. Most form a 2+ ion.
Coloured ions and compounds, when the d subshell is partly filled. Ions with an empty or full d subshell are colourless.
Good catalysts: iron in the Haber process, nickel for adding hydrogen to alkenes, manganese(IV) oxide for decomposing hydrogen peroxide.
They form complex ions with water, ammonia and other ligands.
Many are only moderately reactive: copper, silver and gold do not react with dilute hydrochloric or sulfuric acid.
Strictly, a transition element forms an ion with a partly filled d subshell, so zinc ( $\mathrm{Zn^{2+}}$ : $3d^{10}$ ) and scandium ( $\mathrm{Sc^{3+}}$ : $3d^0$ ) are d-block metals but not transition elements.

Ion	Electron configuration	Colour in water
$\mathrm{Cr^{3+}}$	$[\mathrm{Ar}]\,3d^3$	Green
$\mathrm{Mn^{2+}}$	$[\mathrm{Ar}]\,3d^5$	Very pale pink
$\mathrm{Fe^{2+}}$	$[\mathrm{Ar}]\,3d^6$	Pale green
$\mathrm{Fe^{3+}}$	$[\mathrm{Ar}]\,3d^5$	Yellow to orange-brown
$\mathrm{Cu^{2+}}$	$[\mathrm{Ar}]\,3d^9$	Blue
$\mathrm{Zn^{2+}}$	$[\mathrm{Ar}]\,3d^{10}$	Colourless

The configurations follow the rule from Atomic Structure: the 4s electrons leave first.

Practice this conceptself-check · 3 quick reps

Not every transition metal is reactive

Iron rusts and zinc reacts with dilute acids, but copper does not react with dilute hydrochloric or sulfuric acid at all. Statements that all transition metals are reactive, or that all their ions are coloured, are false.

Concept 2 of 2: Finding the oxidation state of a metal in a compound or ion

Oxidation states are bookkeeping charges: you pretend every bond is ionic and give the electrons to the more electronegative atom. Elements with fixed values (oxygen, hydrogen, group 1 and 2 metals) are filled in first, and the metal takes whatever is left so that the total matches the charge on the whole species.

Definition

Rules, applied in this order:

An uncombined element is 0.
The oxidation states in a formula add up to its overall charge (0 for a compound).
Group 1 metals are +1, group 2 metals +2, fluorine −1.
Hydrogen is +1, except in metal hydrides (−1).
Oxygen is −2, except in peroxides such as $\mathrm{H_2O_2}$ (−1).
If the metal appears more than once, divide its total by the number of its atoms.

Sum of oxidation states

\sum \text{oxidation states of all atoms} = \text{overall charge}

Worked example

Find the oxidation state of manganese in potassium manganate(VII),

\mathrm{KMnO_4}

, and of iron in

\mathrm{Fe_2O_3}

Practice this conceptself-check · 4 quick reps

Share the total between the metal atoms

\mathrm{K_2Cr_2O_7}

the two chromium atoms together carry +12, so each is +6. Answering with the total is a classic distractor whenever the metal appears more than once in the formula.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

Finding the oxidation state of a metal in a compound or ion
Sum of oxidation states
$\sum \text{oxidation states of all atoms} = \text{overall charge}$

Reference tables (1)

Properties of the transition metals6 rows

Ion	Electron configuration	Colour in water
$\mathrm{Cr^{3+}}$	$[\mathrm{Ar}]\,3d^3$	Green
$\mathrm{Mn^{2+}}$	$[\mathrm{Ar}]\,3d^5$	Very pale pink
$\mathrm{Fe^{2+}}$	$[\mathrm{Ar}]\,3d^6$	Pale green
$\mathrm{Fe^{3+}}$	$[\mathrm{Ar}]\,3d^5$	Yellow to orange-brown
$\mathrm{Cu^{2+}}$	$[\mathrm{Ar}]\,3d^9$	Blue
$\mathrm{Zn^{2+}}$	$[\mathrm{Ar}]\,3d^{10}$	Colourless

The configurations follow the rule from Atomic Structure: the 4s electrons leave first.

Watch out for (2)

Not every transition metal is reactive
→ Properties of the transition metals
Share the total between the metal atoms
→ Finding the oxidation state of a metal in a compound or ion

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