Transition metals: general properties: WASSCE Chemistry

The block of elements in the middle of the periodic table includes iron, copper, chromium, nickel and titanium: the metals of buildings, wires, machines and coloured paints. These transition metals share a set of properties that set them apart from the metals of groups 1 and 2: many colours, several oxidation states, and the ability to act as catalysts.

Learning objectives

By the end of this lesson you should be able to:

define a transition element and write the configurations of the first transition series
describe the general physical properties of transition metals
explain variable oxidation states, coloured compounds, catalytic activity and complex-ion formation
give examples of coloured ions and catalysts
compare transition metals with group 1 and 2 metals

What is a transition element?

The first transition series runs from scandium (Sc, 21) to zinc (Zn, 30) in period 4. In these elements the 3d sub-shell is being filled.

Definition

A transition element is a d-block element that forms at least one stable ion with a partially filled d sub-shell.

Element	Configuration	Common ions
Sc	[Ar] 3d¹ 4s²	Sc³⁺ (3d⁰)
Ti	[Ar] 3d² 4s²	Ti⁴⁺
V	[Ar] 3d³ 4s²	V³⁺, VO²⁺
Cr	[Ar] 3d⁵ 4s¹	Cr³⁺
Mn	[Ar] 3d⁵ 4s²	Mn²⁺, MnO₄⁻
Fe	[Ar] 3d⁶ 4s²	Fe²⁺, Fe³⁺
Co	[Ar] 3d⁷ 4s²	Co²⁺
Ni	[Ar] 3d⁸ 4s²	Ni²⁺
Cu	[Ar] 3d¹⁰ 4s¹	Cu⁺, Cu²⁺
Zn	[Ar] 3d¹⁰ 4s²	Zn²⁺ (3d¹⁰)

By the strict definition, scandium (Sc³⁺ is d⁰) and zinc (Zn²⁺ is d¹⁰) are not true transition elements, though they are d-block elements. Zinc shows few typical transition properties: its compounds are white and it has only one oxidation state.

General physical properties

Hard and strong, with high melting points and boiling points (many delocalised electrons, from both 4s and 3d)
High densities
Good conductors of heat and electricity
Malleable and ductile, and form useful alloys
Iron, cobalt and nickel are strongly magnetic (ferromagnetic)

Characteristic chemical properties

1. Variable oxidation states

The 4s and 3d electrons have similar energies, so different numbers can be lost.

Element	Common oxidation states	Examples
Iron	+2, +3	FeSO₄ (green), FeCl₃ (yellow-brown)
Copper	+1, +2	Cu₂O (red), CuSO₄ (blue)
Manganese	+2, +4, +7	MnSO₄, MnO₂, KMnO₄
Chromium	+3, +6	Cr₂O₃ (green), K₂Cr₂O₇ (orange)

Group 1 and 2 metals have only one oxidation state each (+1 or +2).

2. Coloured compounds

Most transition-metal ions are coloured in solution, because of their partly filled d sub-shells.

Ion	Colour in solution
Cu²⁺ (aq)	Blue
Fe²⁺ (aq)	Pale green
Fe³⁺ (aq)	Yellow-brown
Ni²⁺ (aq)	Green
Co²⁺ (aq)	Pink
Mn²⁺ (aq)	Very pale pink
MnO₄⁻	Purple
Cr₂O₇²⁻	Orange
CrO₄²⁻	Yellow
Zn²⁺, Sc³⁺	Colourless (d¹⁰ and d⁰)

Compounds of group 1 and 2 metals are usually white, or colourless in solution.

3. Catalytic activity

Transition metals and their compounds are important catalysts, because they can change oxidation state and provide surfaces where reactions happen.

Catalyst	Process
Iron	Haber process (ammonia)
Vanadium(V) oxide	Contact process (sulphuric acid)
Nickel	Hydrogenation of vegetable oils (margarine)
Platinum / rhodium	Ostwald process (nitric acid); catalytic converters in cars
Manganese(IV) oxide	Decomposition of hydrogen peroxide

4. Complex ions

Transition-metal ions form complex ions, in which molecules or ions (ligands) such as water, ammonia or cyanide donate lone pairs to the metal ion through dative bonds.

Complex ion	Colour
[Cu(H₂O)₆]²⁺	Blue (copper(II) sulphate solution)
[Cu(NH₃)₄]²⁺	Deep blue (Cu²⁺ with excess ammonia)
[Fe(SCN)]²⁺	Blood red (test for Fe³⁺ with thiocyanate)
[Fe(CN)₆]³⁻	Gives a dark-blue precipitate with Fe²⁺ (a test for iron(II))

Transition metals compared with group 1 and 2 metals

Property	Group 1 and 2 metals	Transition metals
Hardness, melting point	Soft, low to moderate	Hard, high
Density	Low	High
Reactivity with water	Reactive	Much less reactive (iron rusts slowly)
Oxidation states	One (+1 or +2)	Several
Colour of compounds	White or colourless	Coloured
Catalysts	Rarely	Often
Complex ions	Rarely	Readily

In Sierra Leone

Rutile, mined in southern Sierra Leone, is titanium(IV) oxide, TiO₂. Titanium is a transition metal. Its oxide is the brilliant white pigment in house paints, toothpaste and plastics. Titanium metal is light, strong and corrosion-resistant, and is used in aircraft and in artificial hip joints.

Common mistake

Zinc is in the d-block but is not a typical transition metal: its only ion, Zn²⁺, has a full d sub-shell (3d¹⁰), so its compounds are white and it has one oxidation state.

Key points

The first transition series (Sc to Zn) fills the 3d sub-shell; a transition element forms a stable ion with a partly filled d sub-shell.
Transition metals are hard, dense and high-melting, conduct well and form alloys.
Characteristic properties: variable oxidation states, coloured ions (Cu²⁺ blue, Fe²⁺ green, Fe³⁺ yellow-brown, MnO₄⁻ purple, Cr₂O₇²⁻ orange), catalytic activity (Fe, V₂O₅, Ni, Pt, MnO₂) and complex ions.
Zn and Sc are d-block elements but not true transition elements.

WASSCE examination focus

WAEC tests this

Expect to write configurations of first-row transition elements (watch Cr and Cu), to state the characteristic properties of transition metals with examples, to give colours of common ions, to name transition-metal catalysts, and to compare transition metals with s-block metals.

Quick check

Define a transition element.
Give two oxidation states of iron and the colours of their ions in solution.
Name the transition-metal catalyst used in the Haber process.
Why are zinc compounds white?

Answers:

A d-block element that forms at least one stable ion with a partially filled d sub-shell.
+2 (Fe²⁺, pale green) and +3 (Fe³⁺, yellow-brown).
Iron.
Zn²⁺ has a completely filled d sub-shell (3d¹⁰), so it does not absorb visible light.

Transition metals: general properties