Transition Metals

(Separate Chemistry only) Transition metals (spec 4.1.3) are assessed in Separate/Triple Chemistry, not in Combined Science: Trilogy. The whole of this lesson is chemistry-only content.

The transition metals occupy the large central block of the periodic table, between Group 2 and Group 3. They are the metals most people picture when they think of a metal: iron, copper, nickel, gold and many more. The AQA specification names six you should be able to refer to.

Transition metal	Symbol	A familiar use
Chromium	Cr	Shiny plating that resists corrosion
Manganese	Mn	Strengthening steel
Iron	Fe	Structural steel, catalyst in the Haber process
Cobalt	Co	Magnets, rechargeable batteries
Nickel	Ni	Coins, catalyst for hardening oils
Copper	Cu	Wiring and pipes

These six (Cr, Mn, Fe, Co, Ni, Cu) are the examples the exam expects you to use when illustrating transition metal properties.

The specification asks you to compare transition metals directly with the Group 1 alkali metals, and the differences are striking. Transition metals are, as a rule, much more like a "typical" strong metal than the soft, reactive alkali metals are.

Compared with Group 1 metals, transition metals have:

• higher melting points (except mercury);
• higher densities;
• greater strength and hardness.

The alkali metals, by contrast, are soft enough to cut with a knife and have low densities: lithium, sodium and potassium are all less dense than water and float on it.

Property	Transition metals	Group 1 metals
Melting point	High	Low
Density	High	Low (Li, Na, K float on water)
Hardness / strength	Hard and strong	Soft

These physical differences are why transition metals such as iron are used for construction, while a soft, reactive metal like sodium would be useless for the job.

The reactivity difference is just as important. Transition metals react much more slowly, or not at all, with oxygen, water and the halogens, whereas Group 1 metals react quickly and vigorously.

• With oxygen: Group 1 metals tarnish in seconds and burn readily; iron rusts only slowly over days, and copper reacts with oxygen very slowly.
• With water: Group 1 metals fizz and can ignite in cold water; most transition metals react extremely slowly or not at all with water. Iron reacts only slowly, in the presence of both air and water, to rust.
• With halogens: Group 1 metals react vigorously; transition metals react far more slowly.

Compared with Group 1, transition metals react slowly (or not at all) with oxygen, water and halogens. Their low reactivity is one reason metals like copper and iron are useful for everyday objects.

Comparing sodium and iron makes the contrast concrete: sodium must be stored under oil to keep it from air and water, while iron can be left in the open and only slowly rusts.

A typical property of transition metals is that many of them form ions with more than one charge. This is unlike Group 1 metals, which always form a single 1+ ion.

The same element can form different ions, and the charge is written as a Roman numeral in the compound's name to show which ion is present:

Transition metal	Ions it can form	Example compound
Iron	Fe²⁺ and Fe³⁺	Iron(II) chloride, iron(III) chloride
Copper	Cu⁺ and Cu²⁺	Copper(I) oxide, copper(II) oxide
Chromium	Cr²⁺, Cr³⁺ and more	Chromium(III) oxide

The Roman numeral is essential: "iron(II) sulfate" and "iron(III) sulfate" are different compounds with different formulae. Group 1 metals never need a Roman numeral because they only form one ion.

Another typical property is that transition metal compounds are usually coloured. The colour depends on the metal and, for metals with variable charge, on which ion is present. This contrasts sharply with Group 1 compounds, which are white solids that dissolve to give colourless solutions.

Compound	Colour
Copper(II) sulfate	Blue
Iron(II) compounds	Green
Iron(III) compounds	Orange-brown
Sodium chloride (Group 1)	White; colourless in solution

The colours are useful in practice. When sodium hydroxide is added to a solution of a metal ion, a coloured precipitate can identify the metal: a blue precipitate points to Cu²⁺, a green one to Fe²⁺, and a brown one to Fe³⁺.

Coloured compounds are a signature of transition metals. If a metal compound or solution is coloured, it is very likely a transition metal compound, not a Group 1 compound.

Transition metals and their compounds are widely used as catalysts: substances that speed up a chemical reaction without being used up themselves. This is a further property that Group 1 metals do not share.

Two named industrial examples are worth knowing:

• Iron is the catalyst in the Haber process, which makes ammonia from nitrogen and hydrogen.
• Nickel is used as a catalyst in the hydrogenation of vegetable oils, turning unsaturated oils into solid fats such as those in margarine.

Because a catalyst is not consumed, a small amount speeds up huge quantities of reaction, which makes transition metal catalysts valuable in industry.

Catalyst	Reaction it speeds up
Iron	Haber process (making ammonia)
Nickel	Hardening vegetable oils (hydrogenation)
Manganese(IV) oxide	Decomposition of hydrogen peroxide

1. Treating transition metals as Combined Science content

Transition metals (4.1.3) are Separate/Triple Chemistry only. If a question on them appears, it is on a Triple Chemistry paper. Do not expect this topic in Combined Science: Trilogy.

2. Saying transition metals are unreactive because they are inert

They react slowly, not never. Iron still rusts and copper still tarnishes over time; the point is that they react much more slowly than Group 1 metals, not that they never react.

3. Leaving out the Roman numeral

For a metal with variable charge, name the compound with the correct Roman numeral, for example iron(II) chloride or copper(II) sulfate. Writing just "iron chloride" is incomplete because the charge is not shown.

4. Claiming Group 1 metals form coloured compounds

Group 1 compounds are white and give colourless solutions. Coloured compounds are a transition metal property. Do not attribute colour to the alkali metals.

5. Confusing a catalyst with a reactant

A catalyst speeds up a reaction without being used up, so it is not shown as being consumed in the equation. Iron in the Haber process is a catalyst, not a reactant.

6. Comparing to the wrong group

The specification compares transition metals with Group 1. Make the contrast explicit: higher melting points, higher densities, harder, stronger, and less reactive than the alkali metals.