Transition elements

Where they are

The transition elements are the block of metals between Group II and Group III, starting in period 4. The ones that appear in questions are iron, copper, zinc, chromium, manganese, nickel, cobalt and silver.

Compared with the Group I metals

Worked example. Take a 10 cm³ block of each metal.

mass of iron = 10 × 7.9 = 79 g

mass of sodium = 10 × 0.97 = 9.7 g

The iron block is eight times as heavy as the sodium one of the same size, which is what "high density" means in practice and why sodium floats on water while iron sinks.

Coloured compounds

This is the property most often used to identify a transition element, and the colours are recall marks.

Solid compounds carry colours too: copper(II) oxide is black, copper(II) carbonate is green, and potassium manganate(VII) is purple.

With aqueous ammonia instead of sodium hydroxide, copper(II) gives a light blue precipitate that dissolves in excess to a deep blue solution, which is the standard test for copper(II) ions.

Zinc is the exception to the colour rule. Its compounds are white as solids and colourless in solution, so a white precipitate is not evidence against a transition element.

For contrast, two ions that are not transition elements also give white precipitates with sodium hydroxide. Aluminium gives a white precipitate that dissolves in excess, and calcium gives a white precipitate that does not. Zinc, aluminium and calcium are told apart by what happens when more alkali is added, not by the first observation.

Catalysts

Transition elements and their compounds are frequently used as catalysts, and four should be known by name and by process:

• Iron in the Haber process, making ammonia.
• Vanadium(V) oxide in the Contact process, making sulfur trioxide.
• Nickel in the hydrogenation of alkenes to alkanes.
• Manganese(IV) oxide in the decomposition of hydrogen peroxide to water and oxygen.

Notice that two of these are the element itself and two are compounds of it, which is why the syllabus says the elements and their compounds act as catalysts.

Extended only: variable oxidation numbers

A Group I metal forms one ion and only one, always +1. A transition element commonly forms more than one ion, with different oxidation numbers:

• Iron forms Fe²⁺, iron(II), and Fe³⁺, iron(III).
• Copper forms Cu⁺, copper(I), and Cu²⁺, copper(II).
• Manganese appears at +2, at +4 in manganese(IV) oxide and at +7 in potassium manganate(VII).

This is why the names carry Roman numerals. "Iron chloride" is ambiguous, because it could be FeCl₂ or FeCl₃. Naming it iron(II) chloride or iron(III) chloride fixes the formula.

The two ions have different colours and different chemistry, so a change between them is visible. A pale green iron(II) solution turning yellow-brown means Fe²⁺ has been oxidised to Fe³⁺, losing an electron:

Fe²⁺ → Fe³⁺ + e⁻

Going the other way, yellow-brown to pale green, is reduction. This is the reason the sodium hydroxide test distinguishes them: green precipitate for iron(II), red-brown for iron(III). An iron(II) precipitate left standing in air slowly turns brown at the surface because the air oxidises it, which is a favourite observation to be asked to explain.

Uses that follow from the properties

• Iron and steel for construction, because iron is hard, strong and has a high melting point.
• Copper for electrical wiring and for pipes, because it conducts well, is ductile and does not react with water.
• Nickel and chromium in stainless steel and in plating, because they resist corrosion.

Every one of these uses follows from the table at the top: high melting point, hardness and low reactivity are exactly what the soft, reactive Group I metals do not offer.

Common mistakes

• Saying transition elements are more reactive than Group I metals.
• Expecting all transition metal compounds to be coloured, when zinc compounds are white or colourless.
• Giving iron(II) hydroxide as red-brown and iron(III) hydroxide as green, which is the wrong way round.
• Saying copper(II) hydroxide dissolves in excess sodium hydroxide, when it does not.
• Leaving the Roman numeral out of a name where the oxidation number varies.
• Saying transition elements always form +2 ions.
• Naming a catalyst without naming the process it catalyses, which is usually where the mark is.

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