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2.6 The Transition Elements

GCSE Chemistry · AQA · Chapter 2: The Periodic Table

A free GCSE Chemistry lesson for AQA (8462) from OBA School: how transition metals compare with the alkali metals: ions with different charges, coloured compounds and use as catalysts. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Locate the transition metals in the periodic table
  • Compare the physical and chemical properties of transition metals with Group 1 metals
  • Describe how transition metals form ions with different charges and work out formulae of their compounds
  • Recall that transition metal compounds are often coloured and that many are useful catalysts

Key words

  • Transition metals: The block of metals between Group 2 and Group 3, e.g. chromium, manganese, iron, cobalt, nickel and copper.
  • Density: Mass per unit volume: density = mass ÷ volume, in g/cm³.
  • Catalyst: A substance that speeds up a reaction without being used up.
  • Roman numeral: Shows the charge on a metal ion in a compound name, e.g. iron(III) contains Fe³⁺.
  • Malleable: Can be hammered into shape without breaking.

Where are the transition metals?

The transition metals form the wide central block of the periodic table. At GCSE you need to know about typical examples from Period 4: chromium, manganese, iron, cobalt, nickel and copper. They are all metals, and are used for structures, tools, wires and coins.

Comparing with the Group 1 metals

  • Melting point; Group 1 metals low (all below 200 °C); Transition metals high (except mercury)
  • Density; Group 1 metals low (Li, Na, K float on water); Transition metals high
  • Strength and hardness; Group 1 metals soft, weak; Transition metals strong and hard
  • Reactivity; Group 1 metals very reactive; Transition metals much less reactive (see below)
  • Ions formed; Group 1 metals always 1+; Transition metals often more than one charge
  • Compounds; Group 1 metals white; dissolve to give colourless solutions; Transition metals often coloured

How slowly they react. Transition metals need much harsher conditions than Group 1 metals to react with oxygen, chlorine or water:

  • with oxygen; Group 1 metal (e.g. sodium) tarnishes in seconds at room temperature; Transition metal copper must be heated strongly; a black coating of copper(II) oxide slowly forms
  • with chlorine; Group 1 metal (e.g. sodium) burns vigorously once warmed; Transition metal iron must be heated strongly in a stream of chlorine; red-brown iron(III) chloride forms
  • with water; Group 1 metal (e.g. sodium) fizzes and floats; hydrogen given off; Transition metal copper: no reaction; iron: rusts slowly over days (needs water and oxygen)

2Cu(s) + O₂(g) → 2CuO(s)

2Fe(s) + 3Cl₂(g) → 2FeCl₃(s)

Their strength, hardness and very slow corrosion make transition metals (especially iron in steel) ideal structural materials. Mercury is the odd one out: it is a liquid at room temperature.

Worked example: identifying a metal from its density

A cube of metal has sides of 3.0 cm and a mass of 241.9 g. Is it copper (8.96 g/cm³) or iron (7.87 g/cm³)?
volume = 3.0 × 3.0 × 3.0 = 27 cm³
density = mass ÷ volume = 241.9 ÷ 27 = 8.959… g/cm³

Answer: 8.96 g/cm³, so the metal is copper

Ions with different charges

Many transition metals can form ions with different charges. The charge is shown by a Roman numeral in the compound’s name. Chromium (Cr²⁺, Cr³⁺) and cobalt (Co²⁺, Co³⁺) behave in the same way.

Worked example: writing a formula

Write the formula of iron(III) oxide. Iron(III) ions are Fe³⁺; oxide ions are O²−.
The charges must cancel: 2 × (+3) = +6 and 3 × (−2) = −6.

Answer: Fe₂O₃

  • Iron; Common ions Fe²⁺, Fe³⁺; Example compounds iron(II) chloride FeCl₂; iron(III) chloride FeCl₃; iron(III) oxide Fe₂O₃
  • Copper; Common ions Cu⁺, Cu²⁺; Example compounds copper(I) oxide Cu₂O; copper(II) oxide CuO
  • Chromium; Common ions Cr²⁺, Cr³⁺; Example compounds chromium(III) oxide Cr₂O₃ (dark green)
  • Manganese; Common ions Mn²⁺ and others; Example compounds manganese(II) sulfate MnSO₄; manganese(IV) oxide MnO₂

Coloured compounds and catalysts

Transition metal compounds are often coloured. They are used to colour pottery glazes, stained glass and paints. The colour depends on the ion, which also helps chemists identify ions in tests.

Many transition metals and their compounds are catalysts:

  • iron in the Haber process to make ammonia
  • nickel for hardening vegetable oils to make margarine (hydrogenation)
  • manganese(IV) oxide to speed up the decomposition of hydrogen peroxide: 2H₂O₂(aq) → 2H₂O(l) + O₂(g)

Key fact: A catalyst is not used up. If you filter off and dry the manganese(IV) oxide after the reaction, it has the same mass as at the start.

Common mistakes

  • Saying transition metals are more reactive than Group 1 metals. They are much less reactive.
  • Thinking iron(III) means three iron atoms. The (III) is the charge on the ion: Fe³⁺.
  • Saying all metals have high melting points and densities. Group 1 metals do not.
  • Saying a catalyst is “used up slowly” or is a reactant. It is unchanged at the end.
  • Saying copper reacts with water like sodium. Copper does not react with water at all; it only reacts with oxygen when heated strongly.
  • Writing iron(II) chloride as FeCl. Fe²⁺ needs two Cl− ions: FeCl₂.

Revision questions

Try these, then check your answers below each one.

Question 1. Which property is typical of transition metals but not of Group 1 metals? (Choose from: High density, Conduct electricity, Form positive ions, Shiny when freshly cut.)

Answer: High density

Working: Transition metals have high densities (e.g. iron 7.87 g/cm³). Group 1 metals have low densities. All metals conduct, form positive ions and are shiny.

Question 2. Iron(III) ions are Fe³⁺ and chloride ions are Cl−. Write the formula of iron(III) chloride.

Answer: FeCl3

Working: One Fe³⁺ needs three Cl− ions: FeCl₃.

Question 3. A block of metal has a volume of 5.0 cm³ and a mass of 39.35 g. Calculate its density.

Answer: 7.87 g/cm³

Working: 39.35 ÷ 5.0 = 7.87 g/cm³ (this is iron).

Related lessons

Continue in the full course

This lesson is section 2.6 (AGCCH0206) of our free GCSE Chemistry course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Chemistry course.

 

 

 

 

 

 

 

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