Transition elements

O Level / IGCSE Chemistry · The Periodic Table. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

The transition elements are the block of metals in the centre of the Periodic Table, between Group II and Group III. Iron, copper, chromium, manganese and nickel are examples.

They are typical metals, but they differ from the Group I metals in several ways. They are hard and strong, with high densities and high melting points. One element can have more than one oxidation number, so iron forms both iron(II) and iron(III) compounds. Their compounds are usually coloured, whereas Group I compounds are usually white and give colourless solutions.

Many transition elements, and their compounds, are also good catalysts: they speed up a reaction and are not used up.

Rules to remember

Common mistake

Students often choose a property that every metal has, such as conducting electricity or forming positive ions, when asked what is special about transition elements. Choose from high density, high melting point, variable oxidation number, coloured compounds or catalysis.

Worked example

Copper forms two oxides, Cu2O and CuO. What is the oxidation number of copper in each?

  1. Oxygen is −2 in both oxides, and the oxidation numbers in each compound add up to zero.
  2. Cu2O: (2 × Cu) + (−2) = 0, so 2 × Cu = +2 and Cu = +1.
  3. CuO: Cu + (−2) = 0, so Cu = +2.

Answer: Copper is +1 in Cu2O and +2 in CuO, which shows a variable oxidation number.

Practice questions

Try each one, then open the answer.

1. Which aqueous ion is coloured?

  1. A
    Mg2+
  2. B
    Cu2+
  3. C
    Zn2+
  4. D
    Na+
Show answer

Answer: B. Copper(II) ions are blue. Ions of Group I and Group II metals are colourless, and so is Zn2+.

2. Compared with sodium, a typical transition element has

  1. A
    a lower density, a higher melting point and white compounds
  2. B
    a lower density, a lower melting point and white compounds
  3. C
    a higher density, a higher melting point and coloured compounds
  4. D
    a higher density, a lower melting point and white compounds
Show answer

Answer: C. Transition elements are hard, dense metals with high melting points. Sodium is soft, floats on water and melts below 100 °C.

3. Which statement about transition elements and catalysis is correct?

  1. A
    transition elements are used up when they catalyse a reaction
  2. B
    transition elements and their compounds can act as catalysts, such as iron in the Haber process
  3. C
    only the Group I metals act as catalysts, such as sodium in the Haber process
  4. D
    transition element catalysts increase the equilibrium yield of a reversible reaction
Show answer

Answer: B. Iron (Haber process) and vanadium(V) oxide (Contact process) are examples. A catalyst is unchanged at the end and changes the rate, not the yield.

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