8.4 Transition Elements

2026 Syllabus Objectives

Core:

  • Describe the transition elements as metals that:
    • (a) have high densities
    • (b) have high melting points
    • (c) form coloured compounds
    • (d) often act as catalysts as elements and in compounds

Supplement:

  • Describe transition elements as having ions with variable oxidation numbers, including iron(II) and iron(III)

What are Transition Elements? 🔑

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Transition elements (also called transition metals) are a block of metallic elements located in the center of the Periodic Table in Period 4. Unlike the main group elements, these elements are best considered as a horizontal row or block rather than a vertical group.

These metals fall outside the main groups (Groups I, II, and III-VIII) and possess distinctive properties that make them among the most useful metallic elements available to us.

Key Point: Transition elements are much less reactive than the metals in Groups I and II, and many have excellent corrosion resistance.

General Features of Transition Metals ⚡

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Transition metals share several important general features that distinguish them from other metals:

Physical Properties:

  • Hard and strong - They have excellent mechanical strength
  • High density - They are much denser than alkali metals or alkaline earth metals
  • High melting and boiling points - They can withstand very high temperatures

Example: Tungsten

Tungsten has an exceptionally high melting point of 3410 °C, which led to its use in the filaments of incandescent light bulbs.

Distinctive Properties of Transition Metals 📌

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Transition metals have several unique characteristics that set them apart from other metallic elements:

1. Formation of Coloured Compounds 🎨

Many compounds of transition metals are brightly coloured. When these compounds dissolve in water, they produce coloured solutions.

This property is one of the most visually striking characteristics of transition elements and is used in various applications, from pigments to analytical chemistry.

2. Catalytic Activity

Transition metals and their compounds often act as catalysts in chemical reactions.

Key Example:

  • Iron is used as a catalyst in the Haber process for manufacturing ammonia

Catalysts increase the rate of chemical reactions without being consumed in the process.

3. Variable Oxidation Numbers (Supplement) ⚡

Transition metals can show more than one valency (oxidation state). This means they can form ions with different charges.

Oxidation number: A number given to show whether an element has been oxidised or reduced; the oxidation number of a simple ion is simply the charge on the ion.

Important Examples:

Iron can form two different types of ions:

  • Iron(II): Fe2+\mathrm{Fe}^{2+} - iron in the lower oxidation state (+2)
  • Iron(III): Fe3+\mathrm{Fe}^{3+} - iron in the higher oxidation state (+3)

The Roman numerals in the name (II or III) indicate the oxidation number of the metal ion.

4. Formation of Multiple Oxides

Because transition metals can exist in different oxidation states, they can form more than one type of oxide.

Nature of Oxides:

  • Lower oxidation state oxides are basic ionic oxides

    • Examples: copper(II) oxide, chromium(II) oxide

  • Highest oxidation state oxides tend to be covalent and produce acidic solutions in water

    • Example: chromium(VI) oxide

  • Amphoteric oxides can react with both acids and bases

    • Chromium(III) oxide (Cr2O3\mathrm{Cr}_2\mathrm{O}_3) is similar to aluminium oxide (Al2O3\mathrm{Al}_2\mathrm{O}_3) in being an amphoteric oxide

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