Transition Elements

Transition elements are the metals in the middle block of the Periodic Table (between Groups 2 and 3). Common examples include iron, copper, nickel, and chromium. They share important properties that make them useful in everyday life.

Key Properties

• High densities – they are heavy for their size.
• High melting points – most need very high temperatures to melt.
• Form coloured compounds – many of their compounds are brightly coloured.
• Variable oxidation numbers – they can form ions with different charges, for example iron forms iron(II) $\text{Fe}^{2+}$ and iron(III) $\text{Fe}^{3+}$.
• Often act as catalysts – they speed up reactions without being used up.

Understanding the Ideas

Coloured compounds

Think of transition metal ions like stained glass that absorbs some colours and lets others through. Examples: copper(II) sulfate is blue, potassium dichromate(VI) is orange, and potassium manganate(VII) is purple.

Variable oxidation numbers

Main-group metals usually form one common ion (e.g. Group I forms +1). Transition metals can “choose” different charges to suit different reactions. The Roman numerals in names show the ion’s charge: iron(II) = $\text{Fe}^{2+}$, iron(III) = $\text{Fe}^{3+}$.

Catalysts in action

Catalysts are like shortcuts on a journey. They help reactions happen faster and are not used up. Examples: iron is the catalyst in the Haber process for making ammonia:

$$\mathrm{N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)}$$

Nickel is used to harden vegetable oils into margarine. Manganese(IV) oxide speeds up the decomposition of hydrogen peroxide.

Real-World Connections

• Iron and steel for buildings, bridges, and cars.
• Copper for electrical wires and water pipes.
• Coloured compounds used as pigments and in testing chemicals.
• Catalysts used in making fertilizers and fuels.

Common Misconceptions

• “Coloured compounds” does not mean the metals themselves are always colourful; it refers to many of their compounds.
• A catalyst is not used up; it can be reused.
• Oxidation number (charge) is not the same as the number of atoms; iron(II) and iron(III) are different charges, not different amounts of iron.