8.4 Transition Elements Revision Notes

1. Overview

The transition elements are a collection of metals located in the central block of the Periodic Table, between Groups II and III. These elements are vital in construction and the chemical industry due to their high strength and ability to act as catalysts to speed up industrial reactions.

Key Definitions

Transition Element: A metal found in the central block of the Periodic Table that forms at least one stable ion with a partially filled d-subshell (often simplified at IGCSE as "metals in the middle of the table").
Catalyst: A substance that increases the rate of a chemical reaction by providing an alternative pathway with lower activation energy, without being chemically changed at the end of the reaction.
Oxidation Number: A value assigned to an atom in a compound that represents the number of electrons lost or gained (e.g., in FeCl₂, Iron has an oxidation number of +2).

Core Content

Transition elements are distinct from Group I (alkali metals) and Group II (alkaline earth metals) because of their specific physical and chemical properties.

Physical Properties

High Densities: They are much "heavier" for their size compared to Group I metals. For example, Iron (Fe) has a density of 7.87 g/cm³, whereas Sodium (Na) is only 0.97 g/cm³.
High Melting Points: They remain solid at much higher temperatures. For example, Iron melts at 1538°C, while Sodium melts at only 98°C.

Chemical Properties

Form Coloured Compounds: Unlike Group I metals which form white compounds, transition metals form brilliantly coloured salts.
- Copper(II) sulfate: Blue
- Iron(II) salts: Pale green
- Iron(III) salts: Orange/Brown/Yellow
Act as Catalysts: Both the pure elements and their compounds are used to speed up reactions.
- Iron (Fe): Used in the Haber Process to make ammonia.
- Manganese(IV) oxide (MnO₂): Used to decompose hydrogen peroxide.

Example Reaction: Decomposition of Hydrogen Peroxide

Word Equation: Hydrogen peroxide $\rightarrow$ water + oxygen (using Manganese(IV) oxide catalyst)
Symbol Equation: $2H_2O_2(aq) \rightarrow 2H_2O(l) + O_2(g)$

📊A diagram showing the middle block of the Periodic Table highlighted, labeled "Transition Elements", contrasting them with the highly reactive Group I metals on the far left.

Extended Content (Extended Only)

Variable Oxidation Numbers

Transition elements do not have a fixed number of electrons in their outer shell that they lose. Instead, they can form multiple stable ions with different charges. This is referred to as having variable oxidation numbers.

Common Examples:

Iron (Fe):
- Iron(II) ions: $Fe^{2+}$ (forms green compounds)
- Iron(III) ions: $Fe^{3+}$ (forms reddish-brown compounds)
Copper (Cu):
- Copper(I) ions: $Cu^{+}$
- Copper(II) ions: $Cu^{2+}$

Reactivity with Sodium Hydroxide (aq): This property is often tested by adding aqueous sodium hydroxide to identify the specific ion present via a precipitation reaction:

Iron(II) reaction: Iron(II) chloride(aq) + Sodium hydroxide(aq) $\rightarrow$ Iron(II) hydroxide(s) + Sodium chloride(aq) $FeCl_2(aq) + 2NaOH(aq) \rightarrow Fe(OH)_2(s) + 2NaCl(aq)$ (Result: Green precipitate)
Iron(III) reaction: Iron(III) chloride(aq) + Sodium hydroxide(aq) $\rightarrow$ Iron(III) hydroxide(s) + Sodium chloride(aq) $FeCl_3(aq) + 3NaOH(aq) \rightarrow Fe(OH)_3(s) + 3NaCl(aq)$ (Result: Red-brown precipitate)

Key Equations

Reaction	Balanced Symbol Equation	Notes
Haber Process	$N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$	Uses Iron (Fe) catalyst
Contact Process	$2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g)$	Uses Vanadium(V) oxide ($V_2O_5$) catalyst
Peroxide Breakdown	$2H_2O_2(aq) \rightarrow 2H_2O(l) + O_2(g)$	Uses Manganese(IV) oxide ($MnO_2$) catalyst

Key to Symbols:

$N_2$ = Nitrogen gas
$H_2$ = Hydrogen gas
$NH_3$ = Ammonia gas
$\rightleftharpoons$ = Reversible reaction
$(aq)$ = Aqueous (dissolved in water)
$(s)$ = Solid

Common Mistakes to Avoid

❌ Wrong: Saying transition metals are "reactive" like Group I.
- ✓ Right: Transition metals are generally much less reactive than Group I and II metals (e.g., Gold and Platinum are very unreactive).
❌ Wrong: Describing the metals themselves as coloured.
- ✓ Right: While some are (Copper is pinkish-brown), the key property is that they form coloured compounds.
❌ Wrong: Forgetting the Roman numerals in names.
- ✓ Right: Always write Iron(II) or Iron(III) to indicate the oxidation state, as "Iron chloride" is ambiguous.

Exam Tips

Command Word "State": This topic has appeared 29 times as a "State" question. If asked to "state three properties of transition metals," simply list: High density, high melting point, and form coloured compounds.
Identification Questions: If a question mentions a "blue solution" or "green precipitate," immediately think of Transition Elements (Copper and Iron respectively).
Catalyst Contexts: Expect questions on the Haber Process. Remember that the catalyst (Iron) does not appear in the chemical equation itself but is written over the arrow.
Typical Values: In calculation questions, be ready to see atomic masses such as 56.0 (Fe), 63.5 (Cu), or 65.0 (Zn).
Frequency: With 44 past paper appearances, this is a high-yield topic. Ensure you can distinguish transition metals from Group I metals (who have low density and low melting points).