Topic 15: Transition Metals — Edexcel A-Level Chemistry

Transition metals are elements found in the d-block of the periodic table, specifically in groups 3-12, that form at least one stable ion with a partially filled d-subshell. This topic explores their unique properties, including variable oxidation states, formation of coloured compounds, catalytic activity, and complex ion formation. Understanding transition metals is crucial for A-Level Chemistry as it connects electronic configuration to observable chemical behaviour, and has real-world applications in industrial catalysis, biological systems (e.g., haemoglobin), and materials science.

The key to mastering this topic lies in the d-orbital splitting in complex ions, which explains colour and magnetic properties. You'll learn about ligand substitution reactions, the chelate effect, and how to predict stability constants. This topic builds on earlier work on atomic structure, bonding, and redox reactions, and is assessed through both multiple-choice and extended-response questions, often requiring you to apply concepts to unfamiliar contexts.

Transition metals are not just theoretical; they are essential in everyday life. For example, iron in haemoglobin transports oxygen, platinum catalysts are used in catalytic converters, and vanadium(V) oxide is used in the Contact process. By the end of this topic, you should be able to explain why these elements are so versatile and how their properties arise from their electronic configurations.

• →Electronic configuration of transition metals and their ions: the 4s orbital fills before 3d, but is lost first when forming ions (e.g., Fe: [Ar] 3d⁶4s², Fe²⁺: [Ar] 3d⁶, Fe³⁺: [Ar] 3d⁵).
• →Variable oxidation states: due to the small energy difference between 3d and 4s orbitals, transition metals can lose different numbers of electrons (e.g., Mn from +2 to +7).
• →Formation of coloured compounds: caused by d-d electron transitions when light is absorbed; the colour observed is complementary to the absorbed wavelength.
• →Catalytic behaviour: transition metals and their compounds act as catalysts by providing a surface for adsorption (heterogeneous) or by changing oxidation states (homogeneous).
• →Complex ion formation: ligands (e.g., H₂O, NH₃, Cl⁻) donate lone pairs to the central metal ion, forming coordinate bonds; shapes include octahedral, tetrahedral, and square planar.

• Correct calculation of oxidation numbers in compounds and ions, including peroxides and metal hydrides.
• Correct identification of oxidation and reduction based on electron transfer and oxidation number changes.
• Correct identification of oxidising and reducing agents.
• Correct identification of disproportionation reactions.
• Correct use of Roman numerals to indicate oxidation numbers.
• Correct construction of full ionic equations from ionic half-equations.

• 💡Always check that the sum of oxidation numbers in a neutral compound equals zero and in an ion equals the charge of the ion.
• 💡Remember that oxidising agents are reduced (gain electrons) and reducing agents are oxidised (lose electrons).
• 💡When balancing half-equations, ensure the total charge on both sides is equal.
• 💡Practice identifying oxidation numbers in various contexts, especially for s- and p-block elements.
• 💡When explaining colour, always state that light is absorbed to excite an electron from a lower d-orbital to a higher d-orbital, and the complementary colour is transmitted/reflected. Mention that the energy gap (ΔE) depends on the ligand and oxidation state.
• 💡For catalysis questions, distinguish between heterogeneous (different phase, e.g., Fe in Haber process) and homogeneous (same phase, e.g., Fe²⁺/Fe³⁺ in the iodide-peroxodisulfate reaction). Explain how the catalyst provides an alternative pathway with lower activation energy.
• 💡In complex ion questions, remember to state the coordination number, shape, and bond angle. For example, [Cu(H₂O)₆]²⁺ is octahedral (coordination number 6), but [CuCl₄]²⁻ is tetrahedral (coordination number 4) due to Jahn-Teller distortion or ligand size.

• Confusing the direction of electron transfer in oxidation and reduction.
• Incorrectly assigning oxidation numbers in complex ions or species.
• Failing to balance both atoms and charges when constructing ionic half-equations.
• Misidentifying the species being oxidised or reduced in a disproportionation reaction.
• Misconception: All d-block elements are transition metals. Correction: Only those that form ions with a partially filled d-subshell are transition metals. Scandium (Sc³⁺: [Ar]) and zinc (Zn²⁺: [Ar] 3d¹⁰) are not transition metals.
• Misconception: The 4s orbital is filled before 3d, so it is always higher in energy. Correction: In isolated atoms, 4s is lower in energy, but in ions, 4s is higher, so electrons are removed from 4s first.
• Misconception: Colour in transition metal compounds is always due to d-d transitions. Correction: Some colours arise from charge transfer (e.g., permanganate ion, MnO₄⁻, is purple due to O→Mn charge transfer).