Transition Elements

A transition element is a d-block element whose atom has an incomplete d subshell, or which forms a cation with an incomplete d subshell. Incomplete means between one and nine d electrons. The definition tests electron configuration; being coloured or forming a complex is supporting behaviour, not the definition.

For first-row transition-metal cations, write the neutral atom first, then remove 4s electrons before 3d electrons. The order used to build a neutral-atom configuration is not a universal order of removal. Chromium and copper are the familiar neutral-atom exceptions to a simple 4s² pattern.

From scandium towards copper, nuclear charge rises while electrons are added mainly to 3d. These added electrons partly shield the outer electrons from the increasing nuclear charge, and the atoms do not acquire a new principal shell. The two changes largely offset one another: atomic radii and first ionisation energies vary relatively little compared with a full main-group period. Radii generally contract modestly and first ionisation energies show small, irregular changes; neither quantity is exactly constant.

Do not explain a high melting point using strong intermolecular forces: these are metallic structures. Do not explain density only by atomic mass; the volume occupied is also essential.

Transition elements commonly have variable oxidation states because their 3d and 4s electrons have comparable energies and different numbers can participate in bonding. Fe commonly forms +2 and +3 compounds; copper commonly forms +1 and +2 compounds. Calcium, by contrast, usually forms +2: removing more electrons would disrupt a much more tightly bound noble-gas core.

A configuration can suggest likely oxidation states by accounting for 4s electrons and, where chemically feasible, additional 3d electrons. Early members can use more of these electrons in high oxidation states. This is a prediction of possibilities, not proof that every intermediate state is equally stable in water.

The oxidising agent is reduced. Acidified manganate(VII) and dichromate(VI) accept electrons from reducing agents such as Fe²⁺, which becomes Fe³⁺. Acid is a reactant in both oxyanion half-equations, not just a label above the arrow. Changing pH can change both the potential and the manganese or chromium product.

A ligand is an ion or molecule that donates a lone pair to a central metal atom or ion to form a coordinate bond. A complex contains that central metal bonded to its surrounding ligands. H₂O and NH₃ are neutral donors through O and N respectively; Cl^- is a negatively charged donor. The metal accepts the pair: it does not supply both electrons in that coordinate bond.

Coordination number counts donor atoms directly bonded to the metal. For these one-donor ligands it equals the number of ligands. Determine oxidation state separately from overall complex charge: the negative chloride complex still contains copper in the +2 state.

Excess ammonia: [Cu(H₂O)₆]²⁺(aq) + 4NH₃(aq) ⇌ [Cu(NH₃)₄(H₂O)₂]²⁺(aq) + 4H₂O(l). Ammonia replaces four coordinated water molecules; the solution becomes deep blue.

Concentrated chloride: [Cu(H₂O)₆]²⁺(aq) + 4Cl^-(aq) ⇌ [CuCl₄]^2-(aq) + 6H₂O(l). High chloride concentration favours the yellow complex. Dilution favours the blue aqua form. These are ligand exchanges, not redox reactions.

A few drops of aqueous ammonia supply OH^- through NH₃ + H₂O ⇌ NH₄⁺ + OH^-. Copper(II) first forms blue Cu(OH)₂(s): Cu²⁺(aq) + 2OH^-(aq) → Cu(OH)₂(s). In excess ammonia, complex formation removes dissolved copper ions and the precipitate dissolves to the deep-blue ammine solution. NaOH does not provide that ammonia ligand, so the precipitate remains in excess NaOH.

Chromium(III) hydroxide dissolves in excess hydroxide by forming a soluble hydroxo complex: Cr(OH)₃(s) + 3OH^-(aq) ⇌ [Cr(OH)₆]^3-(aq). Chromium remains +3. In contrast, the browning of freshly formed iron(II) or manganese(II) hydroxide in air involves oxidation. Record the initial colour and subsequent change separately.

In haemoglobin, oxygen binds reversibly to an iron-containing centre. Carbon monoxide competes for the binding site and binds much more strongly, displacing oxygen and reducing oxygen transport. A simplified exchange is HbO₂ + CO ⇌ HbCO + O₂. This illustrates ligand competition; it is not a reaction in which CO burns with oxygen.

For an isolated metal ion, the five d orbitals have equal energy: they are degenerate. In an octahedral complex, six ligands approach along the positive and negative x, y and z axes. Their electron pairs repel d-electron density unevenly because the orbitals point in different directions.

If a suitable lower d level is occupied and a higher one can receive an electron, light of energy equal to the gap can cause a d-d transition. Absorbing selected visible wavelengths leaves the complementary mixture to be transmitted or reflected, giving the observed colour. A blue solution transmits blue light; it does not look blue because it absorbs blue most strongly.

Changing the ligand, metal or oxidation state can change the splitting and the wavelength absorbed. This explains why ligand exchange can change colour while oxidation state remains fixed. You do not need a memorised ranking of ligand field strengths for this syllabus.

Transition metals and their compounds can offer a pathway with lower activation energy. Their ability to interact with reactant electron density helps surface catalysis; accessible oxidation states help electron-transfer cycles. These are chemical reasons for catalytic activity, not a claim that every transition metal catalyses every reaction.

In a catalytic converter, transition-metal surfaces also help CO react with NO: 2CO(g) + 2NO(g) → 2CO₂(g) + N₂(g). Adsorption holds reactants near each other and facilitates bond changes. The catalyst does not change the reaction enthalpy or equilibrium constant.

Named redox electron counts: Fe³⁺/Fe²⁺: 1; MnO₄^-/Mn²⁺: 5; Cr₂O₇^2-/Cr³⁺: 6. The latter two equations require acid. Copper: water blue; excess ammonia deep blue; concentrated chloride favours yellow complex, often green in a mixture. Copper remains +2 in those exchanges.