d- and f- Block Elements

The One Thing

The distinctive chemistry of d- and f-block elements arises from the progressive filling of d and f orbitals and, particularly for transition elements, the comparable energies of the ns and (n−1)d orbitals. These electronic features account for variable oxidation states, colour, magnetism, catalysis, complex formation and characteristic redox behaviour.

Reactions, Processes and Experiments

Key Terms

• d-block elements: Elements in which the differentiating electron enters a d-subshell of the penultimate shell; they occupy Groups 3 to 12.
• Transition element: An element whose atom or at least one common ion has a partially filled d-subshell. Zinc, cadmium and mercury are generally excluded because their atoms and common ions have completely filled d-subshells.
• Electronic configuration of transition elements: The general configuration is (n−1)d1–10 ns0–2. Important exceptions include chromium, [Ar] 3d5 4s1, and copper, [Ar] 3d10 4s1, because half-filled or completely filled d-subshells provide extra stability.
• Transition series: The four series are Sc to Zn, Y to Cd, La or Hf to Hg depending on classification, and Ac or Rf to Cn in the extended periodic table.
• Variable oxidation states: Different numbers of ns and (n−1)d electrons can participate in bonding because the orbitals have similar energies. Manganese shows oxidation states from +2 to +7.
• Atomic and ionic radii: Radii generally decrease initially across a transition series and then change only slightly because increasing nuclear charge is partly offset by d-electron shielding.
• Ionisation enthalpy: Ionisation enthalpies generally increase across a transition series, but the trend is irregular because electronic configurations and half-filled or filled subshell stability vary.
• Colour of transition-metal ions: Visible light can promote d-electrons between split d-orbitals. Ions with d0 or d10 configurations are usually colourless; charge-transfer transitions can also produce colour.
• Magnetic behaviour: Unpaired electrons cause paramagnetism, whereas substances with all electrons paired are diamagnetic.
• Spin-only magnetic moment: μ = √[n(n + 2)] BM, where n is the number of unpaired electrons and BM means Bohr magneton.
• Catalytic activity: Transition metals and their compounds catalyse reactions by changing oxidation state, adsorbing reactants and providing alternative reaction pathways.
• Complex formation: Transition-metal ions form coordination compounds because of their small size, high charge density and vacant orbitals. Examples include [Fe(CN)6]4− and [Cu(NH3)4]2+.
• Alloy formation: Transition elements form alloys because similar atomic sizes allow atoms to replace one another in metallic lattices. Stainless steel and brass are examples involving transition metals.
• Interstitial compounds: Small atoms such as hydrogen, carbon or nitrogen occupy spaces in a metal lattice. These compounds are often hard, have high melting points and retain metallic conductivity; examples include TiC and Fe3C.
• Lanthanoids: The fourteen elements following lanthanum in which 4f orbitals are progressively filled, commonly represented from cerium to lutetium. Their general configuration is [Xe] 4f1–14 5d0–1 6s2.
• Lanthanoid contraction: The gradual decrease in atomic and ionic radii from La3+ to Lu3+ caused by poor shielding by 4f electrons and increasing effective nuclear charge.
• Consequences of lanthanoid contraction: It produces similar lanthanoid properties, makes separation difficult, decreases the basicity of lanthanoid hydroxides from La(OH)3 to Lu(OH)3 and accounts for similarities between Zr and Hf.
• Oxidation states of lanthanoids: The most common oxidation state is +3. Some elements also show +2 or +4 when these states produce stable f0, f7 or f14 configurations, as in Ce4+, Eu2+ and Yb2+.
• Actinoids: The fourteen elements following actinium in which 5f orbitals are progressively filled, commonly represented from thorium to lawrencium. Their general configuration is [Rn] 5f0–14 6d0–1 7s2.
• Actinoid contraction: The gradual decrease in actinoid atomic and ionic radii across the series because of poor shielding by 5f electrons.
• Radioactivity of actinoids: All actinoids are radioactive because their nuclei are unstable. Thorium, uranium and plutonium are important examples.
• Oxidation states of actinoids: Actinoids commonly show oxidation states from +3 to +6 because 5f, 6d and 7s orbitals have comparable energies.
• Lanthanoids versus actinoids: Lanthanoids mainly form +3 ions and have less variable chemistry, whereas actinoids show greater oxidation-state variability, stronger complex formation and universal radioactivity.
• Potassium permanganate: KMnO4 is a purple crystalline compound and a strong oxidising agent. In acidic medium, MnO4− is reduced to Mn2+; in neutral or weakly alkaline medium, it commonly forms MnO2.
• Permanganate reduction in acidic medium: MnO4− + 8H+ + 5e− → Mn2+ + 4H2O.
• Potassium dichromate: K2Cr2O7 is an orange crystalline compound and a strong oxidising agent, especially in acidic medium, where dichromate ion is reduced to Cr3+.
• Dichromate reduction in acidic medium: Cr2O7^2− + 14H+ + 6e− → 2Cr3+ + 7H2O.
• Chromate–dichromate equilibrium: In alkaline medium, Cr2O7^2− + 2OH− ⇌ 2CrO4^2− + H2O; in acidic medium, 2CrO4^2− + 2H+ ⇌ Cr2O7^2− + H2O.
• Oxidising power of oxoanions: Oxidising behaviour depends strongly on the medium; acidic conditions generally favour reduction of permanganate and dichromate to lower oxidation-state products.

Easily Confused

• d-block element and transition element: All transition elements are d-block elements, but zinc, cadmium and mercury are generally excluded from the transition elements because their atoms and common ions have completely filled d-subshells.
• Paramagnetic and diamagnetic: Paramagnetic substances contain unpaired electrons; diamagnetic substances have all electrons paired.
• Lanthanoids and actinoids: Lanthanoids involve progressive filling of 4f orbitals and mainly show +3 oxidation states, whereas actinoids involve 5f filling, show more variable oxidation states and are all radioactive.
• Lanthanoid contraction and actinoid contraction: Lanthanoid contraction results from poor shielding by 4f electrons, whereas actinoid contraction results from poor shielding by 5f electrons.
• Permanganate and dichromate: KMnO4 is purple and MnO4− accepts five electrons in acidic medium; K2Cr2O7 is orange and Cr2O7^2− accepts six electrons.
• Chromate and dichromate: Chromate is yellow and favoured in alkaline medium; dichromate is orange and favoured in acidic medium.
• Colourless ions and coloured ions: Sc3+ is d0 and Zn2+ is d10, so both are generally colourless and diamagnetic; ions with partially filled d-orbitals are often coloured and paramagnetic.
• ns and (n−1)d electron removal: In forming transition-metal ions, ns electrons are removed before (n−1)d electrons, as shown by Fe, Fe2+ and Fe3+.

What Gets Asked

• Electronic-configuration questions: Students may be asked to write configurations for transition elements and ions, especially chromium [Ar] 3d5 4s1, copper [Ar] 3d10 4s1, Fe [Ar] 3d6 4s2, Fe2+ [Ar] 3d6 and Fe3+ [Ar] 3d5. The main mark-losing error is removing d-electrons before ns-electrons.
• Magnetic-moment questions: A question may require the number of unpaired electrons or use of μ = √[n(n + 2)] BM. The common error is confusing the number of unpaired electrons with the total number of d-electrons.
• Colour and magnetism questions: Students may need to explain why ions such as Ti3+, V2+, Cr3+, Mn2+, Fe2+, Fe3+ and Cu2+ are coloured, and why Sc3+ and Zn2+ are generally colourless and diamagnetic. The key distinction is between partially filled d-orbitals and d0 or d10 configurations.
• Catalysis and complex-formation questions: Questions may require the specific examples Fe in N2 + 3H2 ⇌ 2NH3, V2O5 in 2SO2 + O2 ⇌ 2SO3, Ni in hydrogenation of vegetable oils, [Fe(CN)6]4− or [Cu(NH3)4]2+. Marks are lost by giving a generic catalyst or complex without the named example.
• Lanthanoid and actinoid trend questions: Students may be asked to explain lanthanoid contraction, decreasing basicity from La(OH)3 to Lu(OH)3, the similarity of Zr and Hf, or the greater oxidation-state variability and radioactivity of actinoids. The common error is attributing 4f-based contraction to actinoids or stating that lanthanoids have the more variable oxidation states.
• Redox and equilibrium questions: Questions may require the half-reactions for KMnO4 and K2Cr2O7, electron-acceptance values, preparation equations for KMnO4, or the colour change between chromate and dichromate. Marks are lost by using the wrong reduction product or assigning yellow to dichromate and orange to chromate.