The d-and f-Block Elements

The One Thing

The characteristic chemistry of the d- and f-block elements arises from partially filled or readily accessible d- and f-subshells, together with small energy differences between relevant orbitals. These features account for variable oxidation states, colour, magnetism, complex formation, catalysis, alloy formation, and contraction across the lanthanoid and actinoid series.

Reactions, Processes and Experiments

Key Terms

• d-block elements: Elements in which the differentiating electron enters a d-orbital of the penultimate shell; they occupy groups 3 to 12.
• Transition element: An element whose atom or at least one stable ion has an incomplete d-subshell. Zinc, cadmium, and mercury are generally excluded because their atoms and common ions have completely filled d-subshells.
• General electronic configuration: The general configuration of d-block elements is (n−1)d1–10 ns0–2.
• Variable oxidation states: The ability of transition elements to show several oxidation states because ns and (n−1)d electrons have similar energies.
• Colour in ions: Colour produced when electrons absorb visible light and undergo d–d transitions between split d-orbital energy levels. Charge-transfer transitions can also produce intense colours.
• Magnetic behaviour: Unpaired d-electrons cause paramagnetism; species with all electrons paired are diamagnetic.
• Magnetic moment: The spin-only magnetic moment is μ = √[n(n+2)] BM, where n is the number of unpaired electrons and BM means Bohr magneton.
• Complex formation: The formation of coordination compounds when transition-metal ions accept lone pairs from ligands.
• Catalytic activity: Catalysis resulting from variable oxidation states, reactant adsorption, and the formation of intermediate complexes.
• Alloy formation: Formation of solid mixtures in which similarly sized metal atoms can replace one another in a crystal lattice.
• Interstitial compounds: Compounds formed when small atoms such as hydrogen, carbon, or nitrogen occupy spaces in a metal lattice.
• Lanthanoids: The fourteen elements from cerium to lutetium in which 4f orbitals are progressively filled; lanthanum is often included because of its similar chemistry.
• Lanthanoid electronic configuration: [Xe] 4f0–14 5d0–1 6s2.
• Lanthanoid contraction: The gradual decrease in atomic and ionic radii from La to Lu caused by poor shielding by 4f electrons and increasing effective nuclear charge.
• Lanthanoid oxidation states: The most common oxidation state is +3; +2 and +4 may occur when they provide especially stable f0, f7, or f14 arrangements.
• Actinoids: The fourteen elements from thorium to lawrencium in which 5f orbitals are progressively filled; actinium is commonly discussed with the series.
• Actinoid electronic configuration: [Rn] 5f0–14 6d0–1 7s2.
• Actinoid contraction: The gradual decrease in atomic and ionic radii across the actinoids due to poor shielding by 5f electrons and increasing effective nuclear charge.
• Actinoid oxidation states: Actinoids commonly show +3, while +4, +5, and +6 occur particularly among earlier members.
• Disproportionation: A redox process in which the same oxidation state is simultaneously oxidised and reduced.
• Potassium permanganate: KMnO4, a strong oxidising agent whose reduction product depends on the reaction medium.
• Potassium dichromate: K2Cr2O7, an important orange oxidising agent in acidic medium.
• Lanthanides versus actinides: Actinides generally show greater oxidation-state variability, form more complex compounds, and are radioactive, whereas most lanthanoids are non-radioactive except for naturally occurring radioactive members such as promethium.

Easily Confused

• d-block elements vs transition elements: All transition elements are d-block elements, but zinc, cadmium, and mercury are d-block elements generally excluded from the transition elements because their atoms and common ions have completely filled d-subshells.
• d–d transitions vs charge-transfer transitions: d–d transitions involve promotion between split d-orbital levels, whereas charge-transfer transitions involve transfer of electron density and can produce more intense colours.
• Paramagnetic vs diamagnetic: Paramagnetic species contain unpaired electrons; diamagnetic species have all electrons paired.
• Lanthanoids vs actinoids: Lanthanoids involve progressive filling of 4f orbitals, whereas actinoids involve progressive filling of 5f orbitals; actinoids are generally more radioactive and show more variable oxidation states.
• Lanthanoid contraction vs actinoid contraction: Both result from poor shielding by f-electrons, but actinoid contraction is greater because 5f electrons are less effective at shielding and participate more in bonding.
• Chromate vs dichromate: Chromate, CrO4^2−, is generally yellow, whereas dichromate, Cr2O7^2−, is generally orange; their interconversion is governed by Cr2O7^2− + H2O ⇌ 2CrO4^2− + 2H+.
• Permanganate products in different media: Acidic medium gives Mn2+, neutral or weakly basic medium commonly gives MnO2, and strongly basic medium can give MnO42−.
• Lanthanoids vs lanthanum: The fourteen-element 4f-filling sequence is commonly stated as cerium to lutetium, although lanthanum is often included because of its similar chemistry.
• Alloy compounds vs interstitial compounds: Alloys involve substitution of similar metal atoms in a lattice, whereas interstitial compounds involve small atoms occupying spaces within a metal lattice.

What Gets Asked

• Define a transition element and identify exclusions: State that an atom or at least one stable ion must have an incomplete d-subshell; zinc, cadmium, and mercury are excluded because their relevant d-subshells are complete.
• Write electronic configurations: Questions may require the general configuration (n−1)d1–10 ns0–2 or the exceptions Cr: [Ar] 3d5 4s1 and Cu: [Ar] 3d10 4s1. A common error is failing to remove ns electrons before (n−1)d electrons when forming ions.
• Explain colour and magnetism: Link colour to d–d or charge-transfer transitions and paramagnetism to unpaired electrons. Marks are lost by treating all transition-metal ions as coloured or by omitting the magnetic-moment equation μ = √[n(n+2)] BM.
• Explain catalytic activity and alloy formation: Use the stated examples—Fe in the Haber process, V2O5 in the Contact process, Ni in hydrogenation of vegetable oils, and MnO2 in decomposition of hydrogen peroxide. Do not substitute an unrelated catalyst.
• Explain lanthanoid contraction and its consequences: Include poor 4f shielding, decreasing Ln3+ size, decreasing hydroxide basicity, difficult separation, and the similar sizes of zirconium and hafnium.
• Compare lanthanoids and actinoids: State the different orbitals being filled, the greater oxidation-state variability and radioactivity of actinoids, and the relevant configurations.
• Complete or interpret redox equations: The specified half-reactions for acidic permanganate and dichromate must be reproduced accurately, and the reaction medium must be matched to the correct permanganate product.
• Interpret chromate–dichromate colour changes: Use the equilibrium Cr2O7^2− + H2O ⇌ 2CrO4^2− + 2H+ and distinguish orange dichromate from yellow chromate.