Where they sit
The d-block occupies groups 3 to 12 of the periodic table. Here the d orbitals of the penultimate shell are being filled. Its elements are called transition elements because their properties lie between those of the reactive s-block metals and the p-block.
There are four transition series: 3d (Sc to Zn), 4d (Y to Cd), 5d (La, Hf to Hg) and 6d (Ac, Rf to Cn).
The f-block has two rows placed below the main table: the lanthanoids (4f, Ce to Lu) and the actinoids (5f, Th to Lr). These are the inner transition elements.
Electronic configuration
The general configuration of d-block elements is (n − 1)d¹⁻¹⁰ ns¹⁻².
Two exceptions in the 3d series, due to the extra stability of half-filled and fully filled d subshells:
Cr: [Ar] 3d⁵ 4s¹ (not 3d⁴ 4s²)
Cu: [Ar] 3d¹⁰ 4s¹ (not 3d⁹ 4s²)
Zn, Cd and Hg have a full d¹⁰ configuration in their ground state and in their common oxidation states. They are not strictly transition elements, which by definition have an incompletely filled d subshell in the atom or in a common ion.
General properties
Metallic character. All are metals: hard, lustrous, with high melting and boiling points (except Zn, Cd, Hg), and good conductors. Strong metallic bonding involves many unpaired d electrons. Melting points rise to a maximum around the middle of each series (d⁵), then fall.
Atomic radii. Radii decrease only slightly across a series, because added d electrons shield the outer electrons poorly. Going from the 4d to 5d series, radii barely increase: Zr (160 pm) and Hf (159 pm) are almost the same size. This is due to the lanthanoid contraction (see below).
Ionisation enthalpies increase gradually across a series, with irregularities linked to stable configurations.
Variable oxidation states. Because the (n − 1)d and ns energies are close, transition elements can use different numbers of electrons in bonding. Manganese shows every state from +2 to +7. The highest oxidation states occur in the middle of the series and are often found in oxides and fluorides (MnO₄⁻, CrO₄²⁻), because oxygen and fluorine are highly electronegative and small.
Standard electrode potentials. Most 3d metals (except copper) have negative E° values for M²⁺/M and can release hydrogen from dilute acids. Copper's positive E° (+0.34 V) explains why it doesn't.
Coloured ions
Most transition metal compounds are coloured. When light falls on an ion with an incomplete d subshell, an electron absorbs energy and jumps between d orbitals of slightly different energy (d-d transition). The colour we see is the complementary colour of the light absorbed.
Ion | Configuration | Colour |
|---|---|---|
Sc³⁺ | 3d⁰ | colourless |
Ti³⁺ | 3d¹ | purple |
Cu²⁺ | 3d⁹ | blue |
Mn²⁺ | 3d⁵ | pink |
Zn²⁺ | 3d¹⁰ | colourless |
Ions with d⁰ or d¹⁰ configurations are colourless, as no d-d transition is possible.
Magnetic properties
Substances with unpaired electrons are paramagnetic (attracted to a magnetic field). Those with all electrons paired are diamagnetic (weakly repelled).
The spin-only magnetic moment is
μ = √(n(n + 2)) BM, where n is the number of unpaired electrons (BM = Bohr magneton).
Example. Fe²⁺ (3d⁶) has 4 unpaired electrons: μ = √(4 × 6) = √24 ≈ 4.90 BM. Mn²⁺ (3d⁵) has 5: μ = √35 ≈ 5.92 BM.
Other characteristic properties
Complex formation: small, highly charged ions with vacant d orbitals form many complexes, such as [Fe(CN)₆]³⁻ and [Cu(NH₃)₄]²⁺.
Catalytic activity: variable oxidation states and the ability to adsorb reactants make them excellent catalysts: iron in the Haber process, V₂O₅ in the contact process for sulphuric acid, nickel in hydrogenation of oils.
Interstitial compounds: small atoms like H, C and N get trapped in the gaps of the metal lattice (e.g. steel). These compounds are hard and have high melting points.
Alloys: transition metals have similar atomic sizes, so they readily form alloys such as brass (Cu–Zn) and bronze (Cu–Sn).
Important compounds
Potassium dichromate, K₂Cr₂O₇, is an orange crystalline solid made from chromite ore (FeCr₂O₄).
Chromate (yellow) and dichromate (orange) interconvert with pH: in acid, 2CrO₄²⁻ + 2H⁺ → Cr₂O₇²⁻ + H₂O.
In acidic solution, it is a strong oxidising agent: Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O. The colour changes from orange to green. It oxidises iodide to iodine and Fe²⁺ to Fe³⁺, and is used in volumetric analysis.
Potassium permanganate, KMnO₄, forms dark purple crystals. It is made from pyrolusite (MnO₂).
In acidic solution: MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O (purple to almost colourless).
In neutral or weakly basic solution: MnO₄⁻ + 2H₂O + 3e⁻ → MnO₂ + 4OH⁻ (a brown precipitate).
It oxidises Fe²⁺, oxalates, iodide and many organic compounds, and is used as a disinfectant. In titrations, it acts as its own indicator.
The lanthanoids
The lanthanoids (Ce to Lu, with La often included) fill the 4f orbitals. Their common oxidation state is +3; Ce also shows +4 and Eu and Yb +2.
Lanthanoid contraction: across the series, atomic and ionic radii steadily decrease. The 4f electrons shield each other poorly from the increasing nuclear charge, so the outer electrons are pulled in more strongly.
Consequences:
The 5d elements are almost the same size as the 4d elements above them (Zr and Hf), so they have very similar properties and are hard to separate.
Lanthanoids themselves are very similar, making them difficult to separate from one another.
Mischmetall, an alloy of lanthanoids (about 95%) with iron, is used in lighter flints.
The actinoids
The actinoids (Th to Lr) fill the 5f orbitals. All are radioactive; many after uranium are made artificially. They show a wider range of oxidation states than the lanthanoids (up to +7 for Np and Pu), because the 5f, 6d and 7s levels are close in energy. The actinoid contraction is greater than the lanthanoid contraction because 5f electrons shield even more poorly.
Summary
d-block configuration (n − 1)d¹⁻¹⁰ ns¹⁻²; Cr and Cu are exceptions.
Transition metals show variable oxidation states, coloured ions (d-d transitions), paramagnetism and catalysis.
μ = √(n(n + 2)) BM for n unpaired electrons.
K₂Cr₂O₇ and KMnO₄ are strong oxidising agents in acid.
Lanthanoid contraction makes 4d and 5d elements nearly equal in size.