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Transition elements: Class 12 Chemistry
Position and electronic configuration of d-block elements, general properties, variable oxidation states, colour, magnetism, catalysis, complex formation, important compounds such as KMnO₄ and K₂Cr₂O₇, and uses of transition metals.
Key points
The d-block elements lie in groups 3 to 12 of the periodic table. A transition element is one that has an incomplete d subshell in its atom or in at least one of its common ions. By this definition zinc, cadmium and mercury (d¹⁰) are d-block elements but are not true transition elements. The first series runs from scandium (Z = 21) to zinc (Z = 30) and fills the 3d orbitals.
General electronic configuration is (n−1)d¹⁻¹⁰ ns¹⁻². Chromium and copper are exceptions because half-filled and fully filled d subshells are extra stable. Cr is [Ar] 3d⁵ 4s¹ and Cu is [Ar] 3d¹⁰ 4s¹. When these atoms form ions, the ns electrons leave first, then the (n−1)d electrons. So Fe³⁺ is [Ar] 3d⁵.
Physical properties of transition metals:
- They are hard, strong, lustrous metals with high melting and boiling points, because both ns and (n−1)d electrons take part in metallic bonding.
- They are good conductors of heat and electricity and are malleable and ductile.
- They have high densities because atomic size is small and atomic mass is large.
- Mercury is the only metal that is liquid at room temperature.
Atomic radii decrease slightly across a series and then stay almost constant. The added electrons enter the inner d subshell and shield the outer electrons well, so the effective nuclear charge rises only a little. This is why neighbouring transition metals such as iron, cobalt and nickel are so similar.
Variable oxidation states are a main feature. The ns and (n−1)d electrons have similar energies, so different numbers of electrons can be lost. Manganese shows states from +2 to +7. The +2 state is common for most first-row metals because the two 4s electrons are lost. The highest state of a metal in the middle of the series equals the total of its ns and d electrons, as in Mn(+7) in MnO₄⁻.
Most transition metal ions are coloured. In a compound the d orbitals split into two groups of slightly different energy. An electron absorbs visible light to jump between them (a d-d transition) and we see the complementary colour. Ions with empty d orbitals (Sc³⁺, Ti⁴⁺) or full d orbitals (Zn²⁺, Cu⁺) are colourless. Examples: Cu²⁺ (aq) is blue, Fe³⁺ is yellow to brown, MnO₄⁻ is purple and Cr₂O₇²⁻ is orange.
Many transition metals and their ions are paramagnetic because they have unpaired electrons. Substances with all electrons paired are diamagnetic. The magnetic moment depends on the number of unpaired electrons n and is found from μ = √(n(n + 2)) in Bohr magnetons (BM). Iron, cobalt and nickel are ferromagnetic, which is a much stronger effect.
Catalytic activity is common. Transition metals can use empty d orbitals and change oxidation state, so they give a surface or an easy reaction path. Examples: iron in the Haber process, V₂O₅ in the contact process, nickel in hydrogenation of oils and platinum in catalytic converters.
Transition metals form complex ions by accepting lone pairs from ligands into their empty orbitals, as in [Fe(CN)₆]⁴⁻ and [Cu(NH₃)₄]²⁺. They also form interstitial compounds, where small atoms such as H, C or N fit in gaps of the metal lattice, and they form alloys easily because their atoms have similar size. Steel, brass (Cu and Zn) and bronze (Cu and Sn) are common examples.
Important compounds:
- Potassium dichromate K₂Cr₂O₇ is an orange crystalline solid and a strong oxidising agent in acid solution. Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O. The orange colour changes to green. It is used in titrations and in leather tanning.
- Potassium permanganate KMnO₄ is a dark purple solid. In acid solution MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O, so purple turns colourless. It acts as its own indicator in titrations and is used as a disinfectant.
- In the dichromate and chromate equilibrium, adding alkali gives yellow CrO₄²⁻ and adding acid gives orange Cr₂O₇²⁻.
Uses of transition metals include iron and steel in construction, copper in wires, zinc for galvanising, titanium in aircraft and chromium for plating and stainless steel. Rusting of iron needs oxygen and water and is prevented by painting, galvanising or alloying.
Study tip: Learn the first-row configurations and the two exceptions by heart, because most numericals start there. Practise the oxidation number method on KMnO₄, K₂Cr₂O₇ and complex ions. For long questions on colour or catalysis, always mention the d orbitals, since examiners look for that key point.
Definitions and flashcards
Say the meaning first, then open the card to check.
Transition element
An element with an incomplete d subshell in its atom or in one of its common ions.
Variable oxidation state
The ability of an element to show more than one oxidation number in its compounds.
Paramagnetic
Attracted by a magnetic field because the species has unpaired electrons.
Diamagnetic
Weakly repelled by a magnetic field because all electrons are paired.
d-d transition
Movement of an electron between split d orbitals by absorbing visible light, giving colour.
Ligand
A molecule or ion that donates a lone pair to a metal ion to form a complex.
Interstitial compound
A compound in which small atoms occupy the gaps in a metal lattice.
Alloy
A mixture of a metal with other elements, with properties better than the pure metal.
Formula sheet
| Name | Formula | What it means |
|---|---|---|
| General configuration | (n−1)d¹⁻¹⁰ ns¹⁻² | Valence electron arrangement of d-block atoms |
| Spin-only magnetic moment | μ = √(n(n + 2)) BM | n is the number of unpaired electrons |
| Oxidation number sum | Sum of oxidation numbers = charge on species | Used to find the oxidation state of a metal |
Short questions with model answers
These are the kind of questions asked for 2, 3 and 5 marks. Try each one before opening the answer.
Why is zinc not counted as a transition element? (2 marks)
Zinc has the configuration [Ar] 3d¹⁰ 4s² in its atom and 3d¹⁰ in its common ion Zn²⁺. The d subshell is full in both, so it is never incomplete.
Why do transition elements show variable oxidation states? (2 marks)
The 4s and 3d electrons are close in energy, so different numbers of electrons from both subshells can take part in bonding. This gives several oxidation states, for example manganese from +2 to +7.
Why are most transition metal compounds coloured? (3 marks)
In a compound the d orbitals split into two energy levels. An electron absorbs light of one visible wavelength to move to the higher level. The light that is not absorbed reaches the eye as the complementary colour. If the d subshell is empty or full, no such jump is possible and the compound is colourless.
Explain why transition metals are good catalysts. (3 marks)
They have empty or partly filled d orbitals that can bond with reactant molecules and hold them on the surface in a favourable position. They can also change oxidation state easily, which gives a new reaction path with lower activation energy. Examples are iron in the Haber process and V₂O₅ in the contact process.
Describe the properties and two uses of potassium dichromate. (5 marks)
K₂Cr₂O₇ is an orange crystalline solid, soluble in water. In acid solution it is a strong oxidising agent: Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O, and the orange solution turns green. Uses: oxidising agent in titrations and in the tanning of leather. Alkali converts it to yellow chromate, and acid reverses this.
Why do the atomic radii of the first series change very little across the series? (3 marks)
The new electrons enter the inner 3d subshell. These electrons shield the 4s electrons from the nucleus, which cancels most of the extra nuclear charge. So the effective nuclear charge on the outer electrons rises only slightly and the size stays almost constant.
Numericals with worked steps
Write the electronic configuration of Fe³⁺ (Z = 26) and find its number of unpaired electrons and spin-only magnetic moment.
- Fe atom: [Ar] 3d⁶ 4s².
- Remove the two 4s electrons: Fe²⁺ is [Ar] 3d⁶.
- Remove one 3d electron: Fe³⁺ is [Ar] 3d⁵.
- Five d orbitals each hold one electron, so n = 5.
- μ = √(n(n + 2)) = √(5 × 7) = √35 = 5.92
Answer: Fe³⁺ = [Ar] 3d⁵, 5 unpaired electrons, μ = 5.92 BM
Calculate the spin-only magnetic moment of Cu²⁺ (Z = 29).
- Cu atom: [Ar] 3d¹⁰ 4s¹.
- Cu²⁺ loses the 4s electron and one 3d electron: [Ar] 3d⁹.
- 3d⁹ has one unpaired electron, so n = 1.
- μ = √(1 × 3) = √3 = 1.73
Answer: μ = 1.73 BM
Find the oxidation number of manganese in KMnO₄ and of chromium in K₂Cr₂O₇.
- KMnO₄: K is +1, O is −2. Let Mn be x.
- (+1) + x + 4(−2) = 0, so x − 7 = 0 and x = +7.
- K₂Cr₂O₇: 2(+1) + 2y + 7(−2) = 0.
- 2 + 2y − 14 = 0, so 2y = 12 and y = +6.
Answer: Mn = +7 in KMnO₄ and Cr = +6 in K₂Cr₂O₇
Find the spin-only magnetic moment of Ni²⁺ (Z = 28).
- Ni atom: [Ar] 3d⁸ 4s².
- Ni²⁺ loses the two 4s electrons: [Ar] 3d⁸.
- In 3d⁸ the five orbitals hold 8 electrons, so three are paired and two are unpaired, n = 2.
- μ = √(2 × 4) = √8 = 2.83
Answer: μ = 2.83 BM
Common mistakes
- Removing 3d electrons before 4s electrons when writing ions. The 4s electrons are lost first.
- Writing Cr as [Ar] 3d⁴ 4s² and Cu as [Ar] 3d⁹ 4s². The correct forms are 3d⁵ 4s¹ and 3d¹⁰ 4s¹.
- Calling all d-block elements transition elements. Zn, Cd and Hg are d-block but not transition elements.
- Forgetting the 'BM' unit, or using n instead of the number of unpaired electrons in the magnetic moment formula.
- Saying colour comes from s-p transitions or from the metal being 'shiny'. It comes from d-d transitions.