28.1 Physical and chemical properties of first-row transition elements
- Syllabus
- 9701–2028–2029
- Topic
- 28.1
- Level
- A2
Learning objectives
28.1.1Transition element• Define a transition element as a d-block element which forms one or more stable ions with incomplete d orbitals28.1.2Sketch the shape of a 3dxy orbital and 3dz²• Sketch the shape of a 3dxy orbital and 3dz² orbital28.1.3Transition elements have properties• Understand: transition elements have the following properties:: (a) they have variable oxidation states; (b) they behave as catalysts; (c) they form complex ions; (d) they form coloured compounds28.1.4Why transition elements have variable• Explain why transition elements have variable oxidation states in terms of the similarity in energy of the 3d and the 4s sub-shells28.1.5Why transition elements behave as catalysts• Explain why transition elements behave as catalysts in terms of having more than one stable oxidation state, and vacant d orbitals that are energetically accessible and can form dative bonds with ligands28.1.6Why transition elements form complex ions• Explain why transition elements form complex ions in terms of vacant d orbitals that are energetically accessible
A transition element forms at least one stable ion with an incomplete d subshell
A transition element is a d-block element that forms at least one stable ion with an incomplete d subshell.
| Element | Relevant stable ion | d configuration | Transition element? |
|---|---|---|---|
| Sc | Sc³⁺ | 3d⁰ | no: the d subshell is empty |
| Fe | Fe²⁺ or Fe³⁺ | 3d⁶ or 3d⁵ | yes: the d subshell is incomplete |
| Zn | Zn²⁺ | 3d¹⁰ | no: the d subshell is complete |
Classify the element by the electron configuration of its stable ion or ions, not by the neutral atom alone. Only one stable ion with a partly filled d subshell is required.
Being in the d block is necessary but is not sufficient under this definition: scandium and zinc are d-block elements but are not transition elements because their stable ions are d⁰ and d¹⁰ respectively.
The 3dxy and 3dz² orbitals have distinct orientations
| Orbital | Plane / axis | Electron-density shape | How to sketch it |
|---|---|---|---|
| 3dxy | xy plane | four lobes between the x and y axes | draw labelled x and y axes, then place one lobe in each quadrant between them |
| 3dz² | z axis and xy plane | two lobes along z plus a torus around the centre in the xy plane | draw a labelled vertical z axis, two opposing lobes on it, then a ring around the centre |
Put the nucleus at the origin and label the relevant axes. For 3dxy the axes pass between the lobes; for 3dz² the two main lobes lie on the z axis and the ring lies perpendicular to it.
An orbital sketch represents a region of electron probability density. It is not a path followed by an electron, and the 3dxy lobes must not be drawn on the x and y axes.
Transition elements share four characteristic chemical properties
| Property | What it means here | Example |
|---|---|---|
| variable oxidation states | the same element forms stable species in more than one oxidation state | Fe²⁺ and Fe³⁺ |
| catalytic activity | the element or one of its compounds speeds a reaction and is regenerated | Fe in the Haber process |
| complex-ion formation | a central metal ion accepts lone pairs from ligands | [Cu(H₂O)₆]²⁺ |
| coloured compounds | many compounds absorb part of visible light and appear coloured | aqueous Cu²⁺ compounds are blue |
These behaviours arise because 3d and 4s levels are close enough in energy for several electron arrangements and bonding interactions to be accessible.
These are characteristic trends, not a claim that every compound of every transition element displays all four. Detailed explanations of colour and complex geometry belong to later objectives.
Similar 3d and 4s energies allow variable oxidation states
The 3d and 4s sublevels are close in energy. Different numbers of 4s and 3d electrons can therefore be removed or used in bonding without an impossibly large extra energy change, allowing several stable oxidation states.
| Species | Electron configuration | Oxidation state |
|---|---|---|
| Fe | [Ar] 3d⁶ 4s² | 0 |
| Fe²⁺ | [Ar] 3d⁶ | +2 |
| Fe³⁺ | [Ar] 3d⁵ | +3 |
When a first-row transition metal forms a positive ion, its 4s electrons are removed before its 3d electrons, even though 4s fills before 3d in the neutral atom.
Variable oxidation state is not caused by electrons having no energy cost. It reflects the relatively small energy differences among accessible 3d/4s arrangements, balanced by bonding and lattice or hydration stabilisation.
Transition elements catalyse reactions through redox cycling or ligand binding
| Catalytic route | Role of the transition element | Result |
|---|---|---|
| oxidation-state cycling | changes reversibly between stable oxidation states by accepting and donating electrons, such as Fe²⁺ ⇌ Fe³⁺ | provides a lower-energy sequence of redox steps |
| ligand binding | accessible vacant d orbitals accept lone pairs to form temporary dative bonds with reactants | brings reactants together or weakens bonds before products leave |
In either route, the catalyst participates in intermediate steps but is regenerated by the end. The alternative pathway has a lower activation energy, so a larger fraction of collisions is successful at the same temperature.
A catalyst changes reaction rate, not the position of equilibrium or the overall enthalpy change. A proposed mechanism must show both how the catalyst participates and how it is regenerated.
Ligands donate lone pairs into accessible vacant orbitals to form complex ions
A complex ion contains a central metal ion surrounded by ligands. Each ligand donates a lone pair into an energetically accessible vacant orbital on the metal ion, forming a dative covalent bond.
TiX3++6HX2O[Ti(HX2O)X6]X3+
| Part of [Ti(H₂O)₆]³⁺ | Electron-pair role |
|---|---|
| each H₂O ligand | donor: an oxygen lone pair is donated |
| Ti³⁺ central ion | acceptor: an accessible vacant orbital accepts the pair |
| six Ti–O links | dative bonds; both bonding electrons originally came from the ligand |
Water is a neutral ligand, so the complex charge is the Ti³⁺ charge plus six zeros: +3. Once a dative bond has formed, it behaves as a covalent bond; its name records the origin of the shared electron pair.
A complex ion is a bonded coordination entity, not merely a metal ion and nearby counter-ions. Counter-ions may balance its charge but are not ligands unless they donate a lone pair directly to the metal centre.