28.1 Transition Elements- Introduction
Learning outcomes
Candidates should be able to:
- define a transition element as a d-block element which forms one or more stable ions with incomplete d orbitals
- sketch the shape of a 3dxy orbital and a 3dz² orbital
- understand that transition elements have the following properties:
- they have variable oxidation states
- they behave as catalysts
- they form complex ions
- they form coloured compounds
- explain why transition elements have variable oxidation states in terms of the similarity in energy of the 3d and the 4s sub-shells
- 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 ligands
- explain why transition elements form complex ions in terms of vacant d orbitals that are energetically accessible
The 3d block and transition elements
The elements from scandium to zinc inclusive make up the 3d block. The 3d sub-shell has five orbitals, each holding two electrons, so the block contains ten elements.
Apart from scandium and zinc, the other eight are called transition elements, because some of their properties change gradually (transition) from those of reactive calcium in Group 2 to much less reactive gallium in Group 13.
Zn: Zn2+ = [Ar] 3d10 → d sub-shell full, so not a transition element.
When transition elements form ions, their atoms lose electrons from the 4s sub-shell first, then from 3d.
Shapes of 3d orbitals
3dz²: two lobes along the z-axis with a ring (doughnut) around the centre.
Properties of transition elements
Transition elements:
- form some compounds containing ions with an incomplete d sub-shell
- have variable oxidation states
- can behave as catalysts
- form complex ions
- form coloured compounds
Physical properties
Like most metals, they have high melting points and high densities, are hard and rigid (useful as construction materials) and are good conductors of electricity and heat.
- Melting and boiling points are much higher than those of s- and p-block elements. Both the 4s and 3d electrons take part in metallic bonding.
- Good conductors of heat and electricity; copper has particularly high electrical conductivity.
- Metallic radii tend to decrease across the block, as increasing nuclear charge attracts the outer electrons more strongly (with minor variations).
- No obvious trend in M2+ ionic radii, but M3+ ionic radii tend to decrease across the block as nuclear charge increases.
Variable oxidation states
Transition elements form compounds in which the metal shows different oxidation states. Common states of the first-row elements are shown below.
| Element | Sc | Ti | V | Cr | Mn | Fe | Co | Ni | Cu | Zn |
|---|---|---|---|---|---|---|---|---|---|---|
| Common oxidation states | +3 | +2, +3, +4 | +2 to +5 | +2, +3, +6 | +2, +4, +7 | +2, +3 | +2, +3 | +2 | +1, +2 | +2 |
Formation of complex ions
Transition-metal ions form complex ions by accepting lone pairs of electrons from ligands into vacant d orbitals.
Transition elements as catalysts
Transition elements act as catalysts because they can:
- change between different oxidation states, and
- provide vacant d orbitals that can form dative bonds with reactant molecules.
These features provide an alternative reaction pathway with lower activation energy.
Contact process: 2SO2 + O2 ⇌ 2SO3 (V2O5 catalyst)
Hydrogenation of alkenes: alkene + H2 → alkane (Ni catalyst)
Exam tips
- Always quote the full definition: a d-block element that forms one or more stable ions with incomplete d orbitals. Sc and Zn fail because their ions have 3d0 and 3d10.
- Electrons are lost from 4s before 3d when atoms form ions.
- Cr and Cu are the exceptions: 3d54s1 and 3d104s1.
- For catalysis, give both points: more than one stable oxidation state, and vacant d orbitals that form dative bonds with reactants.
Quick summary
- Four key properties: variable oxidation states, catalytic activity, complex-ion formation, coloured compounds.
- Variable oxidation states arise because 3d and 4s are close in energy.
- Complex ions form when ligands donate lone pairs into vacant, accessible d orbitals.
- Physical properties: high melting points and densities, hard, good conductors; metallic radii decrease across the block.