Definition of a transition element

· A transition element is a d-block element that forms one or more stable ions with incomplete d orbitals.
· This definition is about the ions formed, not just the atom’s position in the d-block.
· The first-row transition elements in this syllabus are titanium to copper.
· Copper is included because Cu²⁺ has an incomplete 3d subshell.
· Exam phrase: “forms at least one stable ion with an incomplete d subshell.”

This image helps locate the first row of transition elements visually. For CIE 28.1, focus on Ti to Cu, not the whole row shown. Source

3d orbitals: shapes you must recognise

· Transition elements involve 3d orbitals.
· You must be able to sketch the shape of a 3dxy orbital and a 3dz² orbital.
· 3dxy orbital: four lobes lying between the x- and y-axes.
· 3dz² orbital: two lobes along the z-axis plus a ring/doughnut-shaped region around the middle.
· Keep sketches simple, clearly labelled and symmetrical.

Use this image to compare the required 3dxy and 3dz² orbital shapes. In exam sketches, the key marks are correct lobe direction and clear axis labels. Source

Variable oxidation states

· Transition elements show variable oxidation states.
· This happens because the 3d and 4s sub-shells are similar in energy.
· Electrons from both 4s and 3d sub-shells can be involved in bonding or ion formation.
· Different numbers of electrons can be lost or shared, giving different oxidation states.
· Exam phrase: “the 3d and 4s electrons are close in energy, so different numbers of electrons can be used.”

Catalytic behaviour

· Transition elements and their compounds often act as catalysts.
· They can catalyse reactions because they have more than one stable oxidation state.
· This allows them to accept and donate electrons during a reaction mechanism.
· They also have vacant d orbitals that are energetically accessible.
· These vacant d orbitals can form dative covalent bonds with reacting species, helping to lower the activation energy through an alternative pathway.

Formation of complex ions

· Transition elements form complex ions.
· A complex ion forms when species with lone pairs of electrons donate electron pairs to a central metal ion.
· The donated electron pair forms a dative covalent bond.
· Transition element ions can form complexes because they have vacant d orbitals.
· These vacant d orbitals are energetically accessible, so they can accept lone pairs from ligands.

The figure shows that transition metal ions commonly form complex ions and many are coloured. This links directly to the CIE ideas of complex ion formation and coloured compounds. Source

Formation of coloured compounds

· Transition elements commonly form coloured compounds.
· This is a characteristic property of transition elements.
· In exam answers for 28.1, link colour to the presence of partially filled d orbitals only if the question asks for explanation.
· Do not confuse this with simple s-block ions, which usually form colourless compounds.
· Remember: transition metal compounds are often coloured, especially when ions have incomplete d orbitals.

Key exam explanations to memorise

· Definition: a transition element is a d-block element forming at least one stable ion with an incomplete d subshell.
· Variable oxidation states: due to similar energies of the 3d and 4s sub-shells.
· Catalysts: due to more than one stable oxidation state and vacant, accessible d orbitals.
· Complex ions: form because vacant d orbitals can accept lone pairs from ligands.
· Coloured compounds: a characteristic property of transition elements with incomplete d orbitals.

Common exam traps

· Do not define a transition element as simply “an element in the d-block”; this is incomplete.
· Do not say variable oxidation states happen because of “many electrons”; specify similar 3d and 4s energies.
· Do not forget that 4s electrons are removed before 3d electrons when forming ions.
· Do not say complex ions form because ligands “share electrons equally”; use dative covalent bond language.
· Do not overcomplicate orbital sketches: clear shape + axes is enough.