Transition elements: general properties

The essentials

A transition element has an incompletely filled d subshell in its ground state or in one of its common oxidation states. Zinc, cadmium and mercury have a full d¹⁰ configuration in the ground state and common oxidation states, so they are not regarded as typical transition elements. The general outer configuration is (n−1)d¹⁻¹⁰ns¹⁻².

Key points

• Exceptions in the 3d series: Cr is 3d⁵4s¹ and Cu is 3d¹⁰4s¹, because half-filled and fully filled d subshells are extra stable.
• Transition metals are hard, have high melting points and high enthalpies of atomisation due to strong metallic bonding involving unpaired d electrons. In the 3d series, Zn has the lowest enthalpy of atomisation.
• Atomic radii decrease slowly across a series because added d electrons shield poorly. 4d and 5d elements of the same group have nearly equal radii because of lanthanoid contraction.
• Ionisation enthalpies increase gradually across the series.
• They show variable oxidation states because ns and (n−1)d electrons have similar energies. Manganese shows the widest range, +2 to +7.
• E°(M²⁺/M) for Cu is positive (+0.34 V), so copper does not liberate H₂ from dilute acids. The high positive E°(Mn³⁺/Mn²⁺) reflects the stability of d⁵ Mn²⁺; Cr²⁺ is a strong reducing agent and Mn³⁺ a strong oxidising agent.
• Higher oxidation states are stabilised by fluorine and oxygen (e.g. in MnO₄⁻, CrO₃).

Exam tips

• Count d electrons for an ion by removing 4s electrons first: Fe²⁺ is 3d⁶, Fe³⁺ is 3d⁵.

Common mistakes

• Removing 3d electrons before 4s when forming ions.
• Calling scandium's +3 ion a d¹ ion; Sc³⁺ is d⁰.