General Properties of the Transition Elements (d-Block)

Full Notes

Physical Properties

Transition metals share common properties due to metallic bonding and d-electron behaviour.

• Transition metals are all metallic in nature.
• Exhibit high tensile strength, ductility, and metallic lustre.
• Show good thermal and electrical conductivity.
• Have high melting and boiling points due to strong metallic bonding.
• Most form coloured ions or compounds due to d–d transitions.
• They are often paramagnetic because of unpaired electrons.

Variation in Atomic and Ionic Sizes of Transition Metals

Atomic and ionic radii show trends due to increasing nuclear charge and poor shielding by d-electrons.

• A gradual decrease in atomic radius occurs across the 3d series.
• This is due to increasing nuclear charge which pulls electrons closer.
• The decrease is small and irregular because the shielding effect of d-electrons is poor.
• Ionic sizes also follow a similar trend: Mn²⁺ > Fe²⁺ > Co²⁺ > Ni²⁺ > Cu²⁺ > Zn²⁺

Ionisation Enthalpies

Ionisation enthalpies reflect the energy required to remove electrons and reveal electron stability.

• Ionisation enthalpies increase across the series, but not smoothly.
• Irregularities arise due to stable configurations:
  • Cr: 3d⁵ 4s¹
  • Cu: 3d¹⁰ 4s¹
• Successive ionisation energies are also relatively high.

Oxidation States

Transition metals exhibit multiple oxidation states due to ns and (n–1)d electron availability.

• Exhibit a variety of oxidation states, e.g. Mn (+2 to +7).
• Due to close energy levels of 4s and 3d orbitals.
• Early members show more variety; later ones mostly +2 and +3.
• Examples:
  • Mn: +2, +3, +4, +6, +7
  • Fe: +2, +3
  • Cu: +1, +2
  • Zn: +2 only (full d-subshell)

Trends in the M²⁺/M Standard Electrode Potentials

Standard electrode potentials indicate how easily a metal is oxidised to M²⁺ in solution.

• Most 3d elements have negative E° values meaning they can reduce H⁺ ions in solution from acids and liberate H₂ gas.
• Cu has positive E° = +0.34 V meaning it does not liberate H₂ from acid.
• This trend is affected by:
  • Enthalpy of atomisation
  • Ionisation energy
  • Hydration enthalpy

Trends in the M³⁺/M²⁺ Standard Electrode Potentials

This measures the tendency of M²⁺ to be further oxidised to M³⁺.

• E°(M³⁺/M²⁺) increases across the series.
• Fe³⁺/Fe²⁺ < 1, so Fe³⁺ is less stable than Fe²⁺ in aqueous phase.
• Mn³⁺ is unstable in solution, readily converts to Mn²⁺.
• Later elements (like Co³⁺, Ni³⁺) are also less stable.

Trends in Stability of Higher Oxidation States

• Higher oxidation states become more stable in the middle of the series due to increased nuclear charge.
• Maximum oxidation state increases from Sc to Mn:
  • Sc (+3), Ti (+4), V (+5), Cr (+6), Mn (+7)
• Then decreases:
  • Fe (+6), Co (+5), Ni (+4), Cu (+3), Zn (+2)
• Stabilised by ligands such as oxygen and fluorine:
  • e.g. CrO₃, Mn₂O₇

Chemical Reactivity and E° Values

Reactivity of transition elements is linked to their redox potentials and enthalpies.

• Reactivity varies based on:
  • Ionisation enthalpy
  • Enthalpy of hydration
  • Enthalpy of atomisation
• Zn reacts readily with dilute acids (E° = –0.76 V).
• Fe liberates H₂ and rusts in moist air.
• Cu is less reactive and doesn’t react with dilute acids.

Magnetic Properties

Magnetism in transition metals arises from the presence of unpaired d-electrons.

• Transition elements and their ions show paramagnetism, which is caused by unpaired electrons.
• Magnetic moment is calculated using the spin-only formula: μ = √(n(n+2)) BM.
• Across a series, paramagnetism first increases (due to more unpaired electrons) and then decreases (due to pairing).
For Example:
Fe²⁺ (d⁶) has 4 unpaired electrons → μ = √(4×6) = √24 ≈ 4.9 BM.

Formation of Coloured Ions

Transition metal ions are often coloured due to electronic transitions within the d-orbitals.

• The colour is due to d–d transitions, where electrons are promoted from one d-orbital to another of slightly different energy in a crystal field.
• The presence of unpaired electrons and partially filled d-orbitals is essential.
For Example:
• Ti³⁺ (d¹) appears purple due to such transitions.
• Sc³⁺ (d⁰) is colourless as no d–d transition is possible.

Formation of Complex Compounds

Transition elements readily form complexes with various ligands. Note - complexes are covered in more detail in topic 5, see complex ions).

• High charge-to-size ratio and availability of vacant low-energy d-orbitals enable complex formation.
• They can form coordination compounds with ligands like CN⁻, NH₃, H₂O.
For Example:
• [Fe(CN)₆]³⁻
• [Cu(NH₃)₄]²⁺
• These complexes often exhibit characteristic colours and geometries (e.g., octahedral, square planar).

Catalytic Properties

Transition metals are widely used as catalysts in both homogeneous and heterogeneous catalysis.

• Their variable oxidation states enable them to lend or accept electrons during reactions.
• Their surface properties and ability to form intermediates enhance reaction rates.

Example Fe²⁺ catalysing the reaction between I⁻ and S₂O₈²⁻

Reaction: S₂O₈²⁻ + 2I⁻ → 2SO₄²⁻ + I₂. This reaction is slow because both reactants are negatively charged. Fe²⁺ speeds up the reaction by forming an intermediate. Fe²⁺ is regenerated, so it remains a catalyst.

Formation of Interstitial Compounds

Introduction: Transition metals can trap small atoms in their crystal lattice, forming hard, high-melting compounds.

• Atoms like H, B, C, and N occupy interstitial spaces in metal lattices without disturbing the lattice significantly.
• These compounds are non-stoichiometric and retain metallic conductivity.
For Example: TiC, Fe₃H, VN.
• Interstitial compounds are hard and have high melting points.

Alloy Formation

Transition elements can form alloys due to similar atomic sizes and electronic configurations.

• Atoms of different transition elements can replace each other in the lattice, forming solid solutions or substitutional alloys.
For Example: Bronze is an alloy of Copper (Cu) and Tin (Sn):

Summary

• Transition metals show high melting points, conductivity, coloured ions and paramagnetism.
• Variable oxidation states arise from close ns and (n–1)d energies.
• Magnetic moment depends on unpaired d electrons using μ = √(n(n+2)) BM.
• Complex formation, catalysis and alloying are characteristic of d-block chemistry.