Definition and Properties of Transition Metals

The d-block contains elements whose highest energy electrons occupy d sub-shells. However, not all d-block elements are classified as transition metals. In this lesson, we will establish the precise definition of a transition metal, analyse electron configurations, and outline their characteristic chemical properties.

Electron Configurations of the d-Block Elements

When writing the electron configurations of transition metals from Period 4 (Titanium to Copper), we fill orbitals in order of increasing energy. The 4s orbital is filled before the 3d orbital because the 4s is at a lower energy level in neutral atoms. We can use the noble gas core shorthand [Ar] to represent the inner 18 electrons (1s²2s²2p⁶3s²3p⁶).

Why Scandium and Zinc are NOT Transition Metals

Scandium and zinc sit at the edges of the d-block, and neither meets the formal definition of a transition metal:

• Scandium (Sc): Scandium only forms one stable ion, Sc³⁺. To form this ion, the scandium atom ([Ar]3d¹4s²) loses all three outer electrons, resulting in the electron configuration [Ar] or [Ne]3s²3p⁶3d⁰. Because its only stable ion has an empty d sub-shell (3d⁰), scandium is not a transition metal.
• Zinc (Zn): Zinc only forms one stable ion, Zn²⁺. The zinc atom ([Ar]3d¹⁰4s²) loses its two 4s electrons to form Zn²⁺ = [Ar]3d¹⁰. Because its only stable ion has a completely filled d sub-shell (3d¹⁰), zinc is not a transition metal.

General Properties of Transition Metals

Transition metals exhibit four characteristic physical and chemical properties that distinguish them from s-block metals like calcium:

1. Variable Oxidation States

Unlike Group 2 metals which only form +2 ions, transition metals form stable ions in a variety of oxidation states. For example, manganese can exist in oxidation states ranging from +2 in Mn²⁺ to +7 in the manganate(VII) ion, MnO₄⁻.

This is because the energy levels of the 3d and 4s orbitals are very close. The energy required to remove successive electrons is relatively small and gradual, allowing varying numbers of d electrons to participate in bonding without causing chemical instability.

2. Formation of Coloured Ions

Solutions containing transition metal ions are often intensely coloured. In contrast, solutions of s-block compounds (e.g. NaCl, CaCl₂) and zinc compounds are colourless.

Measuring Concentration with a Colorimeter

The absorption of visible light is used in spectroscopy. A simple colorimeter can be used to determine the concentration of coloured ions in solution: the more concentrated a coloured solution, the more light it absorbs.

1. Choose a filter of the colour that the solution absorbs most: for example, a red filter for blue copper(II) solutions, or a green filter for purple manganate(VII) solutions.
2. Zero the colorimeter with a blank of distilled water.
3. Measure the absorbance of standard solutions of known concentration.
4. Plot a calibration curve of absorbance against concentration.
5. Measure the absorbance of the unknown solution and read its concentration off the calibration curve.

3. Catalytic Activity

Transition metals and their compounds are extremely important industrial catalysts. They can act as heterogeneous or homogeneous catalysts:

• Heterogeneous catalysts: The catalyst is in a different physical phase from the reactants. Transition metals have partially vacant d orbitals that allow reactant molecules to adsorb onto their surface, weakening their covalent bonds and lowering the activation energy. Examples include:
  • Iron (Fe): Catalyses the production of ammonia from nitrogen and hydrogen in the Haber process.
  • Nickel (Ni): Catalyses the hydrogenation of vegetable oils to make margarine.
  • Vanadium(V) oxide (V₂O₅): Catalyses the oxidation of sulfur dioxide to sulfur trioxide in the Contact process.
  • Manganese(IV) oxide (MnO₂): A solid that catalyses the decomposition of aqueous hydrogen peroxide (H₂O₂).
• Homogeneous catalysts: The catalyst is in the same phase as the reactants. Transition metals can act as homogeneous catalysts because they readily switch between oxidation states to provide alternative reaction pathways. Examples include:
  • Iron(II) ions (Fe²⁺): Catalyse S₂O₈²⁻ + 2I⁻ → 2SO₄²⁻ + I₂. This is slow without a catalyst because the two negative ions repel. Each catalysed step is between oppositely charged ions, and Fe²⁺ is regenerated:
    • S₂O₈²⁻ + 2Fe²⁺ → 2SO₄²⁻ + 2Fe³⁺
    • 2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂
  • Manganese(II) ions (Mn²⁺): Autocatalyse the reaction between MnO₄⁻ and C₂O₄²⁻. Mn²⁺ is a product, so the reaction speeds up as it forms:
    • 4Mn²⁺ + MnO₄⁻ + 8H⁺ → 5Mn³⁺ + 4H₂O
    • 2Mn³⁺ + C₂O₄²⁻ → 2Mn²⁺ + 2CO₂

4. Complex Ion Formation

Transition metal ions have a high charge density and empty, low-lying d orbitals. This makes them highly attractive to ligands (molecules or ions with lone pairs of electrons). The ligands donate their lone pairs into the empty d orbitals of the transition metal ion, forming coordinate bonds and creating complex ions.