The transition elements have incompletely filled d-subshells in their ground state or in any of their oxidation states. The transition elements occupy position in between s- and p-blocks in groups 3-12 of the Periodic table. Starting from fourth period, transition elements consists of four complete series : Sc to Zn, Y to Cd and La, Hf to Hg and Ac, Rf to Cn. In general, the electronic configuration of outer orbitals of these elements is (n - 1) d^1-10 ns^1-2. The electronic configurations of outer orbitals of Zn, Cd, Hg and Cn are represented by the general formula (n - 1)d¹⁰ ns². All the transition elements have typical metallic properties such as high tensile strength, ductility, malleability. Except mercury, which is liquid at room temperature, other transition elements have typical metallic structures. The transition metals and their compounds also exhibit catalytic property and paramagnetic behaviour. Transition metal also forms alloys. An alloy is a blend of metals prepared by mixing the components. Alloys may be homogeneous solid solutions in which the atoms of one metal are distributed randomly among the atoms of the other.

Which of the following characteristics of transition metals is associated with higher catalytic activity?

Variable oxidation states

The correct answer is option 2. Variable oxidation states.

The characteristic of transition metals that is most associated with higher catalytic activity is variable oxidation states.

Variable Oxidation States and Catalytic Activity:

Ability to Form Complexes: Transition metals can adopt various oxidation states, which enables them to form a wide range of complexes with different ligands, including reactants in a catalytic process. For instance, a transition metal can coordinate with a reactant molecule, stabilizing it and lowering the activation energy required for the reaction to proceed.

Facilitation of Redox Reactions: Many catalytic processes involve redox reactions, where electrons are transferred from one reactant to another. Transition metals can easily switch between different oxidation states, facilitating these electron transfers. This flexibility makes them effective in processes such as the Haber process for ammonia synthesis (iron catalyst) and the catalytic converters in automobiles (platinum, palladium, and rhodium catalysts).

Stabilization of Reaction Intermediates: During a catalytic reaction, intermediate species are often formed. Transition metals, due to their variable oxidation states, can stabilize these intermediates by temporarily binding to them. This stabilization helps lower the overall energy barrier of the reaction, making it proceed faster.

Example: Catalytic Hydrogenation: In catalytic hydrogenation, transition metals like platinum, palladium, and nickel are used to catalyze the addition of hydrogen to alkenes. These metals can adsorb hydrogen and the alkene onto their surface, facilitating the breaking and forming of bonds by switching between different oxidation states during the process.

Why Other Characteristics Are Less Important for Catalytic Activity:

High Enthalpy of Atomization: While a high enthalpy of atomisation indicates strong metallic bonding, which can correlate with the stability of the catalyst at high temperatures, it is not the primary reason for catalytic activity. It does not directly influence the mechanism by which the catalyst facilitates the reaction.

Paramagnetic Behaviour: Paramagnetism arises from unpaired electrons in the d-orbitals of transition metals. While this is a notable property, it mainly affects magnetic behavior rather than catalytic activity. The presence of unpaired electrons can influence the electronic structure and reactivity, but it is the ability to change oxidation states that is more critical for catalysis.

Colour of Hydrated Ions: The color of hydrated transition metal ions is due to d-d electronic transitions influenced by the ligand field. This property is useful for analytical purposes and identifying complexes but does not directly affect catalytic activity.

Conclusion: The primary reason for the high catalytic activity of transition metals lies in their ability to exhibit variable oxidation states. This property allows them to engage in a variety of redox reactions, form complexes with reactants, stabilize intermediates, and provide multiple pathways for reactions to proceed efficiently. This flexibility is what makes transition metals so effective as catalysts in numerous industrial and biochemical processes.