Explain:(i)Transition elements mostly form the complex compounds. Why?(ii)Transition elements are a good catalyst. Why?(iii)Transition elements show variable valency. Why?

(i) Transition elements mostly form complex compounds because of their unique electronic structure. Transition metals have partially filled d orbitals in their atomic or ionic states, which allows them to exhibit a variety of oxidation states. This characteristic makes them highly versatile in bonding with other atoms or ions, including ligands, to form complex compounds. The formation of complex compounds is driven by the desire of transition metals to maximize their coordination number (the number of ligands surrounding the metal atom or ion) and achieve greater stability.

Transition metals can form coordination complexes by accepting electron pairs from surrounding ligands, which can be neutral molecules or anions. These ligands can donate electron pairs to the transition metal's vacant d orbitals, forming coordinate covalent bonds. This ability to form multiple bonds and coordinate with various ligands leads to the formation of complex structures with a wide range of geometries and properties, making transition metals essential in various fields such as catalysis, medicine, and materials science.

(ii) Transition elements are good catalysts for several reasons. One of the primary factors is their ability to change oxidation states easily due to the availability of multiple valence electrons in their d orbitals. This property allows them to participate in redox reactions, accepting and donating electrons during a chemical reaction without being consumed in the process. The key reasons why transition elements make good catalysts include:

a. Variable oxidation states: Transition metals can exist in multiple oxidation states, which allows them to act as both oxidizing and reducing agents during a reaction. This flexibility is crucial for catalyzing reactions where electron transfer is involved.

b. Surface area: Transition metals often have high surface areas when used as catalysts, which provides more active sites for reactants to adsorb and undergo chemical transformations.

c. Coordination chemistry: Transition metals can form coordination complexes with reactants or intermediates, stabilizing them and lowering the activation energy for a reaction.

d. Tunable reactivity: The choice of transition metal and ligands can be tailored to specific reactions, making it possible to fine-tune the catalytic activity and selectivity.

e. Catalytic cycles: Transition metals can participate in catalytic cycles, where they are regenerated after each reaction step, ensuring their continuous involvement in the reaction.

Overall, the combination of these factors makes transition elements effective catalysts for a wide range of chemical reactions.

(iii) Transition elements show variable valency because of the ability of their electrons to occupy different energy levels and orbitals. This variability in valency is mainly attributed to the presence of partially filled d orbitals in their electron configuration. Here's why transition elements exhibit variable valency:

a. Electrons in d orbitals: Transition metals have electrons in their d orbitals, and these electrons can be easily lost or gained to achieve different oxidation states. The availability of multiple d orbitals with varying energies allows for the transfer of electrons between different orbitals, resulting in a range of possible oxidation states.

b. Transition metal ions: Transition elements often form ions with different charges (oxidation states) by losing or gaining electrons. The number of electrons lost or gained determines the oxidation state. For example, iron (Fe) can exist in the +2 or +3 oxidation state, depending on whether it loses two or three electrons from its neutral state.

c. Redox chemistry: The ability to easily switch between different oxidation states makes transition elements versatile in redox reactions, where electrons are transferred between reactants. This variability in valency is essential for their involvement in catalysis and the formation of complex compounds.

In summary, the variable valency of transition elements is a consequence of their electron configuration, specifically the presence of partially filled d orbitals, which allows them to readily change oxidation states and participate in a wide range of chemical reactions.