Why do transition metals exhibit variable oxidation states?

Transition metals exhibit variable oxidation states due to the presence of unpaired electrons in their d-orbitals.

Transition metals are elements found in the d-block of the periodic table, which includes groups 3 to 12. They are unique in their ability to form compounds with different oxidation states. This is primarily due to the presence of unpaired electrons in their d-orbitals.

In transition metals, the 4s and 3d sub-levels are very close in energy. When these metals form ions, they can lose electrons from both the 4s and 3d sub-levels. This flexibility allows them to exhibit a variety of oxidation states. For example, iron (Fe) can exist in both +2 and +3 oxidation states, depending on whether it loses two or three electrons.

The variable oxidation states of transition metals also contribute to their ability to form complex ions and coloured compounds. The different oxidation states can result in different electron configurations, which can absorb different wavelengths of light, leading to the colour of the compounds.

Furthermore, the variable oxidation states of transition metals make them excellent catalysts. Catalysts work by providing an alternative reaction pathway with a lower activation energy. The ability of transition metals to change their oxidation state allows them to form temporary bonds with reactant molecules, lowering the activation energy of the reaction.

In summary, the variable oxidation states of transition metals are a result of the presence of unpaired electrons in their d-orbitals. This characteristic allows them to form a variety of compounds, contribute to the colour of compounds, and act as effective catalysts.