What defines a transition element in the periodic table?

A transition element is defined as a metallic element in the d-block of the periodic table that forms at least one ion with a partially filled d-orbital.

In more detail, transition elements are found in groups 3-12 of the periodic table. They are also known as transition metals and are characterised by their ability to form stable, coloured compounds and to act as catalysts in chemical reactions. The key feature that defines a transition element is its electronic configuration. Specifically, these elements have partially filled d-orbitals either in their ground state or in one of their oxidation states.

The d-block of the periodic table is where the transition elements are located. This block is named after the d-orbital, which is being filled with electrons as you move across the period from left to right. The d-orbital can hold up to 10 electrons, so the transition elements have a variety of oxidation states. This is because they can lose different numbers of d-electrons when they form ions.

Transition elements are unique in their ability to form coloured compounds. This is due to the presence of unpaired d-electrons that can absorb certain wavelengths of light, resulting in the colour we observe. For example, copper sulphate is a bright blue compound, while potassium permanganate is a deep purple.

Furthermore, many transition elements are known for their catalytic properties. They can speed up chemical reactions without being used up in the process. This is again due to their electronic configuration, which allows them to temporarily bond with reactant molecules and lower the energy needed for the reaction to occur.

In summary, a transition element is a metallic element in the d-block of the periodic table, characterised by its electronic configuration, ability to form coloured compounds, and act as a catalyst in chemical reactions.