25.1 Introduction to Transition Elements

Transition and d-block elements

• A transition element is one which forms one or more stable ions with incompletely filled orbitals

• A d-block element is one which has electrons filling the d-orbitals.

• From the above definition, scandium and zinc are not counted as transition elements, although they are d-block elements. This is because:
  1. scandium forms Sc³⁺ ion, which does not have any d-electrons.
  2. zinc forms Zn²⁺ ion, which has a completely-filled d-orbitals.

Physical properties of transition elements

• 1. Transition elements are all metals which are hard and denser than s-block elements.
  2. This is because the relative atomic masses of transition elements are higher and their radii are smaller compared to s-block elements. Hence, there are more mass per unit volume and the density is higher
  3. For example, iron has a density of 9 g cm⁻³ while calcium has a density of 1.6 g cm⁻³.
• 1. Transition elements have higher melting points than s-block elements.
  2. This is because the metallic bond of transition elements is stronger as both 3d and 4s electrons are involved in the In calcium, for example, only 3s electrons are involved.
  3. This is possible because 3d and 4s and very similar energy levels
  4. For example, iron has a melting point of 1538 °C while calcium has a melting point of 839 °C.

Characteristic properties of transition elements

• The characteristic properties of transition elements are:
  1. they have variable oxidation states
  2. they form coloured ions
  3. they form complex ions which are surrounded by ligands
  4. they often act as catalysts