In the presence of ligands, the d-orbitals of transition metal ions split up into two sets of orbitals having different energies. Thus, the transition of electrons takes place from one set to another. The energy required for these transitions is quite less and falls in the visible region of radiation. The ions of transition metals absorb the radiation of a particular wavelength and the rest is reflected, imparting colour to the solution.

Elements that possess incompletely filled d-orbitals either in their ground state or in any of their oxidation states are known as transition elements. The name transition element given to the elements of d-block is only because of their position between s-block and p-block elements.

Characteristics of transition elements are as follows:

1. They show paramagnetic behaviour.

2. They show variable oxidation states.

3. They exhibit catalytic properties.

4. They generally form complex compounds.

5. They generally form coloured compounds.

The complete chemical equation is given below.

Cu is the only metal in the first transition series (3d series) which shows +1 oxidation state most frequently. This is because the electronic configuration of Cu is 3d¹⁰ 4s¹ and after losing one electron it acquires a stable 3d¹⁰ configuration.

In actinoids, 5 f-orbitals are filled. These 5 f-orbitals have a poorer shielding effect than 4 f-orbitals (in lanthanoids). Thus, the effective nuclear charge experienced by electrons in valence shells in case of actinoids is much more than that experienced by electrons in valence shells in case of lanthanoids. Hence, the size contraction in actinoids is greater than that in lanthanoids.