What is meant by the 'lanthanide contraction'? Mention one important fact that can be consisdered as a consequence of the lanthanide contraction.

Answer

Lanthanide contraction phenomenon is associated with the intervention of the `4f` orbitals which must ne filled before the `5d` series of element begin. The filling of `4d` before `5d` orbital results in a regular decrease in atomic radii called Lanthanoid contraction. This essentially compensates for the excepted increase in atomic size with increasing atomic number . The net result of the lanthanoid contraction is that the second and the third d-series exhibits similar radii and ahve very similar physical and chemical properties .