Lanthanide contraction: 1) Definition of Lanthanoid Contraction

1) Definition of Lanthanoid Contraction : The Lanthanoid contraction may be defined as gradual (or small) the decrease in atomic and ionic radii of lanthanoids with the increase in atomic number. The lanthanide contraction is the decrease in atomic and ionic radii of the lanthanide elements due to poor shielding of nuclear charge by the 4f electrons. It affects the chemistry and properties of the lanthanides and the following transition metals. Lanthanoid contraction refers to the gradual decrease in the ionic radii of the lanthanide series elements (from lanthanum to lutetium) as we move from left to right across the series. This contraction occurs due to the ineffective shielding of the nuclear charge by the f-electrons. Lanthanide contraction is the phenomenon where the size of the lanthanide ions decreases moving left to right across the periodic table, even though atomic number increases.