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. Lanthanide contraction is the greater-than-expected decrease in ionic radii of the elements in the lanthanide series from atomic number 57, lanthanum, to atomic number 71, lutetium, resulting in smaller-than-expected ionic radii for the subsequent elements beginning with 72, hafnium. 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. 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.
