What is lanthanide contraction?

Lanthanide contraction is a term that is used to refer to the continuous decline of the atoms’ radii for elements in a series known as the Lanthanide series. The series starts from the element with 57 as its atomic number to the one with 71.

These elements are lanthanum and lutetium respectively. The elements, also referred to as block elements, have an extra electron, which enters an orbital called (n-2)f. The trend of the declining ionic radii in the lanthanide causes the elements with an atomic number that is greater than 71 to have ionic radii that are smaller than expected.