Define Lanthanide contraction.

Hint: There are a total 14 elements in the lanthanide series. In the lanthanide series as the atomic number of elements increases, the element's atomic radius decreases.

Complete Step by Step Solution:
Lanthanides are treated as the rare earth elements present in the modern periodic table. The rare earth metals are those whose atomic number ranges from 58 to 71 followed by lanthanum elements.

There are total 14 elements in lanthanide series which includes cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium.

Lanthanoid contraction also termed lanthanide contraction takes place due to a steady decrease in the atomic size or the ionic size of lanthanide ions having +3 valency from lanthanum (La) to lutetium (Lu). This happens because the electrons enter the inner (n-2)f orbitals and increase the nuclear charge.

Therefore, the gradual decrease in the size along with an increase in an atomic number of elements is known as lanthanide contraction.

Additional information: The 4f electrons protect each other with the increase in positive charge of the nucleus which results in an increase in effective nuclear charge attracting electrons as the elements in lanthanide series progress resulting in a decrease in atomic radii.

Note: There are some consequences of lanthanide contraction as the atomic size of the element of the third transition series is approximately the same as the element of the second transition series. Also, there is difficulty in separating the lanthanides as there is minimal change in the ionic radii of the elements in the lanthanides series so their property is similar.