Lanthanoid contraction, also known as lanthanide contraction, refers to the steady decrease in the size of atoms or ions of the lanthanide series elements as you move across the periodic table from left to right. The lanthanide series consists of the 15 elements with atomic numbers 57 to 71, from lanthanum (La) to lutetium (Lu).
The lanthanoid contraction is primarily attributed to the poor shielding effect of 4f electrons. In the lanthanide series, electrons are being added to the 4f orbital, which is generally shielded poorly by the other electron shells. As a result, there is an increase in effective nuclear charge—meaning that the outer electrons experience a stronger pull from the nucleus.
The consequences of the lanthanoid contraction include:
Decrease in Atomic Size: As you move across the lanthanide series, the atomic radii of the elements steadily decrease due to the increased nuclear charge and poor shielding.
Similar Chemical Properties: Despite the decrease in size, the lanthanides often exhibit similar chemical properties. This is because the 4f electrons are largely localized and do not participate significantly in chemical bonding.
Density Increase: The lanthanoid contraction contributes to an increase in density across the lanthanide series. The increased nuclear charge pulls the electrons more closely to the nucleus, resulting in a denser arrangement of atoms.
Understanding the lanthanoid contraction is important in various fields, including chemistry and materials science, as it influences the behavior and properties of lanthanide elements and their compounds. The transition elements following the lanthanides, the actinides, also exhibit a similar contraction known as the actinoid contraction.