Lanthanoid contraction is a concept in chemistry that explains the gradual decrease in the size of atoms and ions of elements in the lanthanide series. This series includes elements from lanthanum (La) to lutetium (Lu). As we move from one element to the next in this series, the atomic number increases, but the size of the atoms slowly becomes smaller instead of larger.
This decrease in size happens mainly because of the increase in nuclear charge. As more protons are added to the nucleus, the positive charge increases. At the same time, electrons are added to the inner 4f orbitals, which do not shield the outer electrons effectively. Because of this poor shielding, the outer electrons feel a stronger attraction towards the nucleus.
As a result, the electrons are pulled closer to the nucleus, and the overall size of the atom or ion decreases. This effect continues gradually across the entire lanthanide series.
Lanthanoid contraction has important effects in chemistry. It explains why elements after the lanthanides have similar sizes and properties. For example, elements like zirconium (Zr) and hafnium (Hf) have almost the same atomic radius even though they belong to different periods.
Lanthanoid contraction means “a gradual shrinking in size of lanthanide elements due to increasing nuclear pull and weak shielding effect.”