Lanthanoid contraction, also called lanthanide contraction, in chemistry, the steady decrease in the size of the atoms and ions of the rare earth elements with increasing atomic number from lanthanum (atomic number 57) through lutetium (atomic number 71). For each consecutive atom the nuclear charge is more positive by one unit, accompanied by a corresponding increase in the number of electrons present in the 4f orbitals surrounding the nucleus. 

Lanthanoid contraction, also called lanthanide contraction, in chemistry, the steady decrease in the size of the atoms and ions of the rare earth elements with increasing atomic number from lanthanum (atomic number 57) through lutetium (atomic number 71).

Lanthanoid Contraction

Lanthanoid contraction refers to the steady decrease in the size of lanthanide elements as the atomic number increases. This phenomenon occurs due to the poor shielding of the 4f electrons, which leads to an increase in effective nuclear charge as more electrons are added to the inner 4f subshell. The decrease in atomic size has significant implications for the chemical and physical properties of these elements.

Causes and Explanation

Lanthanoid contraction is primarily caused by the shielding effect of the 4f electrons. The 4f orbitals are relatively poor at shielding the outer electrons from the increasing nuclear charge, resulting in a stronger attraction between the nucleus and the outer electrons. As a result, the atomic radius decreases across the lanthanide series.

The 4f orbitals are characterized by their spherical shape and are located closer to the nucleus than the 5d and 6s orbitals. Therefore, the increase in nuclear charge as the atomic number increases has a greater impact on the size of the lanthanide atoms compared to elements in other series.

Consequences

The lanthanoid contraction has several important consequences that affect the properties of the lanthanide elements:

1. Trends in Atomic Radii: The lanthanide elements exhibit a steady decrease in atomic radii across the series. This contraction is responsible for the compact nature of lanthanide elements.

2. Chemical Reactivity: The decrease in atomic size due to the lanthanoid contraction affects the chemical reactivity of the elements. It results in increased ionization energies and decreased metallic radii, leading to the formation of more stable and less reactive compounds.

3. Coordination Chemistry: The lanthanoid contraction plays a crucial role in the coordination chemistry of lanthanide elements. It affects the size of the metal ion, which determines the coordination number and geometry in complex compounds.

4. Rare Earth Extraction: The lanthanoid contraction also affects the separation and extraction of rare earth elements. Since the atomic radii of the lanthanide elements are similar, it becomes challenging to separate them based solely on size differences.

Conclusion

Lanthanoid contraction is a significant phenomenon observed in the lanthanide series, which leads to a decrease in atomic size as the atomic number increases. This contraction is primarily caused by the poor shielding of the 4f electrons. The consequences of the lanthanoid contraction include trends in atomic radii, changes in chemical reactivity, impacts on coordination chemistry, and challenges in rare earth element extraction. Understanding the lanthanoid contraction is crucial for studying the properties and applications of the lanthanide elements.