Question

What is meant by lanthanide contraction? The actinoid contraction is greater than that of lanthanoid contraction. Why?

Answer 1

We need to first know what are lanthanoids and actinoids and study their contractions. As the number of electrons in one atom increases, unexpected behaviors emerge. Because of the way electron energy levels function, certain inner levels fill after one or more outer levels. This may be seen in two families of elements: lanthanides and actinides. Lanthanides and actinides both have empty $f$ orbitals.

Complete Step By Step Answer:
Lanthanide contraction is the gradual decrease in the atomic and ionic sizes of lanthanide elements as their atomic number increases. This is a little reduction with some anomalies. From $\left( {165{\text{ }}pm} \right)$ to $Lu{\text{ }}\left( {156{\text{ }}pm} \right)$ , the atomic radii drops
From $C{e^{3 + }}{\text{ }}\left( {103{\text{ }}pm} \right)$ to $L{u^{3 + }}{\text{ }}\left( {85{\text{ }}pm} \right)$ , ionic radii ( $+ 3$ oxidation state) decreases. The following are the effects of lanthanoid contraction:
( $1$ ) From $Ce{\left( {OH} \right)_3}$ to $\;Lu{\left( {OH} \right)_3}$ , basicity decreases.
( $2$ ) From $Ce{\left( {OH} \right)_3}$ to $Lu{\left( {OH} \right)_3}$ , the covalent nature increases.
( $3$ ) The electronegativity of the ${M^{3 + }}$ ion increases.
( $4$ ) ${E^0}$ values are slightly higher.
$5f$ orbitals are filled in actinoids. These $5f$ orbitals provide less shielding than $4f$ orbitals (in lanthanide). Thus, in the case of actinides, the effective nuclear charge experienced by electrons in valence shells is significantly greater than that experienced by lanthanides. As a result, the size contraction in actinoids is higher than that in lanthanide.

Note:
Lanthanides have long been employed as alloying elements to increase the strength and hardness of metals. Cerium is the primary lanthanide utilized for this purpose, along with trace quantities of lanthanum, neodymium, and praseodymium. These metals are also frequently utilized in the petroleum sector to refine crude oil into gasoline. The actinides are important largely due to their radioactivity. These elements can be utilized as energy sources for a variety of purposes, including cardiac pacemakers and the creation of electrical energy for lunar equipment. Uranium and plutonium have been used in nuclear weapons and nuclear power reactors, respectively.