Question

What is meant by Lanthanide contraction $?$ What is the general configuration of inner transition elements $?$

Answer 1

Hint : Lanthanides and actinides are f-block transition elements that are placed separately in the periodic table. They fill up the $4f$ and $5f$ orbitals respectively. There are $15$ elements in each series. Lanthanides are elements from Lanthanum(La) to Lutetium(Lu) and actinides are elements from Actinium(Ac) to Lawrencium(Lr).

Complete Step By Step Answer:
Lanthanides (Ln) are f block transition metals also known as rare earth elements. There are $15$ lanthanide elements that start from Lanthanum (La) – atomic number $57$ to Lutetium (Lu) – atomic number $71$ . As we go from La to Lu, we can observe a decrease in the atomic radii. The reason behind this shrinkage along the series is the poor shielding of the f orbital. From left to right, the atomic number is increasing, so is the nuclear charge. Due to poor screening and increasing nuclear charge, the hold of the nucleus on the outermost electrons of the f orbital becomes strong and thus the nucleus pulls the orbital towards it. As a result, the size of atoms decreases. This phenomenon of decrease in the atomic radii is called Lanthanide contraction.
Lanthanides and actinides are the inner transition metals, also known as f block elements. The general electronic configuration of inner transition elements are:
Lanthanides : $[Xe]4{f^{1 - 14}}5{d^{0 - 1}}6{s^2}$
Actinides : $[Rn]5{f^{1 - 14}}6{d^{0 - 1}}7{s^2}$

Note :
Due to lanthanide contraction, as we go along the series, the size of the metal atom decreases. Therefore, the electronegativity and ionisation energy of the metal increases since they are inversely proportional to the size of the atom. In addition to it, the basicity decreases from La o Lu. This is because, as the size of the tripositive metal ion decreases in the series, the covalent character increases. Thus, $La{(OH)_3}$ is more basic than $Lu{(OH)_3}$ .