Question

The Lanthanide contraction relates to:
A. Oxidation states
B. Magnetic state
C. Atomic radius
D. Valence electrons

Answer 1

Hint : In the periodic table, on moving from left to right (across a period), the ionic radii decrease. This occurrence is due to the additional electrons being added to the same orbital along the period. An increase in the nuclear charge is seen. In lanthanides, the additional electrons enter the $4f$ orbital and shielding effect by $4f$ electrons is seen. This results in the outer electrons being attracted more by the nucleus.

Complete Step By Step Answer:
The f-block elements consist of elements with atomic numbers $57$ to $71$ and $89$ to $103,$ the two series called Lanthanoids and Actinoids respectively. The f block elements are called the inner transition elements. This is due to their placement in the periodic table in accordance with electronic configurations. These elements are called f-block elements as the valence electrons enter the antepenultimate energy shell, or the $\left( {n - 2} \right)f$ orbital. The general electronic configuration of these elements is:
$\left( {n - 2} \right){f^{(0 - 14)}}\left( {n - 1} \right){d^{(0 - 1)}}n{s^2}$
In Lanthanides, a phenomenon named lanthanide contraction is observed. Lanthanide contraction is the steady decrease in atomic size of the elements in the lanthanide series. This results in an atomic radii which is much lesser than the expected value for atomic radii of these elements. The reason for this phenomenon to occur is due to the poor shielding effect of $4f$ electrons. The poor shielding lets the outer electrons experience more attractive force from the nucleus, and they get closer to the nucleus. Consequently, the atomic radii decreases from La to Lu. Therefore, The Lanthanide contraction relates to C. Atomic radius.

Note :
The periodic table is a tabular recording of the chemical elements discovered. The arrangement is in the order of increasing atomic number, which directly implies that the arrangement is in accordance with electronic configuration. Also, recurring chemical properties are seen. There are certain 'trends' which are seen in the periodic table. The $18$ groups and $7$ periods, composed of $118$ chemical elements, make the periodic table. The periodic table is composed of four blocks, namely 's', 'p', 'd' and 'f'.