Question

The hardest lanthanide element is:
(A) $Sm$
(B) $La$
(C) $Gd$
(D) $Dy$
(E) $Yb$

Answer 1

In this question, we are finding the hardest lanthanide element. Initially, lanthanide or lanthanide series of chemical elements comprises the ${\text{15}}$ metallic chemical elements with atomic number ${\text{57 - 71}}$, from lanthanum through lutetium.

Complete step by step answer:
Hardest lanthanide element is lutetium $\left( {{}_{71}Lu} \right)$ but from among the given options gadolinium $\left( {Gd} \right)$ is the hardest lanthanide. The hardness of a few lanthanide elements on Vickers scale is given below. Vickers test is used for all the metals and it is one of the widest scales among hardness tests. It is used to measure the hardness of the materials.

Lanthanide/lanthanides hardness is:
${}_{57}La = 491,\;{}_{62}Sm = 412,\;{}_{64}Gd = 570,{}_{66}Dy = 540,\;{}_{70}Yb = 206$
Lanthanides are involved in the filling of ${\text{4f}}$ orbitals. The bonding energy of ${\text{4f}}$ electrons is comparatively less than that of ${\text{5f}}$- electrons. Actinides are involved in the filling of ${\text{5f}}$-orbitals. The shielding effect of ${\text{5f}}$-electrons is less effective as compared to that of ${\text{4f}}$- electrons.

Lanthanides are less reactive because of their small size compared to actinides. Let us consider an example, lanthanides show less tendency towards complex formation and less tendency towards the formation of oxides and hydroxides.

The lanthanides and actinides together are sometimes called the inner transition elements. The color of lanthanide ions is due to the presence of partly filled f-orbitals. As a result, it is possible to absorb certain wavelengths from the visible regions of the spectrum.

Hence, option C is correct.

Note: The elements in the lanthanide series undergo steady decrease in the size of the atoms and ions of the rare earth elements with increasing atomic number from lanthanum and this is known as lanthanide contraction.