Question

Fourteen elements following Lanthanum are called Lanthanoids:
(a) What is Lanthanoid contraction? Give reason for it.
(b) $KMn{O}_4$ is a purple coloured crystal and it acts as an oxidant. How will you prepare $KMn{O}_4$ from $Mn{O}_2$?

Answer 1

Lanthanide Contraction: The steady decrease in the size of lanthanide ions (Tri positive metal ion) with the increase in atomic no. is called lanthanide contraction.

Reason: As we move from left to right in a lanthanide series, the atomic number increases and for every proton in the nucleus the new electron goes to 4f orbital. The 4f orbital is too diffused to protect the nucleus effectively, thus there is a gradual increase in the effective nuclear charge experienced by the outer electrons. Consequently, the attraction of the nucleus for the electrons in the outermost shell increases with the increase of atomic number, so the size decreases.

b) Potassium permanganate ($KMn{O}_4$) is prepared from pyrolusite ore ($Mn{O}_2$). The finely powdered $Mn{O}_2$ is fused with an alkali metal hydroxide like KOH in the presence of air to give the potassium Manganate (K2MnO4)

$Mn{O}_2+4KOH+O_2\to 2K_{2}Mn{O}_4+2H_{2}O$