Question

Alums are not formed by which alkali metal?
A) ${\text{Li}}$
B) ${\text{K}}$
C) ${\text{Na}}$
D) ${\text{Cs}}$

Answer 1

To solve this we must know that alums are a class of double sulphates of aluminium. Alums are generally hydrates of double sulphate salts of aluminium and contain a metal cation. The size of monovalent cation attached to aluminium should be comparable.

Complete solution:
We know that alums are a class of double sulphates of aluminium. Alums are generally hydrates of double sulphate salts of aluminium and contain a metal cation.
The chemical formula for alum is ${\text{X}} \cdot {\text{A}}{{\text{l}}_{\text{2}}}{\left( {{\text{S}}{{\text{O}}_{\text{4}}}} \right)_{\text{3}}} \cdot {\text{12}}{{\text{H}}_{\text{2}}}{\text{O}}$ where ${\text{X}}$ is a monovalent cation. The size of the monovalent cation i.e. ${\text{X}}$ should be such that it occupies the lattice positions in alum lattice. The alums can be easily formed if the monovalent cation or the alkali metal cation i.e. ${\text{X}}$ is big in size. Too small sized monovalent cations cannot form alum.
We know that lithium i.e. ${\text{Li}}$ is very small in size. Thus, it cannot occupy the lattice positions in alum lattice. The lattice positions in alum lattice are too big for lithium cation. Thus, the lithium alum does not exist.
Other alkali metals like potassium $\left( {\text{K}} \right)$, sodium $\left( {{\text{Na}}} \right)$ and caesium $\left( {{\text{Cs}}} \right)$ have significant sizes and can occupy the lattice positions in alum lattice.
Thus, alums are not formed by the alkali metal ${\text{Li}}$.

Thus, the correct option is (A) ${\text{Li}}$.

Note: The most common and important alums contain potassium and it is known as potassium alum. Potassium alum has many applications. It is used in dyeing, water purification, textiles and leather tanning. Sometimes alums are also formed by trivalent metal cations like chromium.