Question

All of the following share the same crystal structure except:
A.${\text{RbCl}}$
B.${\text{NaCl}}$
C.${\text{LiCl}}$
D.${\text{KCl}}$

Answer 1

The given question explores the properties of first group elements. To answer this question, you must recall the exceptional properties shown, if any, by a first group element. The smaller size of elements sometimes leads to anomalous behaviour.

Complete step by step solution:
We know that lithium is the first element in the alkali metal series but behaves differently from other members of the group. This anomalous behaviour of lithium is due to its extremely small size.
Since it has a small size, we can conclude that lithium is highly electropositive.
In lithium, the valence electron lies very close to the nucleus and as a result, has high ionisation energy. Thus, lithium has less ionic character than the other alkali metals and it forms covalent bonds.
Also, we know that lithium has polarising power and can distort the electron cloud of the relatively large-sized anions. As a result, lithium forms covalent halides.
${\text{LiCl}}$ is a covalent compound while the rest of the alkali metal halides are ionic in nature.
Also, we know that lithium has a very high charge density due to its small size and has very high hydration energy as a cation. As a result, it easily picks up water molecules from the surroundings and lithium chloride crystallises as a hydrate with 2 molecules of water attached to the molecule.
We can say that lithium chloride is deliquescent.

Thus, the correct option is C.

Note: Apart from its anomalous behaviour, Lithium atom also shows the diagonal relationship with alkaline earth metal magnesium. The electropositivity increases down the group while decreases across a period. Thus, due to a combined effect, magnesium metal has similar electro positivity as that of lithium.