Question

The stability of the following alkali metal chlorides follows the order:
A.$LiCl > KCl > NaCl > CsCl$
B.$CsCl > KCl > NaCl > LiCl$
C.$NaCl > KCl > LiCl > CsCl$
D.$LiCl > NaCl > KCl > CsCl$

Answer 1

In case of alkali metals, as we go down the group, size of the cations decreases resulting in decrease of attractive force between cations and anions. It affects stability of the alkali metal chlorides.

Complete step-by-step solution:
- Alkali metal chlorides are compounds made of alkali cations and halide anions. Their stability depends directly on the lattice energy of the compound. Lattice energy is the amount of energy released when an ionic solid is formed from an ionic compound. It is an exothermic process having negative enthalpy.
- Lattice energy is inversely proportional to the size of alkali metals and thus decreases down the group. This can explain the stability of alkali chlorides. As we go down the group in alkali metal series, the size of cation increases and thereby lattice energy decreases as we move from lithium to caesium because the force of attraction decreases in higher alkali metal ions.
- Therefore, the stability of alkyl halides or alkyl chlorides decreases down the group from lithium to caesium. But here is an exception that potassium chloride is more stable than sodium chloride because the atomic size of potassium and chlorine is almost similar and results in strong bonding between them. Due to this, potassium chloride is more stable.

Hence, the correct option is (A). $LiCl > KCl > NaCl > CsCl$

Note: We can also consider electronegativity differences to find the stability order of alkyl chlorides. Those with higher electronegativity differences are unstable such as CsCl and those with lower electronegativity differences are more stable such as LiCl.