Question

Rank the following ionic compounds in order of decreasing melting point. Note: 1 = highest melting point ; 5 = lowest melting point

LiF

Na2S

MgS

BeO

Li2O

Answer 1

Order of decreasing melting point

1. BeO

2. MgS

3. Li₂O

4. Na₂S

5. LiF

Ionic radius and charge on the ions affect the melting point of an ionic compound.

Smaller the ionic radius greater will be the attraction between ions and hence higher melting point.

Greater the charge on the ions greater will be the force of attraction between the ions and higher will be the melting point.

In BeO, Be²⁺ and O^2- has larger charge and ionic radius of Be²⁺ is smaller compared to Mg²⁺. But O^2- has smaller ionic radius than S^2-

In LiF, though the ionic radii are small, since the charges are +1 on Li and -1 on F, it has lowest melting point amongst the given compounds.