Answer:
AlN has highest melting point among given.
Explanation:
As per ionic model, ionic compounds are held together by electrostatic force. The ionic compound having strong electrostatic force will have high melting point.
Electrostatic force is proportional to charge and size of constituent ions.
[tex]F=k \frac{q_+\times q_-}{r^2}[/tex]
In case of MgO
Charge on Mg = +2
Charge on O = -2
[tex]F=-k \frac{(+2)\times (--2)}{r^2}=\frac{4k}{r^2}[/tex]
In case of LiF
Charge on Li = +1
Charge on F = -1
[tex]F=-k \frac{(+1)\times (-1)}{r^2}=\frac{1k}{r^2}[/tex]
In case of AlN
Charge on Al = +3
Charge on N = -3
[tex]F=-k \frac{(+3)\times (-3)}{r^2}=\frac{9k}{r^2}[/tex]
In case of RbF
Charge on Rb = +1
Charge on F = -1
[tex]F=-k \frac{(+1)\times (-1)}{r^2}=\frac{1k}{r^2}[/tex]
In case of CaS
Charge on Ca = +2
Charge on S = -2
[tex]F=-k \frac{(+2)\times (-2)}{r^2}=\frac{4k}{r^2}[/tex]
As magnitude of electrostatic force is highest in AlN, therefor, it has highest melting point.
