Question

Which of the following has the highest melting point?
A. $NaCl$
B. $NaF$
C. $NaBr$
D. $NaI$

Answer 1

Melting point is a temperature at which different forms like solid and liquid forms of a pure substance can exist in equilibrium. When solid is treated with heat, its temperature increases till the melting point is reached.

Complete answer:
Different groups exhibit trends in boiling points. For groups 1 and 2, the boiling and melting points decrease as you move down in group. For the transition metals, boiling and melting points generally increase as we move from top to bottom, but they decrease for the zinc family.
Since lattice energy is directly proportional to the charge present on cation and anion and inversely proportional to the ion radius ( $F < Cl < Br < I$ ). Since lattice energy is highest for $NaF$ hence melting point will also be high.

Hence, option B is correct.

Note:
The melting and boiling points increase down the group because of the van der waals forces. On going down the group the size of molecules increases. The increase in size meant an increase in the strength of the van der waals forces. The strength of the different intermolecular forces is:
Ionic bond > Hydrogen bonding > dipole dipole > van der waals dispersion forces. The influence of attractive forces depends upon the functional groups present. With the increase in the number of carbons, the boiling point also increases. As intermolecular forces become strong in liquid, the more heat it takes and higher the boiling point. All molecules possess a weak intermolecular attraction that is known as London dispersion force. The London dispersion force is a temporary attractive force which acts when the electrons in two adjacent atoms take positions due to which atoms form temporary dipoles. The element present in periodic table tungsten possess the highest boiling point.