Question

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

Answer 1

Hint: Fluorine is the most electronegative element and it has the smallest size among the halides, which makes the bond stronger and difficult to break.

Complete step by step answer:
> In order to answer our question, we need to know about the halides, as well as the group 17 elements and get to know some of their properties. Now, they have the general configuration of $[inert\,gas]n{{s}^{2}}n{{p}^{5}}$. 

Some properties of the halogens are:
- Atomic and Ionic Radii: In a respective period, halogens have the smallest atomic radii due to maximum effective nuclear charge. Down the group from F to I, atomic and ionic radii increases.
- Ionisation Enthalpy: They have high ionization enthalpy, thus have little tendency to lose electrons. Down the group ionization enthalpy decreases due to increase in size.
- Electron Gain Enthalpy: Halogens have maximum negative in corresponding period, as they are just short of one electron from stable noble gas configuration. Down the group enthalpy becomes less negative, but -ve enthalpy of F is less than that of chlorine due to its small size and high interelectronic repulsion in compact 2p orbital of fluorine, the incoming electron does not experience much attraction.
- Electronegativity: Halogens have high electronegativity. Fluorine is the most electronegative element in the periodic table. Down the group electronegativity decreases.

> We know that Fluorine has the smallest size and is the most electronegative element. That means that it has the strongest bonding with the metal Na. So a lot of heat will be required to melt it, and hence, NaF has the maximum melting point. 

Hence, the correct option “D” .

Note: In the group 17 elements, along with fluorine, chlorine, bromine and iodine, another element known as Astatine is present, and it is a radioactive element.