Question

Which has the higher melting point, $$NaCl$$ or $$AlC{l}_3$$ ?

Answer 1

Hint: The type of present in between the atoms is considered to predict the physical properties of the compounds. Generally ionic compounds exhibit higher melting points than covalent compounds.

Complete step by step answer: The given compounds are sodium chloride ($$NaCl$$) and aluminium chloride ($$AlC{l}_3$$ ). The anion counterpart is the same for both the compounds but the cations are different. In $$NaCl$$ the cation is sodium ion $$N{a}^{+}$$ and in $$AlC{l}_3$$ the cation is $$A{l}^{3+}$$ .
Sodium is an element in the periodic table with atomic number $$11$$ and electronic configuration$$\left[Ne\right]3s^1$$. It belongs to alkali earth metals. Sodium is very electropositive in nature and the tendency to lose proton makes it very reactive.
Aluminium is an element in the periodic table with atomic number $$13$$ and electron configuration$$\left[Ne\right]3s^{2}3p^1$$ . Thus the aluminium ion $$A{l}^{3+}$$ is smaller in size compared to sodium as the electrons are pulled more tightly by the nucleus due to higher charge (or more number of protons than electrons).
Thus for a fixed anion $$C{l}^{-}$$ ion due to bigger size of $$N{a}^{+}$$ ion than $$A{l}^{3+}$$ ion, $$N{a}^{+}$$ ion form ionic bond with $$C{l}^{-}$$ ion and an ionic compound is formed. On the other hand the $$A{l}^{3+}$$ ion forms a covalent compound with $$C{l}^{-}$$ ion and the bonds contain shared electrons. As the electrons are shared among the bonded atoms, these compounds exhibit lower melting points and electrical conductivity compared to ionic compounds.
Hence $$NaCl$$ (an ionic compound) has a higher melting point than $$AlC{l}_3$$ (A covalent compound).

Note:
The tendency to form ionic or covalent compounds depends on the electronegativity difference between the bonded atoms. If the difference is more the bond is ionic but if the electronegativity are similar then the bond is covalent.