Question

Suppose you have a crystal composed of elements X and Y in which the radius ratio is approximately ½, so that the sodium chloride lattice is possible. Each ion is about the same size as Na+ and Cl-, respectively, but each ion is doubly charged X++ and Y--. Would XY (s) have a higher or lower melting point than NaCl (s)?

Answer 1

XY would have a higher melting point than NaCl.

Step-by-step explanation:

The electrostatic force (F) between ions in an ionic lattice can be estimated using Coulomb's Law.

`F=k.(q_(1).q_(2))/(r^(2) )`

where,

k is the Coulomb's constant

q₁ and q₂ are the charges of the ions

r is the distance between the ions

X⁺⁺ and Y⁻⁻ are doubly charged with respect to Na⁺ and Cl⁻ so the force between them is 4 times the force between Na⁺ and Cl⁻. Since the force is stronger, it would require more energy to break it to take it to the liquid state, so a higher melting point is expected.