Question

Which of the following has the highest melting point?
A. $Sr{F_2}$
B. $Be{F_2}$
C. $Ba{F_2}$
 D.$Mg{F_2}$

Answer 1

Hint: The melting point is usually defined as the point at which materials change from a solid to liquid. The melting point of a compound depends on the Vander waals force of attraction which further increases as the molecular size increases.

Complete step by step answer:
The temperature at which solid changes its state to liquid at atmospheric pressure is called the melting point of that liquid. Basically this is the point at which both liquid and solid phases exist at equilibrium.
However, the melting point of a compound depends on Vander waals force of attraction. Among the given options, the Sr atom has the largest size, so the Vander waals force of attraction will be greater in $Sr{F_2}$ molecule and it will have the highest melting point. Its melting point is ${1477^ \circ }C$ . The crystal structure is as shown:
It is a stable, brittle, white crystalline solid with a melting point of ${1477^ \circ }C$ and a boiling point of ${2460^ \circ }C$ . The solid adopts the fluorite structure. In the vapor phase the molecule is non-linear with an $F - Sr - F$ angle of approximately ${120^ \circ }C$ .
Now, most of the solids are packed in a tight crystal structure and have strong intermolecular forces of attraction. When the heat is passed, the internal binding energy of the crystal lattice gets overcome by the heat energy and the intermolecular forces of attraction become weak. Due to this weakening of force, there occurs an instability in the crystal lattice and the molecules start moving in the random direction. Moreover, in case of $Mg{F_2}$ , $Ba{F_2}$ and $Be{F_2}$ , they have weak Vander waals forces of attraction as compared to $Sr{F_2}$ . Their melting points are ${1263^ \circ }C$ , ${1368^ \circ }C$ and ${554^ \circ }C$ respectively.
This instability in the crystal lattice initiates the melting of a substance.

Hence, option A is correct.

Note:
In the majority of cases, the presence of a foreign substance will lower the melting point of a pure organic compound. This fact is utilized in the so-called mixed melting point test for the identification of the organic compounds.