Question

Which of the following fluorides have the lowest melting point?
A. $Ba{F_2}$
B. $Sr{F_2}$
C. $Ca{F_2}$
D. $Be{F_2}$

Answer 1

Notice the elements given in the options. They all are elements of the second row in the periodic table. Fluorides are of metals with chemical formula ${F^ - }$ . You need to think about the forces that will act on elements going down the group for example Van der Waals force.

Complete step by step answer:
To find out about the lowest melting point of compounds given in the options we need to have some knowledge about them. So, before discussing the given compounds let’s see what fluorides are and what melting point is.

Melting point is the temperature at which any compound changes state from solid to liquid. At the melting point, the solid and liquid state exist in equilibrium.

Fluorides is an inorganic, monatomic anion with chemical formula. They are white and colourless. They are an important compound which are used in the production of hydrogen fluoride for fluorocarbons. They are weak bases but concentrated fluorides are corrosive and can affect skin. They are minerals found in your bones and teeth and are found naturally in water, soil, plants, rocks and air. This mineral is used by dentists to strengthen enamel which is the outer layer of teeth. They help in preventing cavities. They are also present in toothpaste, mouth rinses and supplements.

A. $Ba{F_2}$ or Barium Fluoride is a colourless inorganic solid that occurs in nature as a rare mineral frankdicksonite. The melting point of Barium fluoride is $1,{368^ \circ }C$ .
B. $Sr{F_2}$ or Strontium fluoride also called strontium difluoride is a stable brittle white solid with a melting point of $1,{477^ \circ }C$ . It is prepared by reaction of strontium chloride with fluorine gas or by action of hydrofluoric acid with strontium carbonate.
C. $Ca{F_2}$ commonly called calcium fluoride is a white insoluble solid. It occurs as mineral fluorite or fluorspar which is deeply coloured owing to impurities. Its melting point is $1,{418^ \circ }C$ .
D. $Be{F_2}$ called Beryllium fluoride is a white compound which is the main component for the manufacture of beryllium metal. It is soluble in water. Its melting point is ${554^ \circ }C$ .

You can notice here that all the elements are in the same row. With Beryllium on the top and Barium at bottom, we can observe that melting point decreases down the group. This is because of Van der Waals forces as size of molecules increases and therefore the force increases. This increases the strength of molecules indeed increasing the melting point.

**Therefore, the answer to our question is option D.

Note: **
Different types of forces act on atoms. These forces affect the nature and properties of the atoms. To have knowledge about all these forces is important and also the reason for every property of atoms in the periodic table.