Question: The melting point of \[Al{{F}_{3}}\] is \[{{1291}^{o}}C\] and that of \[Si{{F}_{4}}\] is\[-{{77}^{o}...

Question

The melting point of $Al{{F}_{3}}$ is ${{1291}^{o}}C$ and that of $Si{{F}_{4}}$ is $-{{77}^{o}}C$ (it sublimes) because:
A) There is a very large difference in the ionic character of the Al-F and Si-F bonds.
B) In $Al{{F}_{3}}$ , $A{{l}^{3+}}$ interaction is possible
C) The silicon ion in the tetrahedral $Si{{F}_{4}}$ is not shielded effectively from the fluoride ions whereas in $Al{{F}_{3}}$ the $A{{l}^{3+}}$ ion is shielded on all sides
D) The attractive forces between the $Si{{F}_{4}}$ molecules are strong whereas those between the $Al{{F}_{3}}$ molecules are weak.

Answer 1

Solution

Hint The melting point of the compound depends mainly on the force of attraction present between the molecules of the component. When the force of attraction between the molecules is high the melting point and the boiling point both get increased. In an ionic compound we have a large array of positive and negative ions which are held by strong electrostatic force of attraction.

Complete solution:
The $Al{{F}_{3}}$ structure is the ionic structure which is formed by the arrangement of positive ions called cation and negative ions called anions. The ions are held by coulombic forces so they are hard, brittle and strong. The components melt when required energy is transferred to them. The energy overcomes the strong force of attraction and melts the compound. The strength of the ionic bond depends directly on the charge of ions. If the charge is high of the ion then the strength of the ionic bond is high. The $Si{{F}_{4}}$ molecule is a covalent molecule formed by sharing of electrons between the atoms called covalent bonding. The melting points of covalent compounds are less due to the force present between the particles and that weak force is called Vander Waal force of attraction.

Hence, the correct option is (A).

Note: In solid state, the ions of the compound are able to move freely due to strong force of attraction. This is the reason due to which they do not conduct electricity. But when the same compound is in molten or aqueous state it can conduct electricity as the ions are free to move.