The melting point of `AlF_3` is `104^@` C and that of `SiF_4` si `–77^@C` (it sublimes) because :

Options

A. there is very large difference in the ionic character of the Al – F and Si – F bonds
B. in `AlF_3 , Al3^+` interacts very strongly with the neighbouring `F^–` ions to give a three dimensional structure but in `SiF_4` no such interaction is a possible
C. the silicon ion in the tetrahedral `SiF_4` is not shielded effectively from the fluoride ions whereas in `AlF_3` , the `Al^(3+)` ion is shielded on all sides
D. The attractive forces between the `SiF_4` molecules are strong whereas those between the `AlF_3` molecules are weak

Answer

B