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Question

The melting point of AlF3 is 1291C and that SiF4 is 77C (it sublimes ) because :

A
there is a very large difference in the ionic character of the AlF and SiF bonds
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B
in AlF3, Al3+ interacts very strongly with the neighbouring F ions to give a three dimensional structure but in SiF4 no such interaction is possible
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C
the silicon ion in the tetrahedralSiF4 is not shielded effectively from the fluoride ions whereas inAlF3, the Al3+ ion is shielded on all sides
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D
the attractive forces between the SiF4 molecules are strong whereas those between the AlF3 molecules are weak
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Solution

The correct option is B in AlF3, Al3+ interacts very strongly with the neighbouring F ions to give a three dimensional structure but in SiF4 no such interaction is possible
Aluminum fluoride is ionic and has high melting point. Large aluminum cation has vacant d orbitals. It attains a coordination number of six towards the relatively small fluorine atom. Due to this, a three dimensional structure is obtained. This is not possible in silicon tetrafluoride. Hence, the melting point of silicon tetra fluoride is much lower than the melting point of aluminum trifluoride.

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