GCSE / O Levels Chemistry

A molecule of tetrafluorosilane, SiF4, is shown in the dot-and-cross diagram. Only the outer shell
electrons are shown.
XX
XX
XX
FX Si
F
●X
XX
X
F
X●
F
XX
Which statement is correct?
A Each molecule of SiF4 has exactly 16 pairs of electrons.
B
In SiF4 both the silicon and the fluorine have the same electronic configuration as neon.
Molten SiF4 will conduct electricity.
C
D
SiF4 has a low melting point.

Choice D is correct because: 

SiF4 is a covalent compound with a molecular structure. 

Such compounds typically have low melting points due to the relatively weak intermolecular forces (specifically, London dispersion forces) acting between their molecules. 

In SiF4, these forces are not strong, meaning less thermal energy is required to change its state, hence its low melting point.

Choice A is incorrect because each molecule of SiF4 does not have 16 pairs of electrons. It has 5 pairs of electrons in total, 4 shared (bonding) pairs and 1 lone pair on the silicon atom.

Choice B is incorrect because in SiF4, the silicon atom attains the electronic configuration of the noble gas neon through sharing of electrons, but the fluorine atoms do not. Fluorine atoms gain an electron to achieve the electronic configuration of the noble gas neon, but in SiF4 they share electrons with silicon, not gain.

Choice C is incorrect because molten SiF4 will not conduct electricity. Covalent compounds like SiF4 do not have free-moving ions in any state, and it is these ions that allow a substance to conduct electricity.

Covalent Bonding | 2022 - M/J - 5070/12 | Q10