[AhBeng]

How can we explain why the melting points of carbon tetrachloride and silicon tetrachloride do not follow the trend in melting points of group 4 compounds down the group?

Down the group the m.p is expected to increase as an increase in size causes a strengthening of the Van der Waal's forces.

However m.p of CCl4 = -23oC and that of SiCl4 = -70oC while the latter has to be higher than the former!

Can someone please explain this puzzling phenomenon?

[Mitch]

Melting points are a tricky beast, it often has more to do with the lattice structure than a superficial general chemistry argument. Is this trend seen in the boiling points?

[AhBeng]

Melting points are a tricky beast, it often has more to do with the lattice structure than a superficial general chemistry argument. Is this trend seen in the boiling points?

Thanks for your reply, Mitch.

Yes, both m.p. and b.p. is higher for CCl4 as compared to SiCl4.

CCl4
Melting point -22.92 °C
Boiling point 76.72 °C

SiCl4
Melting point −68.74 °C
Boiling point 57.65 °C
 
( Source :
http://www.answers.com/topic/carbon-tetrachloride
http://www.answers.com/topic/silicon-tetrachloride )