CHEM要做Essay,有問題唔識,明天要交

carbon tetrachloride CCl4, has ctructure like this:
　　　Ｃｌ
　　　｜
Ｃｌ－Ｃ－Ｃｌ
　　　｜
　　　Ｃｌ　
carbon is bonded to 4 chlorine atoms by 4 single covalent bonds. it's simple, discrete molecule, held together by relatively weak van de waals force.

for a substance to melt, particles inside a solid must overcome the attrction between particles, which held them together regularly as solid. after melting, particles can flow freely.
for ionic compounds, very strong electrostatic attraction between positive charges and negative charges has to be overcome for ions to flow freely. a lot of energy is required, thus melting point is high.
for simple molecular structures, molecules are held by weak van der waals force (may also contain dipole-dipole attraction, but CCl4 doesn't has one). attraction is weak, so less energy is needed for molecules to flow freely. melting point is lower.

electrical conducitivity is about the presence of free-flowing ions. as long as there're free-flowing ions, it can conduct electricity.
for solid ionic compounds, ions are inmobile. it cannot conduct electricity. however, when melted, ions can freely flow around, thus can conduct electricity.
for simple molecular compounds, no matter in solid or liquid form, there're NO ions (they're just uncharged molecules). it CANNOT conduct electricity at any state.
... unless some compounds may dissociate into ions when dissolved in water, e.g. hydrogen chloride (HCl), forming hydrochloric acid (hydrogin ion and chloride ions freely flowing in water. HOWEVER, carbon tetrachloride does not dissolve in water and does not form ions.