Crystals of covalent compounds always have

Covalent compounds have low melting points because

In the solid state, $PCl_{5}$ exists as

If the value of ionic radius ratio $\left( \frac{r_c}{r_a} \right)$ is $0.52$ in an ionic compound,the geometrical arrangement of ions in the crystal is:

Suggest the most important type of intermolecular attractive interaction in the following pairs.
$(i)$ $n$-hexane and $n$-octane
$(ii)$ $I_{2}$ and $CCl_{4}$
$(iii)$ $NaClO_{4}$ and water
$(iv)$ methanol and acetone
$(v)$ acetonitrile $(CH_{3}CN)$ and acetone $(C_{3}H_{6}O)$

Dry ice is an example of