Silicon dioxide (SiO2\text{SiO}_2SiO2), iodine (I2\text{I}_2I2), and poly(ethene) all contain covalent bonds.

Explain which substance has the highest melting point. Use your knowledge of structure and bonding to support your answer.
Sodium chloride is an ionic compound. Explain why ionic compounds can conduct electricity when dissolved in water, but not when solid.
Construct the dot and cross diagram for the ions in sodium oxide, Na2O\text{Na}_2\text{O}Na2O. Show the outer electron shells only.