Silicon (Si\text{Si}Si), phosphorus (P4\text{P}_4P4), and poly(propene) all contain covalent bonds.

Explain which substance has the highest melting point. Use your knowledge of structure and bonding to support your answer.
Magnesium chloride is an ionic compound. Explain why ionic compounds can conduct electricity when molten or dissolved in water, but not when solid.
Construct the dot and cross diagram for the ions in sodium sulfide, Na2S\text{Na}_2\text{S}Na2S. Show the outer electron shells only.