Ionic compounds melt and boil at high temperatures

1. The ions in an ionic solid sit in a giant lattice, each surrounded by ions of the opposite charge.
2. Every one of those attractions is strong, and there are very many of them.
3. Melting takes enough energy to loosen the lattice so that the ions can move past one another.
4. Boiling takes enough energy to separate the ions from one another completely.
5. Sodium chloride melts at $801{}^{\circ }C$ and boils at $1413{}^{\circ }C$.
6. Magnesium oxide melts at $2852{}^{\circ }C$, because ${Mg}^{2+}$ and $O^{2-}$ attract far more strongly than singly charged ions.

Ionic compounds conduct only when their ions can move

1. In the solid, each ion is fixed in its lattice position and cannot travel.
2. No moving charge means no current, even though every particle present is charged.
3. Melting frees the ions, so a molten ionic compound conducts.
4. Dissolving in water separates the ions, so an aqueous solution conducts.
5. The charge is carried by the ions themselves, which is what separates this from conduction in a metal.

Simple molecular substances melt and boil at low temperatures

1. Each molecule is held together internally by strong covalent bonds.
2. Neighbouring molecules attract one another only weakly.
3. Melting and boiling separate whole molecules, so only those weak forces have to be overcome.
4. The covalent bonds inside the molecules are untouched, which is why the gas is still the same substance.
5. Methane boils at $-161{}^{\circ }C$ and water at $100{}^{\circ }C$, both far below any ionic compound.
6. Larger molecules attract one another more strongly, so their melting and boiling points are higher.

Simple molecular substances are poor conductors

1. A molecule carries no overall charge, so it cannot carry current from one place to another.
2. The electrons in each covalent bond are held between two particular atoms and cannot travel through the substance.
3. There are no ions to move either, in the solid or the liquid.
4. Simple molecular substances are poor conductors in every state as a result.
5. A few of them, such as hydrogen chloride, form ions on dissolving, and those solutions do conduct.

The two classes side by side

1. Both classes contain strong bonds, but only one of them has to break those bonds to melt.
2. An ionic compound melts high because ionic bonds run through the whole lattice.
3. A simple molecular substance melts low because only the forces between molecules give way.
4. An ionic compound conducts once its ions can move, while a simple molecular one has no mobile charge at all.
5. Naming the particle that moves, or the force that is overcome, is what turns a property into an explanation.