Schwarzschild Radius here.
So,
The real questions is "Which compound has the lowest boiling point?" The answer would be the compound with the lowest intermolecular attractions.
Typically, the hierarchy of intermolecular attractions is as follows:
Ionic >> polar covalent > nonpolar covalent
Ionic compounds typically have very high boiling points, because melting them involves inputting enough energy to rip apart the positive and negative ions. Sodium chloride, for example, has a boiling point of 1413°C. So potassium bromide would probably have the highest boiling point.
Polar covalent compounds typically have boiling points closer to room temperature. The most familiar polar covalent compound is water, with a boiling point of 100°C. Ammonia is actually similar to water in that its primary intermolecular attraction is hydrogen bonding.
Nonpolar covalent compounds typically have the lowest boiling points. Small alkanes/alkenes/alkynes have very low boiling points. Their main intermolecular attraction is London dispersion forces, which are directly proportional to molecular mass, and thus are very weak in molecules such as methane, ethane/ethene, propane/propene, etc. Thus, we can expect ethane to have the lowest boiling point.
Hope this helps!
