[tamubsa]

I am having trouble understanding how to tell if one molecule has a lower/higher boiling/melting point than another. I know it has to do with intermolecular forces, but consider two molecules with the same kind of intermolecular forces. I had one instructor tell me it had to do with molecular mass (as the mass increases, so does melting/boiling points). This is consistent with the example of HCl and HBr. However, I had another instructor tell me it had to do with the charges on the ions and the distance between them (which is counter intuitive with the first example). This is consistent with the example RbBr and RbCl. Am I missing something? Thank you in advance for your *delete me*

[Borek]

To make things even more confusing, they were both right. However, the first one described covalent molecules, the second ionic ones.

Interactions between ions in ionic compounds are typically orders of magnitude stronger than intermolecular interactions between molecules of covalent compounds.