sodium and chlorine have a large difference in electronegativity (the ability of an element to pull on electrons), resulting in a big difference in electron sharing, producing a molecule with a large “dipole moment” or polarity. The polarity means that the components (ions, in this case) are REALLY attracted to each other, and are so attracted that they stick together (in this case, in a crystal lattice). It takes a LOT of energy to get them to loosen this lattice and melt. All that energy = high temperature.
hydrogen and chlorine don’t have such a large difference in electronegativity, therefore
