Why do simple molecular compounds have low melting and boiling points?

Simple molecular compounds have low melting and boiling points primarily because the intermolecular forces acting between the molecules are very weak. In these compounds, the individual molecules are held together by covalent bonds, which are strong, but the attractions between the molecules themselves—known as intermolecular forces—are much weaker.

These weak intermolecular forces require less energy to overcome, resulting in lower melting and boiling points. When heat is applied, it takes relatively little energy to disrupt these forces leading to a change in state from solid to liquid or from liquid to gas.

While the size of the molecules can influence physical properties, and some compounds may be non-polar, the key factor in determining the melting and boiling points in the context of simple molecular compounds is the strength of the intermolecular forces. The weak interactions allow these substances to exist as gases or liquids at relatively low temperatures.