Chemistry – Atoms, elements and compounds - Simple molecules and covalent bonds | e-Consult

The melting and boiling points of simple molecular compounds are typically low due to the presence of weak intermolecular forces. These compounds are composed of individual molecules held together by relatively weak attractions. Unlike ionic compounds or metals, where strong electrostatic forces are present, the forces between molecules in molecular compounds are significantly weaker.

These weak intermolecular forces arise from the relatively small size and simple structure of the molecules. The primary types of intermolecular forces present are:

• London Dispersion Forces (LDF): These forces are present in all molecules, even nonpolar ones. They arise from temporary, instantaneous fluctuations in electron distribution, creating temporary dipoles. LDFs increase with molar mass and surface area.
• Dipole-Dipole Forces: These forces occur between polar molecules, which have a permanent separation of charge. The positive end of one molecule is attracted to the negative end of another.
• Hydrogen Bonding: This is a special type of dipole-dipole force that occurs when hydrogen is bonded to highly electronegative atoms like oxygen, nitrogen, or fluorine. Hydrogen bonding is particularly strong.

Because these intermolecular forces are weak, only a small amount of energy is required to overcome them and change the state of the substance from solid to liquid (melting) or liquid to gas (boiling). Therefore, simple molecular compounds have low melting and boiling points.