Why do covalent compounds have low melting and boiling points?

Covalent compounds have low melting and boiling points due to their weak intermolecular forces.

Covalent compounds, also known as molecular compounds, are formed when two non-metals combine and share electrons, creating a covalent bond. The strength of a covalent bond is determined by the overlap of the atomic orbitals and the sharing of the electron pair. However, the forces between these molecules, known as intermolecular forces, are relatively weak compared to the forces within the molecules, i.e., the covalent bonds.

The melting and boiling points of a substance are determined by the strength of the forces between its particles. In the case of covalent compounds, these forces are the weak intermolecular forces, not the strong covalent bonds. When a covalent compound is heated, it is these weak intermolecular forces that are overcome to convert the substance from a solid or liquid to a gas, not the covalent bonds. Therefore, less energy (in the form of heat) is required to break these forces, resulting in lower melting and boiling points.

There are three types of intermolecular forces: London dispersion forces, dipole-dipole interactions, and hydrogen bonding. London dispersion forces are the weakest and are present in all covalent compounds. Dipole-dipole interactions occur in polar covalent compounds, and hydrogen bonding, the strongest of the intermolecular forces, occurs in compounds containing a hydrogen atom bonded to a highly electronegative atom (nitrogen, oxygen, or fluorine). Even though hydrogen bonding is the strongest intermolecular force, it is still much weaker than a covalent bond.

In summary, the low melting and boiling points of covalent compounds can be attributed to the weak intermolecular forces that hold the molecules together. These forces are much weaker than the covalent bonds within the molecules, and thus less energy is required to overcome them, leading to lower melting and boiling points.