Small molecules exhibit low melting and boiling points primarily because they are held together by weak intermolecular forces, which require less energy to break compared to the strong covalent bonds within the molecules themselves.

These small molecules, commonly found in gases or simple molecular structures, possess low melting and boiling points due to the nature of the forces that bind them. The forces in question, known as intermolecular forces, are relatively weak when compared to the robust covalent bonds that exist within the molecules.

The melting and boiling points of a substance are determined by the energy needed to overcome the forces that hold its particles together. For small molecules, these are the weak intermolecular forces; during melting or boiling, it is these forces that are being overcome, not the strong covalent bonds. Consequently, less energy is required to change the state of a substance made up of small molecules, leading to their characteristic low melting and boiling points.

There are three primary types of intermolecular forces:

1. London dispersion forces, which are the weakest and present in all molecules.
2. Dipole-dipole interactions, which are stronger and occur in polar molecules.
3. Hydrogen bonding, which is a particularly strong form of dipole-dipole interaction that occurs when hydrogen is bonded to highly electronegative elements like nitrogen, oxygen, or fluorine.

Despite the presence of dipole-dipole interactions and hydrogen bonding, even the strongest of these intermolecular forces are significantly weaker than covalent bonds.

In many small molecules, the predominant intermolecular forces are often just London dispersion forces, which are notably weak. This explains why substances such as helium, composed of very small molecules, have exceptionally low melting and boiling points. Even when stronger intermolecular forces, such as dipole-dipole interactions or hydrogen bonding, are present, they remain weak in comparison to covalent bonds, allowing small molecules to maintain generally low melting and boiling points.

In summary, the low melting and boiling points of small molecules can be attributed to the weak intermolecular forces that hold them together. These forces require considerably less energy to overcome than the strong covalent bonds within the molecules, resulting in their lower melting and boiling points.