Why do molecular compounds have low melting and boiling points?

Molecular compounds have low melting and boiling points due to the weak intermolecular forces holding their molecules together.

Molecular compounds, also known as covalent compounds, are formed when atoms share electrons to achieve a stable electron configuration. This sharing of electrons results in the formation of covalent bonds, which are strong and require a significant amount of energy to break. However, the forces of attraction between these molecules, known as intermolecular forces, are relatively weak.

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 molecular compounds. Dipole-dipole interactions occur in polar molecules, and hydrogen bonding, the strongest of the three, occurs in molecules containing a hydrogen atom bonded to a highly electronegative atom (such as nitrogen, oxygen, or fluorine).

The melting and boiling points of a substance are determined by the amount of energy required to overcome the forces holding its particles together. In the case of molecular compounds, the energy required to overcome the weak intermolecular forces is relatively low. Therefore, these compounds have low melting and boiling points.

For example, water, a molecular compound, has a relatively low boiling point of 100 degrees Celsius. This is because the energy required to break the hydrogen bonds between water molecules is relatively low. Similarly, carbon dioxide, another molecular compound, sublimates (changes directly from a solid to a gas) at -78.5 degrees Celsius due to the weak intermolecular forces between its molecules.

In contrast, ionic compounds, which are held together by strong electrostatic forces between positively and negatively charged ions, have high melting and boiling points. This is because a large amount of energy is required to overcome these strong forces.

In summary, the low melting and boiling points of molecular compounds can be attributed to the weak intermolecular forces holding their molecules together.