2.2.4 Properties of Small Molecules

In this lesson, we will explore the characteristics of substances that consist of small molecules, including their physical states, melting and boiling points, intermolecular forces, and electrical conductivity.

Physical States and Melting/Boiling Points

Substances that consist of small molecules typically exist as gases or liquids at room temperature. They have relatively low melting and boiling points compared to ionic compounds or giant covalent structures. This is because the forces between the molecules, known as intermolecular forces, are relatively weak. When heat is applied, it is these intermolecular forces that are overcome, not the strong covalent bonds within the molecules themselves.

Intermolecular Forces

Intermolecular forces are the attractive forces between molecules. They arise from the temporary fluctuations in electron distribution, resulting in the formation of temporary dipoles or induced dipoles. The strength of these intermolecular forces increases with the size and shape of the molecules. Substances with larger molecules have stronger intermolecular forces and, as a result, higher melting and boiling points.

Electrical Conductivity

Substances consisting of small molecules do not conduct electricity. This is because the molecules themselves do not have an overall electric charge. While covalent bonds within the molecules involve the sharing of electrons, the lack of charged particles that can move freely within the substance prevents the flow of electric current.

Explaining Bulk Properties

The weak intermolecular forces compared to the strong covalent bonds within the molecules explain the bulk properties of molecular substances. The weak intermolecular forces allow the molecules to move relatively freely, resulting in substances that are gases or liquids at room temperature. The relatively low melting and boiling points are a consequence of the ease with which these weak forces can be overcome by adding or removing heat.

Conclusion

Substances consisting of small molecules exhibit distinct properties. They typically exist as gases or liquids with low melting and boiling points due to weak intermolecular forces. The intermolecular forces increase with the size of the molecules, leading to higher melting and boiling points for larger molecules. These substances do not conduct electricity because the molecules lack an overall electric charge.