Properties of Small Molecules

Instructor: Amanda Robb
This lesson will help you understand the chemical properties of small molecules. We'll go over the typical state of matter of small molecules, as well as the intermolecular forces present and the reason for a lack of electric charge.

What Are Small Molecules?

Bigger is always better, right? Well, not when it comes to molecules. Many important molecules for living things are actually quite small. Small molecules are low molecular weight molecules that are only a few atoms large. Take water for example. Only three atoms make up this molecule, yet it is essential for life. Water is even one of the characteristics scientists look for when searching for the possibility of life on other planets!

But, the importance of small molecules doesn't stop there. Oxygen gas is a diatomic molecule that is crucial for our cells to make energy. Without oxygen, our cells can't do cellular respiration to make energy, and we would die. Additional small molecules include ammonia, methane, monosaccharides, lipids and more.

The properties of these small molecules provide them with unique characteristics that make them so important to living things. Today, we're going to look at some of those properties in detail.

Intermolecular Forces

Small molecules have incredibly strong intramolecular forces. These are the covalent bonds that hold them together. These bonds are very strong and do not easily break. They are not the bonds that break when a molecule changes state. The forces involved in state changes are called intermolecular forces and include any forces involved in interactions between molecules. The intermolecular forces of small molecules include dipole-dipole interactions, London dispersion forces, and hydrogen bonds.

Dipole-dipole interactions occur between polar molecules. Polar molecules are molecules that have a partial charge on their atoms due to an uneven distribution of electrons in the covalent bond. This happens when a very electronegative atom, such as oxygen or nitrogen, is bound to less electronegative atoms, such as hydrogen. The more electronegative atom pulls the electrons closer to it, giving it a partial negative charge. The other atoms get a partial positive charge. These partial charges can interact in an ionic way between different molecules.

London dispersion forces occur in non-polar molecules. Even though these molecules do not possess permanent dipoles like polar molecules do, there are still momentary fluctuations in the electron distribution. This creates temporary dipoles that can be used to generate intermolecular forces.

Hydrogen bonding is a special type of dipole-dipole interaction that involves a dipole moment on a hydrogen atom. Any time more electronegative atoms are bound to hydrogen, they create partial positive charges on the hydrogen atom. This can then be used to form weak intermolecular interactions called hydrogen bonds. Hydrogen bonds are very important for biological molecules such as DNA and protein.

Hydrogen bonds occur between water molecules
hydrogen bonding

States of Matter

Although these intermolecular bonds are important, they are relatively weak. The bonds are easily broken and reformed. This is why small molecules are often found in gaseous or liquid states of matter. To understand this, let's review the states of matter. Solids are states of matter with highly organized molecules with low levels of movement and tend to have strong intermolecular forces. As heat is applied these intermolecular interactions are broken and the molecules move more freely, forming liquids and gases.

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