Intermolecular force

Intermolecular forces are the forces that act between molecules. These forces include attraction or repulsion between atoms and other nearby particles. They are weaker than the forces that hold a molecule together, like covalent bonds. But they are still very important for studying molecules.

The idea of tiny forces was first talked about in a book by Alexis Clairaut. Many scientists have helped study these forces.

There are several types of attractive intermolecular forces, such as hydrogen bonding and Van der Waals forces. We can learn about these forces by measuring things like how thick a liquid is (viscosity). These measurements help us understand how molecules behave. Intermolecular forces are important in areas like biochemistry and molecular biology, because they affect how enzymes and catalysts work with other molecules.

Hydrogen bonding

Main article: Hydrogen bond

A hydrogen bond is a pull between a hydrogen atom and a nearby atom like nitrogen, oxygen, or fluorine. This bond is strong for such a small force. It helps explain why water stays liquid at normal temperatures. Hydrogen bonds also help shape important molecules in our bodies, like proteins and DNA.

Salt bridge

Main article: Salt bridge (protein and supramolecular)

Salt bridges are a type of force between very small particles that have opposite charges, like positive and negative ions. This force is not very strong and is called an intermolecular interaction. In water, these forces help particles stick together. Salt bridges are different from other forces because they don’t point in one direction and depend on the size of the particles.

Dipole–dipole and similar interactions

Dipole–dipole interactions are forces between molecules that have small positive and negative areas. These forces are stronger than some other forces between molecules but weaker than forces between charged particles. They help molecules line up and attract each other more. An example is hydrogen chloride (HCl): the small positive part of one molecule pulls toward the small negative part of another molecule.

Some molecules have parts with charges but balance out, so they don’t seem to have a charge. This happens in symmetric molecules like tetrachloromethane and carbon dioxide. The Keesom interaction is one type of force between molecules and is part of a group called van der Waals forces.

Ion–dipole and ion–induced dipole forces

Ion–dipole and ion–induced dipole forces are interactions between ions—particles with full positive or negative charges—and molecules. These forces are stronger than dipole–dipole interactions because ions have a bigger charge than the small charges in polar molecules.

When an ion meets a polar molecule, they line up so the positive and negative parts are close, creating attraction. An example is when ions dissolve in water: the water molecules surround the ions, which helps keep them stable. An ion–induced dipole force happens when an ion changes the shape of a non-polar molecule’s electron cloud, causing attraction.

Van der Waals forces

Main article: van der Waals force

Van der Waals forces are weak forces between atoms or molecules that are not charged. These forces help things stick together, like why water droplets form or why two pieces of glass can stick when they are very close. There are three main types of these forces.

The first type is called the Keesom force. This happens between molecules that have a tiny magnet-like property, called a dipole. These dipoles attract each other, but this only works between certain molecules.

The second type is the Debye force. This happens when a molecule with a dipole makes a dipole in a nearby molecule, causing them to attract each other.

The third and most common type is the London dispersion force. This happens because all atoms and molecules have tiny, random changes in their electron clouds that create temporary dipoles. These temporary dipoles cause attraction between all kinds of atoms and molecules. This force is important because it works for all materials.

Relative strength of forces

This comparison is only a rough guide. The actual strength can change depending on the molecules involved. For example, water can affect the strength of certain bonds. In general, bonds that hold atoms together inside a molecule, like ionic bonding and covalent bonding, are stronger than the forces between molecules. However, in some cases, like when enzyme molecules work with substrate molecules, the forces between molecules can influence the bonds inside molecules. This helps enzymatic reactions happen, which are very important for living organisms.

Effect on the behavior of gases

Intermolecular forces push molecules apart when they are close and pull them together when they are far apart. In gases, these forces stop molecules from being in the same place. This makes real gases take up more space than ideal gases under the same conditions. The pull between molecules can also make real gases take up less space.

Temperature changes how strong these pulls are. Higher temperature makes the pulls weaker. Squeezing a gas together makes the pulls stronger. If a gas is squeezed enough and the temperature is low, the gas can change into a liquid or solid. In these states, the pulls and pushes between molecules balance each other out.

Quantum mechanical theories

Main article: Covalent bond § Quantum mechanical description

Intermolecular forces between atoms and molecules can be understood in simple ways. One way is by thinking about tiny, changing electric charges called dipoles. Another way is to use quantum mechanics, a special science that helps us understand very small particles. Scientists use these ideas to explain why molecules stick together, such as with hydrogen bonding, van der Waals force, and dipole–dipole interactions. There are tools in quantum chemistry that help us see these forces, like the non-covalent interaction index, which looks at how electrons are spread out in the molecules. Special forces called London dispersion forces are very important here.

Recently, new methods have been developed to study how electrons are arranged. One of these is called IBSI (Intrinsic Bond Strength Index), and it uses something called the IGM (Independent Gradient Model) to learn more about these forces.