Intermolecular force

Intermolecular forces are the forces that act between molecules, including the electromagnetic forces of attraction or repulsion that work between atoms and other nearby particles like atoms or ions. These forces are weaker than the forces that hold a molecule together, such as covalent bonds, which involve sharing pairs of electrons between atoms. Even though intermolecular forces are weaker, they are very important and are part of the forces used in studying molecules.

The idea of microscopic forces was first talked about in a book by Alexis Clairaut called Théorie de la figure de la Terre, published in Paris in 1743. Many scientists, such as Laplace, Gauss, Maxwell, Boltzmann, and Pauling, have helped study these forces.

There are several types of attractive intermolecular forces, including hydrogen bonding, ion–dipole forces, Van der Waals forces, and others. We can learn about these forces by measuring properties like how thick a liquid is (viscosity) or how it behaves under different pressures, volumes, and temperatures. These measurements help connect what we see on a large scale to what is happening on a tiny scale with molecules. Intermolecular forces are important in many areas, especially in biochemistry and molecular biology, because they play a key role in how enzymes and catalysts work with other molecules.

Hydrogen bonding

Main article: Hydrogen bond

A hydrogen bond is the pull between a hydrogen atom, which is stuck to a very powerful atom like nitrogen, oxygen, or fluorine, and another powerful atom nearby. This bond is quite strong for a tiny force and helps explain why water stays liquid at normal temperatures instead of being a gas like other similar substances. Hydrogen bonds also help give shape to 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 that happens between tiny particles with 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 because of changes in energy. Salt bridges are different from other forces because they don’t point in a specific direction and depend mostly on the size of the particles involved.

Dipole–dipole and similar interactions

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

Some molecules have parts with charges but overall balance out, so they don’t act like they 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 that involve ions—particles with full positive or negative charges—along with molecules. These forces are stronger than dipole–dipole interactions because ions have a bigger charge than the slight 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 in solution. 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 that happen 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 if 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 natural tiny magnet-like property, called a dipole. These dipoles attract each other, but this only works between certain molecules and can be affected by temperature.

The second type is the Debye force. This happens when a molecule with a dipole induces a dipole in a nearby molecule, causing them to attract each other. This can happen between polar molecules and non-polar molecules.

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 very close and pull them together when they are far apart. In gases, these forces mainly stop molecules from being in the same place, which 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 than ideal gases.

Temperature affects how strong these pulls are. Higher temperature makes the pulls weaker, while 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 different ways. One way is by thinking about tiny, ever-changing electric charges called dipoles. Another way is to use the ideas of 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 in this area.

Recently, new methods have been developed that 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.