Van der Waals force

Van der Waals forces are named after Dutch scientist Johannes Diderik van der Waals, who first proposed their existence in 1873. These forces are a type of intermolecular force that holds molecules together. They are weaker than covalent and ionic bonds but are essential to understanding the behavior of molecules, especially nonpolar compounds.

Understanding these forces is important in chemistry because they also play a key role in everyday phenomena. Van der Waals forces involve the attractions and repulsions between atoms, molecules, and surfaces. They are caused by correlations in the fluctuating polarizations of nearby particles.

Types of Van der Waals forces

There are mainly three types of van der Waals forces: dipole-dipole interactions, London dispersion forces (also called induced dipole-induced dipole interactions), and dipole-induced dipole interactions. Each has different characteristics and occurs under different conditions.

Dipole–dipole interaction

Dipole-dipole interactions occur between molecules that have permanent dipoles. A molecule with a permanent dipole has one side that is slightly positively charged and the other side that is slightly negatively charged. Water (H 2 O) is a classic example of a molecule with a permanent dipole. In these interactions, the positive pole of one molecule attracts the negative pole of the other molecule.

Example: Consider two water molecules: the oxygen atoms of one molecule will attract the hydrogen atoms of the adjacent molecule.

London dispersion force

London dispersion forces are the weakest type of van der Waals force. They are temporary attractive forces that arise when electrons in two adjacent atoms occupy positions that cause the atoms to become temporary dipoles. These forces exist in all molecules, whether they are polar or nonpolar.

Example: In a nonpolar molecule such as argon gas (Ar), temporary dipoles can form.

Since electrons are constantly moving, at any given time the electron density at one end of an atom may increase, resulting in a temporary dipole. This induces a similar dipole in the neighbouring atom, causing an attraction between the two.

Dipole-induced dipole interactions

Dipole-induced dipole interactions occur when a molecule with a permanent dipole approaches a nonpolar molecule. The electric field of the dipole can cause a distortion in the electron cloud of the nonpolar molecule, causing it to have a dipole moment.

Example: Consider a situation where a molecule of chlorine (Cl 2) comes close to a water molecule (H 2 O).

In this interaction, the electric field generated by water (polar molecule) induces a dipole in the chlorine molecule (nonpolar), leading to interaction between them.

The role of van der Waals forces in nature

Van der Waals forces are everywhere in our daily lives. They may be weak, but they are important and are responsible for many phenomena.

Gecko's feet

A classic and often cited example of how van der Waals forces work is the ability of geckos to walk on walls and ceilings. Tiny hairs on geckos' feet allow them to use van der Waals forces to stick to surfaces.

The forces between the atoms in the setae of the gecko's foot and the surface are sufficient to support its body weight. This adherence is due to the cumulative effect of very weak van der Waals forces at millions of hair contacts.

Condensation of gases

Another example of van der Waals forces is the condensation of gases into liquids. When gas molecules come closer together as the temperature is lowered, the weak van der Waals forces become strong enough to keep the molecules close to each other, resulting in condensation into a liquid.

Strength of Van der Waals forces

The strength of Van der Waals forces depends on several factors:

• Size of molecules: Larger molecules have more electrons and are more polarizable, so they have stronger van der Waals interactions.
• Surface area: Molecules with larger surface area can form stronger van der Waals forces due to the increased contact points.
• Distance: These forces are highly distance dependent, they decrease rapidly as the molecules move apart.

Conclusion

While van der Waals forces are weaker than ionic and covalent bonds, their presence is indispensable in understanding a variety of processes in chemistry and biology. They play a key role in the physical properties of molecules, affecting boiling and melting points, solubility, and mechanical properties. While they are challenging to observe directly due to their weak nature, their collective strength can often be significant, making them essential for interactions at the molecular and larger levels. This understanding highlights the complexity and intricacy of the interactions that govern the natural world.

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