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# Van der Waals Forces

Have you ever been relaxing in bed a looked up to see a spider staring down at you? While your first thought might be about killing it, have you ever wondered how it could climb on the ceiling like that? Unless you have the powers of Spider-man, if you tried that yourself, you would fall off the wall before you even got to the ceiling.

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So how do they do it? The secret is Van der Waals forces. In this article, we will be learning all about these forces and how they work. Maybe after learning about them, you could also learn how to harness their power to climb (though I wouldn't recommend it!)

• First, we will learn the definition of Van der Waals forces
• Then, we will learn about the three types of Van der Waals forces: dipole-dipole, dipole-induced dipole, and London dispersion forces
• Lastly, we will cover why Van der Waals forces are so important, and we will also look at some examples of these forces in action

## Van der Waals Forces Definition

For starters, let's take a look at the definition of Van der Waals forces.

Van der Waals forces are the electrostatic attraction/repulsion between molecules with either permanent or temporary dipoles.

• Dipoles are a set of separated charges within an atom/molecule. They are caused by an uneven distribution of electrons.

When a species has a dipole, it has a partial positive (δ+) and a partial negative (δ-) end. These poles act like those of a magnet and will attract opposite charges while repelling like ones.

Van der Waals forces are essentially the pushing/pulling caused by these polar molecules coming close to each other.

## Types of Van der Waals Forces

There are three types of Van der Waals forces that you need to be familiar with:

1. Dipole-dipole interactions
2. Dipole-induced dipole interactions
3. London dispersion forces (Instantaneous dipole-induced dipole) these forces are classified by whether the two molecules/atoms interacting have a permanent or temporary dipole, which we will discuss later.

Now, let's talk about each of these Van der Waals forces in detail.

### Dipole-Dipole Interactions

The strongest of the Van der Waals forces are dipole-dipole interactions.

Dipole-dipole interactions are the attractive/repulsive forces that exist between two polar molecules (molecules with a permanent dipole).

The way we determine whether a molecule is polar or not is based on electronegativity.

Electronegativity is the tendency for an atom to pull electrons/electron density towards itself.

Electronegativity increases the further to the top right (fluorine) an element is on the periodic table, it decreases the further you get to the bottom right (francium). The difference in electronegativity is what determines bond polarity. If two atoms have a difference >0.5 in electronegativity, then the bond is polar. However, if a molecule is symmetrical, the molecule itself will be non-polar since the polarities will cancel themselves out.

Let's look at HF as an example:

Fig.1 HF is a polar molecule with a permanent dipole.

Fluorine is much more electronegative than hydrogen (3.98 > 2.2), so the electron density is pulled towards it. Because of this, the fluorine side of the molecule is partially negative (δ-). This also means that hydrogen's end is partially positive (δ+) since it is lacking electron density.

Molecules like HF participate in dipole-dipole interactions because of their dipole. Below is what these interactions look like:

Fig.2-Dipole-Dipole attraction and repulsion

The partial positive end is attracted to the partial negative end since it is lacking electron density, while the partial negative end has an excess of it. However, poles with the same charge will repel one another.

In addition, polar molecules can be at any orientation and still experience these forces. Molecules can be on top of each other or facing diagonally, and these forces will still occur as long as the molecules are in close enough proximity.

It's helpful to think of these molecules like magnets. If you hold a magnet above another magnet (opposite ends parallel), the magnets will be attracted to each other and possibly snap together.

### Dipole-Induced Dipole Interactions

Now, let's talk about the second type of Van der Waals forces: dipole-induced dipole interactions.

Dipole-induced dipole interactions are the attractive/repulsive forces between a polar molecule and an atom/molecule with an induced dipole.

• An induced dipole is a temporary dipole caused by the attraction of an atom/molecule's electrons to a polar molecule's partial positive end.

Below is a diagram showing how an induced dipole is formed:

Fig. 3-A polar molecule induces a dipole in a neutral atom

When a neutral atom/molecule approaches a polar molecule's positive end, its electrons will be pulled toward that pole. This causes the electrons to have an uneven distribution, leading to a temporary dipole. It is only temporary since the dipole will reverse once the polar molecule is far enough away.Since these dipoles are temporary, these interactions are weaker than dipole-dipole forces.

### London Dispersion Forces

Next, we have London dispersion forces.

London dispersion forces are the attractive/repulsive forces between a nonpolar atom/molecule with an instantaneous dipole and a nonpolar atom/molecule with an induced dipole

• An instantaneous dipole is a temporary dipole that forms when the electrons orbiting an atom/molecule are distributed unevenly.

In a non-polar molecule, electrons are spread evenly, however, as the electrons are moving around the electron cloud, they can randomly be spread out unevenly as shown below.

Fig.4 A neutral species forms an instantaneous dipole

Unlike induced dipoles, which only exist while a polar molecule is present, instantaneous dipoles both appear and disappear at random and on their own.While they are weak, an instantaneous dipole can induce dipoles. This is why non-polar molecules have some forces between them, despite being non-polar. London dispersion forces are the interaction between the instantaneous dipole and the induced dipole, as shown below.

Fig.5 The interaction between the instantaneous dipole and the induced dipole.

Since both species were originally non-polar, London dispersion forces are the weakest of the Van der Waals forces.

Van der Waals forces and gases

When we are calculating different variables for gases, we often use the “Ideal gas equation”. One of the problems with this formula is that it ignores the Van der Waals forces between gas particles.

While this might not be a big deal for some gases, gases like xenon, for example, have significant Van der Waals forces.

To predict gas properties for “real” gases, we use the Van der Waals equation:

$$(P+\frac{n^2a}{V^2})(V-nb)=nRT$$

Where:

• P: pressure
• n: number of moles
• V: volume
• T: temperature
• R: gas constant
• a: magnitude of Van der Waals attraction
• b: volume of gas particles

## Factors Affecting Intermolecular Van der Waals forces

Van der Waals forces are affected by three things:

1. Number of electrons

• The greater number of electrons, the more likely instantaneous dipoles will form.

• Ex: Argon (18 electrons) has a lower boiling point than xenon (54 electrons), which means argon has weaker forces

2. Shape of the molecule

• Long, unbranched molecules have stronger forces than short, branched ones.

• Ex: Isobutane has a lower boiling point than butane, since isobutane is branched, while butane isn't

3. Distance

• The farther apart molecules/atoms are, the weaker their interactions

• Coulomb's law states that the force between two charges weakens the farther they are apart

• $$F=k\frac{q_1q_2}{r^2} \text{where k is a constant, q1 and q2 are different charges and r is the distance between them}$$

While London dispersion forces are not affected by temperature, dipole-dipole interactions are.

## Importance of Van der Waals forces

Van der Waals forces are important for several reasons. Here are just some examples:

• They impact the properties of various organic compounds and molecular solids.

• They are why non-polar molecules/atoms can become solids and liquids

• They are used in several fields such as structural biology and polymer science

• They stabilize protein structures

Just based on this small list alone, you can see why these forces are so important!

## Van der Waals Forces Examples

As I just mentioned, Van der Waals forces are very important, and we can see evidence of that in our daily lives.

Geckos and spiders have small hair-like bristles on their feet which they use to stick to smooth surfaces and even be upside-down! This is because of the van der Waals forces of attraction between these bristles and the surfaces they are walking on. Scientists have tried to replicate this phenomenon by making “Geckskin” which also uses these forces. While tests are still ongoing, we may one day have a real-life Spider-Man!

As another example, Van der Waals forces are what keep the “rungs” of the DNA ladder together. Without these forces, DNA wouldn't have the necessary stability and would be prone to falling apart, which would be a big deal for us!

## Van der Waals Forces - Key takeaways

• Van der waals forces are the electrostatic attraction/repulsion between molecules with either permanent or temporary dipoles.
• Dipoles are a set of separated charges within an atom/molecule. They are caused by an even distribution of electrons.
• Dipole-dipole interactions are the attractive/repulsive forces that exist between two polar molecules (molecules with a permanent dipole).
• Dipole-induced dipole interactions are the attractive/repulsive forces between a polar molecule and an atom/molecule with an induced dipole.

• An induced dipole is a temporary dipole caused by the attraction of an atom/molecule's electrons to a polar molecule's partial positive end.

• London dispersion forces are the attractive/repulsive forces between a nonpolar atom/molecule with an instantaneous dipole and a nonpolar atom/molecule with an induced dipole

• An instantaneous dipole is a temporary dipole that forms when the electrons orbiting an atom/molecule are distributed unevenly.

• Van der waals forces are affected by

• the number of electrons (more=stronger)

• the shape of the molecule (longer=stronger)

• the distance between species (farther=weaker)

#### Flashcards in Van der Waals Forces 48

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What are van der waals forces?

Van der Waals Forces are intermolecular forces that are very weak and have a short range of action and their intensity decreases rapidly with increasing distance.

What do van der waals forces do?

They occur between atoms of the same molecule or of different molecules without the formation of a chemical bond.

Why are van der waals forces important?

Van der Waals forces have strong macroscopic impacts on cohesion, adhesion, and friction.

How do van der waals forces hold molecules together?

Van der Waals forces hold molecule together through electrostatic reactions

What is an example of van der waals forces?

An example of Van der Waals forces is the bonds that are formed amongst proteins that help them to build the 3D structure of DNA.

## Test your knowledge with multiple choice flashcards

Rank the types of Van der waals forces from weakest to strongest

The positive end of a dipole will ___ the positive end of another dipole while ___ the negative end of another dipole

True or False: Dipole-Dipole interactions are only between polar molecules

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