Intermolecular Forces

Intermolecular Forces

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Carbon and oxygen are two elements that are quite alike. They both have a similar atomic mass and make molecules through covalent bonds. You might know carbon from diamond or graphite, and oxygen from dioxygen molecules. But even though they are similar, they have very different boiling and melting points. Oxygen melts at a chilly -218.8°C, but diamond doesn't melt at all in normal conditions. Instead, it only turns into a gas at a scorching 3700°C. So why is there such a difference in their physical properties? The answer is all about intermolecular and intramolecular forces.

Intermolecular forces are the forces that work between molecules, while intramolecular forces are the ones that work within a molecule. Understanding intermolecular forces is key to understanding why diamond and oxygen behave so differently. And that's what we're going to explore next!

Intramolecular forces vs intermolecular forces

Let's take a closer look at how carbon and oxygen bond. Carbon is a giant covalent structure, which means that it is made up of a lot of atoms that are all held together in a repeating pattern by many covalent bonds. Covalent bonds are a type of force that work within a molecule. On the other hand, oxygen is a simple covalent molecule. This means that two oxygen atoms bond together using just one covalent bond, and there are no covalent bonds between molecules. Instead, there are only weak intermolecular forces. To melt diamond, we need to break these strong covalent bonds that hold it together. But to melt oxygen, we just need to overcome the weak intermolecular forces that exist between the oxygen molecules. As you're about to see, breaking intermolecular forces is much easier than breaking intramolecular forces. So, let's dive deeper into the world of intramolecular and intermolecular forces.

Intramolecular forces

As we defined above, intramolecular forces are forces within a molecule. They include ionic, metallic, and covalent bonds. You should be familiar with them. (If not, check out Covalent and Dative Bonding, Ionic Bonding, and Metallic Bonding.) These bonds are extremely strong and breaking them requires a lot of energy.

Intermolecular forces

An interaction is an action between two or more people. Something that is international occurs between multiple nations. Likewise, intermolecular forces are forces between molecules. These are weaker than intramolecular forces, and don’t require as much energy to break. They include van der Waals forces (also known as induced dipole forces, London forces or dispersion forces), permanent dipole-dipole forces, and hydrogen bonding. We’ll explore them in just a second, but first we need to revisit bond polarity.

A diagram showing the relative strengths of intramolecular and intermolecular forces
A diagram showing the relative strengths of intramolecular and intermolecular forces

Bond polarity

The type of intermolecular force a molecule will experience depends on the bond polarity. If the bonding pair of electrons is not evenly spaced between two atoms joined with a covalent bond, one atom will attract the pair more strongly than the other due to differences in electronegativities. This creates a polar bond and the molecule contains a dipole moment. The three main types of intermolecular forces are Van der Waals forces, permanent dipole-dipole forces, and hydrogen bonding. A molecule with polar bonds may not be polar overall, as the dipoles may act in opposite directions and cancel each other out, leaving the molecule with no overall dipole moment.

Types of intermolecular forces

A molecule will experience different types of intermolecular forces depending on its polarity. Let’s explore them each in turn.

Van der Waals forces

Van der Waals forces, also known as London forces, induced dipole forces, or dispersion forces, are the weakest type of intermolecular force. They are found in all molecules, including non-polar ones. These forces arise due to the constantly moving electrons within a molecule. This movement creates a temporary dipole, which can induce a dipole in a nearby molecule. This attraction between molecules is known as van der Waals forces. As molecule size increases, the strength of van der Waals forces increases as well, because larger molecules have more electrons and create a stronger temporary dipole. These forces are also known as London dispersion forces. Overall, van der Waals forces are a type of intermolecular force that is present in all molecules and arise due to the constantly moving electrons within them.

Permanent dipole-dipole forces

As we mentioned above, dispersion forces act between all molecules, even ones that we would consider non-polar. However, polar molecules experience an additional type of intermolecular force. Molecules with dipole moments that do not cancel each other out have something we call a permanent dipole. One part of the molecule is partially negatively-charged, while another is partially positively-charged. Oppositely-charged dipoles in neighbouring molecules attract each other and similarly-charged dipoles repel each other. These forces are stronger than van der Waals forces as the dipoles involved are larger. We call them permanent dipole-dipole forces. Permanent dipole-dipole forces are a type of intermolecular force found between two molecules with permanent dipoles.

Hydrogen bonding

Hydrogen bonds are a type of intermolecular force that arises due to the electrostatic attraction between a hydrogen atom covalently bonded to an extremely electronegative atom, and another electronegative atom with a lone pair of electrons. Only three elements are electronegative enough to form hydrogen bonds: fluorine, oxygen, and nitrogen. Hydrogen bonds are much stronger than permanent dipole-dipole forces and dispersion forces, requiring more energy to overcome. Common molecules that form hydrogen bonds include water, ammonia, and hydrogen fluoride. The strength of hydrogen bonds explains why hydrogen fluoride has a much higher boiling point than hydrogen bromide, despite being a smaller molecule. However, hydrogen bonds are still only about 1/10th as strong as covalent bonds, which is why a lot of energy is needed to break the strong covalent bonds between atoms in substances like carbon.

Examples of intermolecular forces

In summary, intramolecular forces are forces within molecules, while intermolecular forces are forces between molecules. Polarity of molecules determines the type of intermolecular forces between them. Van der Waals forces, also known as London forces or dispersion forces, are found between all molecules and are caused by temporary dipoles. Permanent dipole-dipole forces are found between molecules with an overall dipole moment and are stronger than van der Waals forces. Hydrogen bonds are the strongest type of intermolecular force and are found between molecules containing a fluorine, oxygen, or nitrogen atom bonded to a hydrogen atom. Understanding intermolecular forces is essential in predicting the physical properties of substances, such as boiling point and solubility. 

Intermolecular Forces

What are intermolecular forces?

 Intermolecular forces are forces between molecules. The three types are van der Waals forces which are also known as dispersion forces, permanent dipole-dipole forces, and hydrogen bonding.

 Does diamond have intermolecular forces?

 Diamond forms a giant covalent lattice, not simple covalent molecules. Although there are weak van der Waals forces between individual diamonds, in order to melt diamond you must overcome the strong covalent bonds within the giant structure.

What are the intermolecular forces of attraction?

The three types of attraction are van der Waals forces, permanent dipole-dipole forces, and hydrogen bonding.

Are intermolecular forces strong?

 Intermolecular forces are weak compared to intramolecular forces such as covalent, ionic, and metallic bonds. This is why simple covalent molecules have much lower melting and boiling points than ionic substances, metals, and giant covalent structures.

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