Chemistry » States of Matter » Intermolecular And Interatomic Forces

# Intermolecular and Interatomic Forces

If there were no intermolecular forces than all matter would exist as gases and we would not be here. The next set of lessons introduces you to a new concept called an intermolecular force. It is easy for one to become confused as to whether they are talking about bonds or about intermolecular forces, particularly when the intermolecular forces in the noble gases are discussed.

For this reason, we will try and use the word bond or bonding to refer to the interatomic forces (the things holding the atoms together) and intermolecular forces for the things holding the molecules together. Labeling some of the bonds and intermolecular forces on diagrams of molecules will help you to come to grips with the terminology.

A brief overview of the topics covered next include:

• What are intermolecular and how do they differ from bonds (interatomic forces).

• Physical state and density.

• Particle kinetic energy and temperature.

• The chemistry of water.

## Intermolecular forces

All around us we see matter in different phases. The air we breathe is a gas, while the water you drink is a liquid and the chair you are sitting on is a solid. In this section, we are going to look at one of the reasons that matter exists as solids and liquids.

In previous lessons, we discussed the different forces that exist between atoms (interatomic forces). When atoms are joined to one another they form molecules, and these molecules in turn have forces that bind them together. These forces are known as intermolecular forces.

Intermolecular forces allow us to determine which substances are likely to dissolve in which other substances and what the melting and boiling points of substances are. Without intermolecular forces holding molecules together we would not exist.

Note that we will use the term molecule throughout this section as the compounds we are looking at are all covalently bonded and do not exist as giant networks (recall from grade $$\text{10}$$ that there are three types of bonding: metallic, ionic and covalent). Sometimes you will see the term simple molecule. This is a covalent molecular structure.

#### Tip:

Interatomic (between atoms) forces are also known as intramolecular (within molecules) forces. You can remember this by thinking of international which means between nations.

## Intermolecular and interatomic forces

### Definition: Intermolecular forces

Intermolecular forces are forces that act between molecules.

You will also recall from previous lessons, that we can describe molecules as being either polar or non-polar. A polar molecule is one in which there is a difference in electronegativity between the atoms in the molecule, such that the shared electron pair spends more time close to the atom that attracts it more strongly. The result is that one end of the molecule will have a slightly positive charge ($$δ^{+}$$), and the other end will have a slightly negative charge ($$δ^{-}$$). The molecule is said to be a dipole.

A dipole molecule is a molecule that has two (di) poles. One end of the molecule is slightly positive and the other is slightly negative. We can depict this very simply as an oval with one positive side and one negative. In reality however, the molecules do not look like this, they look more like the images in the figure below.

A different representation of dipole molecules. The red region is slightly negative, and the blue region is slightly positive.

It is important to remember that just because the bonds within a molecule are polar, the molecule itself may not necessarily be polar. The shape of the molecule may also affect its polarity. A few examples are shown in the table below to refresh your memory. Note that we have shown tetrahedral molecules with all the terminal atoms at $$\text{90}$$$$\text{°}$$ to each other (i.e. flat or 2-dimensional), but the shape is really 3-dimensional.

 Molecule Chemical formula Bond between atoms Shape of molecule Polarity of molecule Hydrogen $$\text{H}_{2}$$ Non-polar covalent Non-polar Hydrogen chloride $$\text{HCl}$$ Polar covalent Polar Carbon tetrafluoride $$\text{CF}_{4}$$ Polar covalent Non-polar Trifluoro-methane $$\text{CHF}_{3}$$ Polar covalent Polar

Table: Polarity in molecules with different atomic bonds and molecular shapes.