Introduction and types of intermolecular forces

Intermolecular forces

In the previous chapter, we discussed the different forces that exist between atoms (intramolecular 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 , and we are going to look at them in more detail in this next section.

Intermolecular forces

Intermolecular forces are forces that act between stable molecules.

You will also remember from the previous chapter, 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 ( ${\delta }^{+}$ ), and the other end will have a slightly negative charge ( ${\delta }^{+}$ ). The molecule is said to be a dipole . However, 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 [link] to refresh your memory!

Molecule	Chemical formula	Bond between atoms	Shape of molecule	Polarity of molecule
Hydrogen	H ${}_2$	Covalent		Non-polar
Hydrogen chloride	HCl	Polar covalent		Polar
Carbon tetrafluoromethane	CF ${}_4$	Polar covalent		Non-polar

Types of intermolecular forces

It is important to be able to recognise whether the molecules in a substance are polar or non-polar because this will determine what type of inermolecular forces there are. This is important in explaining the properties of the substance.

1. Van der Waals forces These intermolecular forces are named after a Dutch physicist called Johannes van der Waals (1837 -1923), who recognised that there were weak attractive and repulsive forces between the molecules of a gas, and that these forces caused gases to deviate from 'ideal gas' behaviour. Van der Waals forces are weak intermolecular forces, and can be divided into three types:
   1. Dipole-dipole forces [link] shows a simplified dipole molecule, with one end slightly positive and the other slightly negative.

      

      When one dipole molecule comes into contact with another dipole molecule, the positive pole of the one molecule will be attracted to the negative pole of the other, and the molecules will be held together in this way ( [link] ). Examples of materials/substances that are held together by dipole-dipole forces are HCl, FeS, KBr, SO ${}_2$ and NO ${}_2$ .

      

   2. Ion-dipole forces As the name suggests, this type of intermolecular force exists between an ion and a dipole molecule. You will remember that an ion is a charged atom, and this will be attracted to one of the charged ends of the polar molecule. A positive ion will be attracted to the negative pole of the polar molecule, while a negative ion will be attracted to the positive pole of the polar molecule. This can be seen when sodium chloride (NaCl) dissolves in water. The positive sodium ion (Na ${}^{+}$ ) will be attracted to the slightly negative oxygen atoms in the water molecule, while the negative chloride ion (Cl ${}^{-}$ ) is attracted to the slightly positive hydrogen atom. These intermolecular forces weaken the ionic bonds between the sodium and chloride ions so that the sodium chloride dissolves in the water ( [link] ).

      

   3. London forces These intermolecular forces are also sometimes called 'dipole- induced dipole' or 'momentary dipole' forces. Not all molecules are polar, and yet we know that there are also intermolecular forces between non-polar molecules such as carbon dioxide. In non-polar molecules the electronic charge is evenly distributed but it is possible that at a particular moment in time, the electrons might not be evenly distributed. The molecule will have a temporary dipole . In other words, each end of the molecules has a slight charge, either positive or negative. When this happens, molecules that are next to each other attract each other very weakly. These London forces are found in the halogens (e.g. F ${}_2$ and I ${}_2$ ), the noble gases (e.g. Ne and Ar) and in other non-polar molecules such as carbon dioxide and methane.
2. Hydrogen bonds As the name implies, this type of intermolecular bond involves a hydrogen atom. The hydrogen must be attached to another atom that is strongly electronegative, such as oxygen, nitrogen or fluorine. Water molecules for example, are held together by hydrogen bonds between the hydrogen atom of one molecule and the oxygen atom of another ( [link] ). Hydrogen bonds are stronger than van der Waals forces. It is important to note however, that both van der Waals forces and hydrogen bonds are weaker than the covalent and ionic bonds that exist between atoms .

   

Types of intermolecular forces

1. Complete the following table by placing a tick to show which type of intermolecular force occurs in each substance:

   Formula	Dipole-dipole	Momentary dipole	Ion-dipole	hydrogen bond
   HCl
   CO ${}_2$
   I ${}_2$
   H ${}_2$ O
   KI(aq)
   NH ${}_3$

2. In which of the substances above are the intermolecular forces...
   1. strongest
   2. weakest