Dipole-dipole forces

Intermolecular and intramolecular bonding

Intermolecular bonding is the bonding between different molecules while intramolecular bonding is the bonding within the molecule. The bonding which holds giant structures such as diamond, graphite and ionic lattices as well as small molecular substances together is likely to either ionic or covalent or metallic in the case of metals. These different types of bonding are all very strong, now this type of bonding is often called the intramolecular bonding. That is the bonding "within" the molecule or giant structure.

However there is another type of bonding which acts between different covalent molecules and individual atoms such as those found in the noble gases. This type of bonding is called intermolecular bonding and it is very weak in comparison to covalent, ionic or metallic bonding, that is the intramolecular bonding.

In gcse chemistry we simply learned about the existence of this intermolecular bonding between different molecules and atoms and the affect it had on some of the physical properties of small molecules such as their melting, boiling points viscosity and solubility.

In A-level chemistry you will need to be more familiar with the different types of intermolecular bonding and how each type acts and the various effects it has on many of the properties of atoms and molecules.

The forces of attraction or intermolecular bonding between covalent molecules and individual atoms such as those found in the noble gases are often referred to as Van der Waals forces or Van der Waals bonding. There are three main types of intermolecular bonding or Van Der Waals bonding that you need to become familiar with, these are:

Permanent dipole-dipole forces

Polar Molecules

Most covalent compounds involve bonding between elements with different electronegativity values. This means that the electrons in any bonds formed will NOT be shared EQUALLY. In an ionic compound the ions formed have positive and negative charges because there is complete transfer of an electron from the metal to the non-metal atom. In a polar covalent bond the electron is shared unequally between the two atoms or we could simply say one atom in the bond has a much larger share of the electron. This means that the atoms involved in the polar covalent bond will have partial charges and not full positive and negative charges. The Greek symbol delta (lowercase) δ is used to show the resultant partial positive (δ+) and negative (δ-) charges that result from the unequal sharing of the electrons in a polar covalent bond.  Molecules which have this unequal sharing of electrons which leads to a slight positive charge (δ+) on one side of the molecule and a slight negative charge (δ-) on the other are said to be polar molecules and the bonds within the molecules are said to be polar covalent bonds. These polar molecules are said to have a permanent dipole, this simply means they have charged regions, one region with a partial positive charge and another region with a partial negative charge within the molecule.

The image below shows three polar molecules and hopefully you will see that the (δ-) end of the molecule is towards the more electronegative element while the (δ+) end of the molecule is towards the less electronegative atoms in the polar covalent bond. (note: the polarity of the bond is not the only factor that will determine if a molecule is polar or not, the shape of the molecule is also important. To be polar the molecule must be asymmetric. For more information click here.)

Water, ammonia and hydrogen chloride molecules are all polar molecules with polar covalent bonds

A polar covalent bond or simply a polar bond can be thought as a sort of halfway house between covalent and ionic bonding in that the electrons in the bond are shared but not equally as is the case in a covalent bond and the polar covalent bond contains charges, similar to those found in an ionic compound. However the charges are only partial charges and not full charges as is the case with ionic compounds. The charges on the atoms are only partial charges simply because the electrons are not completely transferred, just shared unequally.

The larger the difference in the electronegativity between the two atoms in the covalent bond the more polar (polar covalent) the bond will be. If the bond between two different atoms has a very large difference in electronegativity values of say 1.7-2.0 or more it will be an ionic bond. If the differences in the electronegativity values is around 0.5 or less then the bond is likely to be covalent. Click on the link for bond character above or here for more information.

The way in which different molecules interact with each other depends on the type of bonding present within the molecule, that is the intramolecular bonding and also on the atoms present in the molecule. Some molecules have a permanent dipole (charged ends) while other molecules are non-polar. Some molecule may have a hydrogen atom bonded to nitrogen, oxygen or a fluorine atom, in this case a very strong type of intermolecular bonding called hydrogen bonding will be present between these molecules. Even non-polar molecules can interact with each other by the presence of temporary induced dipoles which are produced by the movement of electrons within atoms or molecules or the movement of electrons as the molecules or atoms approach each other. Let's start by looking a type of intermolecular bonding called dipole-dipole bonding. Click the links at the top or bottom of the page for information on the other types of intermolecular bonding.

Permanent dipole-dipole interactions

Hydrogen chloride is a polar molecule which has a dipole moment. Molecules with permanent dipoles are one where there are differences in the electronegative values of between 0.5 and 1.9 between the bonding atoms. This difference in electronegativity leads to formation of polar covalent bonds and the molecule will have partially charged ends (dipoles). For example hydrogen chloride gas consists of small covalent molecules. Hydrogen has an electronegativity value of 2.1 and chlorine has a value of 3.0; this means that the difference in electronegativity is large enough to produce a permanent dipole in the hydrogen chloride molecule. The electrons in the H-Cl bond will spend most of their time closer to the chlorine atom; this means it will have a partial (δ-) negative charge and the hydrogen atom will have a partial positive charge (δ+). This is shown in the image below:

This means that when different hydrogen chloride molecules approach each other there will be an electrostatic attraction between the partially charged δ+ and δ- ends or poles of the molecules. This type of intermolecular bonding between polar molecules is called dipole-dipole bonding. This is shown in the diagram below:

The intermolecular bonding between polar molecules such as hydrogen chloride is called dipole-dipole bonding.

Key Points

Practice questions

Check your understanding - Questions on dipole-dipole bonding

Next