dipole moments

If your are not sure about bond dipoles and electronegativity then it would be wise to review this before reading this page on dipole moments.

Dipole moments in molecules

Polar molecules

Explanation of how polar molecules will be aligned if they are placed between two oppositely charged plates. If a molecule contains polar covalent bonds then there will be a bond dipole present; that is the molecule will contain atoms with partial charges. As long as the molecule is non-symmetrical then these bond dipoles will result in the molecule being a polar one. If the molecule is highly symmetrical it is likely the bond dipole will cancel each other out due to the geometry and shape of the molecule, this means that the molecule will be non-polar.

If a polar molecule is placed between two electrically charged plates, as shown opposite, then the molecules will align themselves up in such a way that the partially positively charged end of the polar molecule (δ+) will align with the negatively charged plate and the δ- end of the molecule will align itself with the positively charged plate. Non-polar molecules will simply arrange themselves in a random way if placed between two charged plates.

The overall net polarity of a molecule is called the dipole moment. It is given the Greek symbol µ (pronounced mu). The dipole moment of a molecule depends on the magnitude (size) of the charges at the end of the polar bond and on the distance between these two oppositely charged atoms in the polar covalent bond.
As an example consider the hydrogen halides: HF, HCl, HBr and HI; these are shown in the table below. You can clearly see that as the difference in electronegativity increases, which means a larger partial positive charge (δ+) on the hydrogen atom a negative partial charge (δ-) on the halide atom, then the dipole moment increases in size. It is worth mentioning that all these molecules are non-symmetrical and have permanent bond dipoles and therefore the molecules will be polar.

Substance structure difference in electronegativity dipole moment
hydrogen fluoride hydrogen fluoride molecule 4.0-2.1= 1.9 1.8
hydrogen chloride Hydrogen chloride molecule 3.0-2.1= 0.9 1.1
hydrogen bromide Hydrogen bromide molecule 2.9-2.1= 0.8 0.8
hydrogen iodide Hydrogen iodide molecule. 2.6-2.1= 0.5 0.44

Calculating dipole moments

The dipole moment of a molecule is calculated using the formula:

µ= Q x r

Where: Dipole moments are usually recorded in units of debye's (D), where 1 Debye is 3.34 x10 -30 coulomb-metres. As an example consider an electron, charge 1.6 x 10-19 coulombs; moving a distance of 1 x 10-10 metres, which is around the length of a typical covalent bond. Then moving this amount of charge would create a dipole moment of:

Formula for calculating the dipole moment in a molecule.

Example 2- Calculating the size of the partial charges in a polar bond

Hydrogen chloride is a polar molecule with a dipole moment of 1.1D, the H-Cl bond length is 1.27 x 10-10m. Calculate the size of the charge on the hydrogen and chlorine atoms in this polar molecule. So simply start with the equation above for calculating the dipole moment and re-arrange for Q.

Examples on how to calculate the size of the dipole moment in a molecule.

We can easily extend this calculation to work out the % ionic character in the H-Cl bond:

Worked examples and calculations on how to find the size of the dipole moment in a molecule and the size of the charges on the ends of the dipole.

Testing the size of the dipole moment in liquids.

Demonstration how polar lquids can be deflected by charged rods.

You may have carried out a practical investigation in class where you can compare the size of the dipole moment present in various liquids simply by measuring how far the liquids are deflected by a charged rod.
The set-up is very simple. The liquids under test are placed in a burette. The tap in the burette is opened and a slow stream of liquid is allowed to flow out. Next the plastic rod is charged by rubbing it with a dry cloth or fur rag. The rod will become either positively or negatively charged. The charged rod is then brought up to the slow flowing liquid from the burette and using a ruler you can estimate how far the liquid is deflected. This is shown opposite.

Deflection of polar liquids - an explanation

We can offer an explanation as to why some liquids will be deflected by the charged rod. In the example below the first 2 burettes are filled up with water. Water is a polar molecule with a dipole moment. This means that when the charged rod is placed near the stream of water, the water molecules will orientate themselves in such a way that the oppositely charged portion of the molecule is directed towards the charged rod. If you look at the diagram closely you will see that the water molecules are orientated differently in each of the first 2 water streams from the burette. This is simply because the charge on the rod in each example has been changed. In the final example it does not matter whether you use a rod with a positive or negative charge. Tetrachloromethane is a non-polar molecule and will not be attracted to the rod; no matter what charge is on it.

Three burettes filled up with polar and non-polar liquids- the amount of deflection of the liquids when a charged rod is brought close to the liquid flow can be easily measured.

Key Points