Dipole: Interaction, Molecules & Forces

You certainly know the typical red-blue magnet, where the plus pole is drawn on one side and the minus pole on the other side. Exactly this concept of plus and minus pole also exists in chemistry and is called a dipole.

as dipole refers to two spatially separated poles within a molecule or atom, each of which has a different sign, namely either + or -. These poles are created by the shifting of electrons in the atomic shell. All electrons happen to be on one side of the atom/molecule. So this side is negatively charged. The opposite side is positively charged because there are no more electrons there.

The dipole molecule

A dipole is created in a molecule by different electronegativity of the atoms. The higher the electronegativity of a substance, the more that atom attracts the electrons it shares with another atom. As a result, part of the molecule is more negatively charged than the part that attracts the electrons less.

A Dipole arises in a molecule through the different distribution of the electrons. Due to the different electronegativity that the individual elements have, the electrons are attracted to one of the atoms. So this side of the molecule is where all the electrons are, and this side of the molecule is then through those electrons negatively charged. The opposite side, on the other hand, has no more electrons and is therefore positively charged. This is how the dipole molecule is formed.

When the electronegativities are equal, as in an elementary molecule like sulfur, dipoles can only arise by chance. That is, if all the electrons happen to be on one side of the molecule.

Depending on the direction of electronegativity, the dipole moments of the various bonds in the molecule add up and can therefore cancel or reinforce each other.

Electronegativity shows the ability of an element/atom to attract electrons in a bond. You can find out how large the electronegativity of an element is in the periodic table.

In the case of a water molecule, the polarization of the covalent bond results in an electric dipole in which the hydrogen atoms become partially positive and the oxygen atoms partially negative, creating a dipole molecule. Only because it within the H2O molecule comes to a slight charge transfer and thus seen electrically, a dipole is formed, water can form hydrogen bonds and interact with other molecules.

Figure 1: Water molecule with the corresponding poles Source: bs-wiki.de

Here’s a very simple rule of thumb to remember: Molecules with an asymmetric structure and an electronegativity difference (ΔEN) less than 1.7 but greater than 0.5 according to Pauling behave like dipolar molecules.

That is, although they are electrically neutral to the outside, they have a (measurable) dipole moment. When ΔEN is greater than 1.7, an ionic bond is assumed. However, the limit value ΔEN < 1.7 is only a guide value.

Dipole-Dipole Interaction

As with magnets, the creation of a dipole and thus a positive and a negative pole also means that these poles attract opposite poles. This attraction results from the Coulomb’s laws. This interaction leads to the dipole-dipole forces that bind two molecules or two atoms together. The dipole-dipole interaction is one of the chemical bonds.

Figure 2: Dipole-dipole forces Source: Lernort-mint.de

Here you can also see this interaction in the picture. A and B attract each other because they are charged differently. In this way, the two actually separate molecules bond and form a bond via the dipole-dipole interaction.

If you want to know more about the dipole-dipole interaction, just have a look at the appropriate explanation: dipole-dipole forces.

dipole moment

A dipole can occur not only with molecules. So-called dipole moments can also occur in individual atoms. These occur when the electrons that are outside the nucleus all happen to be on one side of the nucleus at a given time. You can also see this in the picture below.

There you can see that the electrons are on the left side of the molecule. As a result, the molecule also has a positively charged pole, which is marked in red, and a negatively charged pole, which is shown here in blue.

These poles then also attract the opposite poles and can thus also trigger dipoles in other molecules. These are then induced poles. The randomly created dipole is called a temporary dipole. When two atoms with a dipole moment are bound to each other by the different charge, this force is also called van der Waals force.

Figure 3: Temporary and induced dipole Source: u-helmich.de

You can also look at a separate explanation for the Van der Walls force to better understand it: Van der Waals forces.

Calculation of the electric dipole

In order to be able to calculate the size of a dipole, you need different quantities.

First the negative charge (-q), which is at a distance (l) from the positive charge (q) and also the connection vector (), which points from the negative charge in the direction of the positive charge.

In a nutshell is a connection vector, a mathematical one Object the distance of two describes points in the plane (2D) or in space (3D).

The magnitude of the dipole moment results from these magnitudes using the following formula:

The dipole moment is given in the unit Debye (). A Debye corresponds to 3.33564 x 10-30 Coulomb x meter, i.e. electric charge times distance.

The dipole moment increases as the charge q increases and the distance between the charges increases.

physical dipole

A good example of a physical dipole is the bar magnet.

In physics there are physical fields whose sources can also be monopoles. But this is not the case with magnetic fields. Magnetic fields only have dipoles. The best way to remind yourself of this is with the example of the bar magnet. This has two poles, each located at one end of the magnet.

Any electric charge distribution can be assigned to an electric dipole moment if its overall charge is not completely symmetrical. To achieve this one looks for the electrical center of gravity which is positively charged and the electrical center of gravity which is negatively charged. You can then imagine both centers of gravity as dipoles. These centroids are basically just the respective ends of the bar magnet if we come back to our example.

The dipole is determined by the dipole moment. This is influenced by the distance and strength of the poles. The source of a dipole field is the dipole.

Figure 4: Dipole magnets Source: wikipedia.org

In this figure, for example, you can see four electrical focal points. Each magnet has two, which are also color-coded here.

More examples of the dipole

In medicine, dipoles occur in the heart muscle cells: the excitation of the heart muscle cells moves to non-excited cells, i.e. from minus to plus. The direction of excitation is represented by the position of the vector and its length.

The dipoles run in a similar way when loudspeakers are running: They have the same characteristics, namely running from minus to plus.

Dipole – The most important thing

  • The dipole has a negative and a positive pole within an atom or molecule.
  • The poles depend on the electronegativity of the individual atoms.
  • When two dipoles come into contact with each other, a dipole-dipole force or the van der Waals force, i.e. a chemical compound, can form.
  • An example of a dipole is the water molecule.
  • A physical dipole is a magnetic field in the broadest sense.