Instantaneous Dipoles
Many small molecules do not possess permanent dipoles because the atoms that are bonded together all have the same, or very similar, electronegativity. This means that on average, the shared electron cloud is evenly distributed across the atoms. However, a temporary or instantaneous dipole can arise because the electrons are in constant motion and at any particular moment, they may not be evenly distributed over the atoms in the molecule. The end of the molecule where the electrons are more concentrated would have a greater negative charge than the other end and instantaneously, the molecule has developed a dipole.
An example is Br2, which exists as a diatomic gaseous molecule which can develop at any moment, an instantaneous dipole. The larger the atoms in a molecule, the higher the probability that an instantaneous dipole will exist as the electron cloud will be bigger and further from the nuclei so therefore the electrostatic attraction holding the electrons in place will be weaker and they would be more free to move, compared to small molecule where electrons are closely held to the nuclei.
Left alone, the dipole will only last for an instant before the constantly moving electrons readjust the charge, cancelling out the previous negative charge or even reversing the dipole. However, if other molecules are present near to the molecules with instantaneous dipoles, they will be affected and produce induced dipoles.
Induced Dipoles
When an unpolarised molecule such as Cl2 comes into close proximity with a polar molecule such as HCl, the unpolarised molecule may have a dipole induced in it. The dipole attracts or repels electrons in the charged electron cloud of the unpolarised molecule, inducing a dipole.
For example, an unpolarised Cl2 molecule would be affected when near the polar HCl molecule, the hydrogen atom has a slight positive charge and attracts electrons to that end, from the Cl2 molecule. When electrons are attracted to the positive end of the HCl dipole, the chlorine atom near the HCl molecule acquires a slight negative charge, leaving the other chlorine atom with a slight positive charge; a dipole has been induced in the molecule.
Induced dipoles can be brought about by the effect of a permanent dipole, such as HCl, or by the effect of an instantaneous dipole of any molecule – this makes it possible for a whole range of dipoles to be set up in a substance with no permanent dipoles.
Polar-Covalent bonds
This is also a form of dipole, a separation of electronic charge leading to a molecule having an uneven distribution of negative charge and therefore positive charge in the remaining, sedevic regions. To understand this intermediate bond between polar (dipole interactive) and covalent bonds, the Valence shell electron pair repulsion (VSEPR) theory is needed. This is defined as “a model in chemistry used to predict the shape of individual molecules based upon the extent of electron-pair electrostatic repulsion” as per Modern Inorganic Chemistry W.L. Jolly and requires research into.
The importance of dipoles
The occurrence of dipoles is of critical importance to support human life. Dipoles give different liquids (the most abundant atom-economically physical phase in terms of molecular quantity) different and varied qualities which gives rise to the ability to react in a multiplicity of manners.
In additions to different types of dipoles forming between different molecules in different and varied permutations, differences and variation also arises as the result of a differing relative strength of dipoles.
The order of relative dipole strength in qualitative terms
The strongest type of dipole is the hydrogen bond. Following on from this, the next strongest dipole type is the permanent dipole – permanent dipole bond. Following on from this type of dipole, is the permanent dipole – induced dipole bond. And finally, the weakest of these dipoles in purely relative terms, is the instantaneous dipole – induced dipole bond. The polar-covalent dipole slots between the hydrogen bond and the instantaneous dipole – induced dipole bond depending on the conditions and reagents (molecules) involved.
