[/caption]It has long been known that all molecules possess two equal and opposite charges which are separated by a certain distance. This separation of positive and negative charges is what is referred to as an electric dipole, meaning that it essentially has two poles. In the case of such polar molecules, the center of negative charge does not coincide with the center of positive charge. The extent of polarity in such covalent molecules can be described by the term Dipole Moment, which is essentially the measure of polarity in a polar covalent bond.
The simplest example of a dipole is a water molecule. A molecule of water is polar because of the unequal sharing of its electrons in a “bent” structure. The water molecule forms an angle, with hydrogen atoms at the tips and oxygen at the vertex. Since oxygen has a higher electronegativity than hydrogen, the side of the molecule with the oxygen atom has a partial negative charge while the hydrogen, in the center, has a partial positive charge. Because of this, the direction of the dipole moment points towards the oxygen.
In the language of physics, the electric dipole moment is a measure of the separation of positive and negative electrical charges in a system of charges, that is, a measure of the charge system’s overall polarity – i.e. the separation of the molecules electric charge, which leads to a dipole. Mathematically, and in the simple case of two point charges, one with charge +q and one with charge ?q, the electric dipole moment p can be expressed as:p=qd, where d is the displacement vector pointing from the negative charge to the positive charge. Thus, the electric dipole moment vector p points from the negative charge to the positive charge.
Another way to look at it is to represent the Dipole Moment by the Greek letter m, m = ed, where e is the electrical charge and d is the distance of separation. It is expressed in the units of Debye and written as D (where 1 Debye = 1 x 10-18e.s.u cm). A dipole moment is a vector quantity and is therefore represented by a small arrow with a tail at the positive center and head pointing towards a negative center. In the case of a Water molecule, the Dipole moment is 1.85 D, whereas a molecule of hydrochloric acid is 1.03 D and can be represented as:
We’ve also recorded an entire episode of Astronomy Cast all about Molecules in Space. Listen here, Episode 116: Molecules in Space.