How is Polarity of a Molecule Determined

The nature of any substance and its peculiar set of properties can be attributed to its molecular structure. Ergo, to study the substance or chemical compound, you must first know the properties of the molecule. A property of any molecule which has a substantial impact on the macroscopic behavior of the substance is its 'polar' or 'non-polar' nature. The polarity of a molecule is dependent on a lot of factors, which need to be considered in totality. This Buzzle article is a short tutorial on determining the polarity of any molecule.

What is a Polar or Non-Polar Molecule?
Polarity is created within a molecule, because of unequal sharing of electrons between atoms. This unequal sharing results into a net dipole moment for the molecule. So a polar molecule is one which has a net dipole moment because of unequal sharing of electrons.

A non-polar molecule, on the other hand, is one which has no net dipole moment, mostly due to equal sharing of electrons among the chemical species and has a symmetrical structure. Let me talk about the factors which decide whether a molecule will be polar or non-polar, before outlining a procedure for determining polar nature of any molecule.

What Factors Affect Polarity of a Molecule?
One of the prime factors determining molecular polarity is the difference in electronegativities of the atomic species, which are part of the molecule. The more the difference, more unequal is the electron sharing, resulting into a highly polar molecule. Other than this prime factor, the symmetry of the molecule, the total number of atoms, total number of identical atoms around the central atom, number of lone pairs of electrons and overall shape of the molecule will decide whether it's polar or non-polar.

Linear molecules, with disparate electronegativity values are generally polar. Linear molecules with a single hydrogen atom at one end and asymmetrically shaped molecules tend to be polar, while molecules with identical atoms and a symmetric distribution tend to be non-polar.

Determining Molecular Polarity
To determine whether a given compound is polar or non-polar, firstly write down the names of the atoms involved and note down their electronegativity values. Also note down the number of unbonded electron pairs in the molecule. Then determine the shape of the molecule. Evaluate the polarity of all bonds that make the molecule.

If you find that more than one bonds are non-polar in nature due to the large difference in electronegativity (between 0.3 and 1.7), then the molecule is surely going to be polar. In case the difference exceeds 1.7, the bond is almost ionic and therefore polar in nature. If there are more than one unbonded pair of electrons the shape of the molecular is destined to be angular or distorted, making it a polar molecule.

In case there exists an electronegativity difference, but there are identical and equal number of atoms around the central atom, due to which the charge inequality cancels out, the molecule will be polar. This is always true in case of tetrahedral and linear molecules. Triangular pyramid shaped atoms with electronegativity difference are almost always polar. Let me illustrate how to determine the polarity of a molecule with an example. Consider Hydrogen Fluoride molecule (HF). It's a linear molecule with a substantial electronegativity difference between Hydrogen and Fluorine, making it a polar molecule.

To determine polarity, you will need information about the electronegativity values of all the atomic species, that are a part of the molecule. With that information, knowledge of the electronic configuration of all bonded atoms and a study of symmetry of the molecule, you can easily determine whether the given molecule will be polar or non-polar.