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. It is dependent on a lot of factors, which need to be considered in totality.
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 a symmetrical structure.
Factors that Affect Polarity
One of the prime factors 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, the symmetry of the molecule, 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.
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 molecule 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 non-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 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 the symmetry of the molecule, you can easily determine whether the given molecule will be polar or non-polar.