Polarity is the property of a compound that occurs due to the unequal sharing of electrons between the bonded atoms of a molecule.
It relates to other properties such as boiling point, melting point, solubility, and molecular interactions.
Polar and non-polar molecules differ significantly in various ways. In this article, we will learn how to determine polar and non-polar compounds in three steps.
Polar Molecules
Polar molecules are those in which electrons are not shared equally in a covalent bond with a lone pair of electrons on the central atom. They are characterized by permanent dipole moments.
Dipole forms as one part of the molecule are partially positive and the other part is partially negative.
This is due to the difference in the electronegativity of the bonded atoms. If the electronegativity difference between the atoms is 0.5 and 2.0, the atoms form a polar covalent bond. If the electronegativity difference is greater than 2.0, atoms form an ionic bond.
Examples of Polar Molecules
Water-H2O
Ammonia-NH3
Sulphur dioxide-SO2
Hydrogen sulfide-H2S
Non-Polar Molecules
Non-polar molecules are those in which atoms share electrons equally and there is no unshared pair of electrons. Electrons are evenly distributed and there is no net charge across the molecule.
They have identical slides around the central atom and have very small or zero dipole moments.
If the electronegativity difference is less than 0.5, the bond is non-polar. Non-polar molecules also form when atoms that share polar bonds are arranged to cancel out each other.
Examples of Non-Polar Molecules
Noble gases (He, Ne, Ar, Kr, Xe)
Oxygen– O2
Nitrogen (N2)
Carbon dioxide—CO2
Methane – CH4
Carbon tetrachloride-ccl4
Sr No | Comparison | Polar Molecules | Non-Polar Molecules |
1 | Separation of electric charges | A positively charged end and a negatively charged end | No separate charges |
2 | Electron distribution | Asymmetrical | Symmetrical |
3 | Dipole moment | present | Absent |
4 | Electronegativity | Different | Same |
5 | types of bond | Polar covalent bond |
Molecular Polarity is determined in three steps.
Step 1: Draw the Lewis Structure.
This is the first step in determining polar and non-polar molecules. You can tell whether a molecule is polar or non-polar by looking at its Lewis structure.
First, draw the Lewis structure. Count the electron regions around the central atom of a molecule and note which of these are bonding pairs and which are non-bonding pairs. In symmetrical molecules, dipole moments would cancel out each other.
Though molecules with lone pairs are asymmetrical and the dipole moments cancel out each other, so they are non-polar. However, there are some exceptions.
Step 2: Determine the Geometry
To determine the 3D geometry of the molecule. You do this with the help of VSEPR (Valence Shell Electron Pair Repulsion Theory).
Electron groups are arranged around the central atom to minimize repulsion and get far away from each other while turning around the central atom.
Determine which of these categories of shape a molecule falls into; linear, tetrahedral, trigonal planar, bent, trigonal pyramid. The first three have symmetrical shapes, while the last two have asymmetrical shapes.
Sr No. | Shape | Polarity | Example |
1 | Linear | Non-polar | C2H2 |
2 | Tetrahedral | Non-polar | C2H4 |
3 | Trigonal planar | Non-polar | CH4 |
4 | bent/V-shaped | polar | NH3 |
5 | Trigonal Pyramidal | polar | H2O |
Step 3: Dissect the Symmetric Molecules
Let’s dissect the symmetric molecules. If the same atoms are attached to the central atom, the molecule is non-polar. But if different atoms are attached to the central atom, the molecule is polar.
Examine the arrows in Lewis’s structure.
Non-Polar Covalent Bond
Arrows equal in length
The arrangement is symmetrical.
Polar covalent bond
Arrows are different in length.
The arrangement is asymmetrical or uneven.
Summary
Non-polar molecules are symmetric with all the electrons equally shared and having equal or nearly equal electronegativities.
Polar molecules are asymmetric, containing a lone pair of electrons on the central atom or largely different electronegativities of bonded atoms.
Exceptions
Although the above steps are sufficient to determine the polarity of compounds. However, there are some violations of these rules:
The Lewis structure does not apply to hydrocarbons. Lewis’s structure is not suitable for:
Molecules with an odd no. of electrons (e.g., NO)
Molecules with more than or less than eight electrons
Another example is NCL3, which is rare. The bond between N and Cl is non-polar. However, when you see the molecule, you see that the nitrogen atom has a single electron pair. This makes the nature of the molecule polar.
When we compare the polarity of molecules, it tells us about other properties of molecules such as solubility, boiling point, etc. Polarity is of great significance to the biological sciences.
For example,
Water is lighter than oxygen or nitrogen gas, so you could expect it to be a gas considering its molecular weight. The polarity of water makes it stick together very well.
The good thing is that it dissolves and carries nutrients, thus supporting living things. If there was no polarity in water, cohesion, adhesion, and tension would decrease as water won’t stick to other surfaces, thus making life impossible on earth.
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