What is the difference between polar and non-polar molecules?
The distribution of the electronic charge influences the polarity of all the bonds in a molecule.
A bond is polar if it has a non-uniform electronic charge distribution over the molecule.
A bond is non-polar if the electronic charge is uniformly spread over the molecule.
The table below summarises all the major differences between polar and non-polar bonds.
Polar Bonds
Non-polar Bonds
Atoms involved in the bond have a difference in electronegativity
Atoms involved in the bond may have the same or different electronegativity values
There is an unequal electronic charge distribution in the overall molecule
Non-polar bonds have an equal charge distribution overall
Usually, an odd number of lone pairs of electrons present on the central bonded atom
The lone pair of electrons are either not present at all or are evenly distributed throughout the molecule
Molecules containing polar bonds include water (H2O), ethanol (CH3CH2OH), ammonia (NH3), sulfur dioxide (SO2), bromine pentafluoride (BrF5), etc.
Molecules containing non-polar bonds include oxygen (O2), nitrogen (N2), methane (CH4), ethyne (C2H2), benzene (C6H6), etc.
How to tell if a bond is polar or non-polar with electronegativity?
Electronegativity is defined as the ability of an atom to attract a shared pair of electrons from a covalently bonded molecule.
It decreases down the group in the Periodic Table of elements while increasing across a period.
Greater the electronegativity difference between the bonded atoms in a molecule, the higher the bond polarity.
Fluorine (F), belonging to Group VII A ( or 17) of the Periodic Table, is the most electronegative element (E.N = 3.98).
We can tell if a bond is polar or non-polar by looking at the electronegativity difference between the bonded atoms.
According to Linus Pauling’s electronegativity scale, a covalent bond is polar in nature if the difference in electronegativity between the bonded atoms varies from 0.4 to 1.6 units. It is called bond polarity.
One of the two bonded atoms strongly attracts the shared electron pairs. It results in a partial negative charge (δ–) on the atom, while the other atom develops a partial positive (δ+) charge due to electron deficiency. Consequently, oppositely charged poles develop in the molecule.
A bond in which the involved atoms have an electronegativity difference of fewer than 0.4 units will be non-polar because of the equal sharing of the electron cloud.
In contrast, if the difference in electronegativity is between 0.4 to 1.6 units, the bond is considered polar covalent in nature.
How to tell if a bond is polar or non-polar with electronegativity?
Examples of different polar and non-polar molecules
An example of a polar molecule with polar Br-F bonds is boron trifluoride (BrF3 ).
Each Br-F bond in the boron trifluoride molecule is polar due to an electronegativity difference of 1.02 units between the Br and F atoms. The electron cloud is not evenly distributed in the molecule overall. Thus, BrF3 is polar.
Similarly, water (H2O) is a polar molecule.
Each O-H bond in the water molecule is polar due to a high electronegativity difference of 1.24 units between the bonded atoms. The electron cloud stays non-uniformly distributed in the molecule overall. As a result, H2O is polar.
Chlorine fluoride (ClF) is also a polar molecule
The Cl-F bond in the ClF molecule has an electronegativity difference of 0.82 units making it a polar molecule.
On the other hand, fluorine ( F2 ) is a non-polar molecule.
It is made up of two identical F atoms, and the electronegativity difference is 0. Both have an equal grip on the shared electrons in the linear shape of the molecule.
Ethyne ( C2H2 ) is also a non-polar molecule.
It comprises two non-polar C-H and one non-polar C≡C bond. The C-H bond has an electronegativity difference of only 0.35 units making the C-H bonds in the C2H2 molecule non-polar. On the other hand, the C≡C bond is non-polar as it is made up of two identical atoms.
Similarly, methane ( CH4 ) is a non-polar molecule.
Each C-H bond in the methane molecule is non-polar due to an electronegativity difference of only 0.35 units between the bonded atoms. The electron cloud stays uniformly distributed in the molecule overall. As a result, CH4 is non-polar.
How can we tell if a bond is polar or non-polar with electronegativity?
According to Linus Pauling’s electronegativity scale, any bond’s polarity depends on the electronegativity difference between the bonded atoms.
The bond is polar if the difference in electronegativity between the bonded atoms is between 0.4 to 1.6.
For example, each N-H bond has an electronegativity difference of 0.84 units in the ammonia (NH3) molecule. Thus, it is polar.
The bond is considered non-polar if the electronegativity difference between the bonded atoms is less than 0.4.
For example, The Cl-Cl bond has no electronegativity difference in the chlorine (Cl2) molecule. Thus, it is non-polar.
HCl is polar, while H2 and Cl2 are non-polar. How?
HCl is polar because the H-Cl bond has an electronegativity difference of 0.96 units. As a result, the electron cloud stays non-uniformly distributed in the molecule overall. Thus, HCl is polar.
H2 and Cl2 are non-polar because they comprise two identical H and Cl atoms. There is no electronegativity difference between the bonded atoms in each molecule. Both the atoms in each linear-shaped molecule have an equal grip on the shared electrons.
How does the polarity of a molecule affect its solubility?
It is a general principle that like dissolves like. Polar molecules dissolve in polar solvents such as water (H2O), while non-polar molecules are miscible with non-polar solvents such as benzene (C6H6).
For example, ethyl alcohol and ammonia are soluble in water, while gasoline and octane are soluble in benzene.
What are some examples of polar and non-polar molecules?
Some examples of polar molecules are chloroform (CHCl3), ammonia (NH3), water (H2O), sulfur dioxide (SO2), hydrogen cyanide (HCN), methyl bromide (CH3Br), etc.
Some examples of non-polar molecules are hydrogen (H2), fluorine (F2), carbon dioxide (CO2), carbon tetrachloride (CCl4), boron trifluoride (BF3), sulfur trioxide (SO3), etc.
Are there any exceptions in the polar versus non-polar concept according to Pauling’s electronegativity scale?
A few exceptions exist in some molecules whose polarity cannot be justified by Pauling’s electronegativity scale (∆E.N < 0.4 = non-polar molecule, (∆E.N 0.4 – 1.6 = polar molecule).
For example, hydrogen sulfide (H2S) is a slightly polar molecule, although there is an electronegativity difference of only 0.38 units between the bonded S and H atoms, thus an H-S bond is nonpolar but the polarity of H2S can be justified by the bent shape of the molecule, with two lone pairs present on the central S atom.
Ozone (O3) comprises three identical oxygen atoms with no electronegativity difference, but it is still a polar molecule. The asymmetric, bent shape of O3 can justify the polarity. The O-O the electron cloud is delocalized and shifts continuously from one position to the other in the O3. This leads to the polar nature of the molecule overall.
Summary
A bond is polar if it has asymmetrical electronic charge distribution over the molecule.
A bond is non-polar if the electronic charge is uniformly spread over the molecule.
The polarity of a bond depends on the difference in electronegativity between the bonded atoms.
If the difference in electronegativity between the bonded atoms in a molecule varies from 0.4 to 1.6 units, the bond is polar.
If the electronegativity difference is less than 0.4 units, the bond is considered non-polar.
If the difference in electronegativity between the bonded atoms in a molecule is greater than 1.6, the bond is ionic.
Vishal Goyal is the founder of Topblogtenz, a comprehensive resource for students seeking guidance and support in their chemistry studies. He holds a degree in B.Tech (Chemical Engineering) and has four years of experience as a chemistry tutor. The team at Topblogtenz includes experts like experienced researchers, professors, and educators, with the goal of making complex subjects like chemistry accessible and understandable for all. A passion for sharing knowledge and a love for chemistry and science drives the team behind the website. Let's connect through LinkedIn: https://www.linkedin.com/in/vishal-goyal-2926a122b/
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