Both polar and nonpolar molecules are found in covalent substances. Some covalent molecules have the ability to get polarized and some do not. Polar molecules and non-polar molecules interact with each other in different ways. Polar molecules interact with each other by forces such as dipole-dipole interactions whereas nonpolar molecules interact with each other through London dispersion forces. Let’s have a look at how these molecules differ from each other in nature and how they interact with each other.
This article explains,
1. What are Polar Molecules?
– Definition, Characteristics and Examples
2. What are Nonpolar Molecules?
– Definition, Characteristics and Examples
3. How Do Polar and Nonpolar Molecules Interact With Each Other?
What are Polar Molecules
Polar molecules are a result of asymmetrically dispersed electrons in a molecule. A covalent bond is formed by sharing two electrons between two atoms. These atoms can be of the same element or of two different elements. When there are two different elements involved, they might have similar electronegativities (the ability to attract electrons), or different electronegativities. If the electronegativity difference between two atoms is 0.4<, there is a great tendency for the more electronegative atom to pull the shared pair of electrons towards itself. Hence, there will be a slight negative charge (δ-) induced upon it, leaving the other atom slightly positive (δ+). This process is called polarization.
Figure 1: Permanent dipole of water molecule
Water molecule is one fine example of polar molecules. The electronegativity difference between O and H is 1.5; hence the pair of electrons shared are attracted more towards the oxygen atom which is more electronegative. Therefore, the water molecule is said to be polarized.
Some other examples of polar molecules are ammonia (NH3), Hydrogen sulfide (H2S) and Sulfur dioxide (SO2).
What are Nonpolar Molecules
Nonpolar molecules have symmetrically distributed electrons; hence, there is no charge separation. Basically, this happens when two atoms of similar electronegativity come together to make a covalent bond. Hence, the pair of electrons they share is almost not biased towards any of the participating atoms. No charge separation can be seen in such molecules. However, even if there is charge separation, the shape of some molecules cancel out the charges. CO2 is a typical example.
Figure 2: Lewis Structure of Carbon Dioxide
Even though there is sufficient electronegativity difference between C and O atoms to qualify for a polar bond, the charges are cancelled out due to the linear shape of the molecule resulting in a net dipole of zero. Hence, the carbon dioxide molecule is considered as a non-polar molecule.
Examples of non-polar compounds are mainly diatomic gas molecules such as N2, Cl2 and O2. Hydrocarbon liquids are also non-polar most of the time. Toluene, Gasoline, Pentane and Hexane are some examples.
How Do Polar and Nonpolar Molecules Interact With Each Other
The two types of molecules interact with each other differently.
How Do Polar Molecules Interact With Each Other
Figure 3: Dipole-Dipole interaction between two HCl molecules
Polar molecules interact with each other by forces such as dipole-dipole interactions. It was earlier discussed that polar molecules have uneven charge distribution due to asymmetric electron dispersion. Therefore, the slightly positive end of one polar molecule is attracted towards the slightly negative end of another molecule. The above figure (3) shows the interaction clearly.
The slightly positive H atom of one molecule is attracted towards the slightly negative Cl atom of the second molecule. The attraction force between the two molecules is known as a dipole-dipole interaction.
There’s a special kind of dipole-dipole interaction which is called hydrogen bonding. This interaction involves a hydrogen donor, which is a highly electronegative atom of a molecule which donates its hydrogen to form a bond with another highly electronegative atom with a lone pair of electrons, from another molecule. The latter is called a hydrogen acceptor. The following figure (4) illustrates the Hydrogen bonding in water.
Figure 4: Hydrogen Bonding in Water
The oxygen atom labeled B is accepting hydrogen from the oxygen atom A and makes a bond between the two water molecules. Oxygen atom A is the hydrogen donor whereas oxygen atom B is the hydrogen acceptor.
How Do Non-polar Molecules Interact With Each Other
Non-polar molecules can’t form dipole-dipole interactions. Instead, they interact with each other by forming London dispersion forces.
Electrons of a molecule move randomly. When the electrons are collected towards one end of the non-polar molecule, a slight negative charge is induced at that particular end. It makes the other end of the molecule slightly positive. This leads to a temporary charge separation on the molecule. When another non-polar molecule comes to the neighbourhood, the former molecule has the ability to induce a dipole on the latter as well. This occurs due to the repulsion of like charges.
The electron density of the negative end of the molecule A, repels the electrons of the adjacent end of molecule B, inducing a positive charge on that end. Then a weak bond is formed during the two ends.
Interaction Between Polar and Nonpolar Molecules
London dispersions are called much weaker than dipole-dipole forces of polar molecules. Therefore, the tendency for polar molecules to interact with non-polar molecules is minimum. Because the energy released by the formation of dispersion forces between polar and non-polar molecules are not enough to break strong dipole-dipole interactions between polar molecules. Therefore non-polar solutes cannot be dissolved in polar solvents.
Reference:
Image Courtesy:
“Dipoli acqua” By Riccardo Rovinetti – Own work (CC BY-SA 3.0) via Commons Wikimedia
“Carbon-dioxide-octet-dot-cross-colour-coded-2D” By Ben Mills – Own work (Public Domain) via Commons Wikimedia
“Dipole-dipole-interaction-in-HCl-2D” By Benjah-bmm27 – Own work (Public Domain) via Commons Wikimedia
“Hydrogen-bonding-in-water-2D” (Public Domain) via Commons Wikimedia
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