Difference Between Polar and Nonpolar Molecules

Main Difference – Polar vs Nonpolar Molecules

Atoms of different or same elements come together to form molecules. The bond which is formed by sharing a pair of electrons between two atoms is called a “Covalent Bond”. Different atoms show attraction to electrons in various degrees. Their ability to pull electrons towards them is called electronegativity.  Atoms such as F, Cl, O show greater electronegativity comparative to atoms like C, P, S. When two atoms with 0.4< electronegativity difference are bonded, polar molecules are formed. If the electronegativity difference between the atoms is <0.4 the molecule becomes non-polar. The main difference between polar and nonpolar molecules is net dipole moment. The net dipole moment is formed on the atoms of polar molecules, but not on non-polar molecules.

This article explains,

1. What are Polar Molecules
      – Definition, Formation, Properties, Examples

2. What are Nonpolar Molecules
      – Definition, Formation, Properties, Examples

3. What is the difference between Polar and Nonpolar Molecules

What are Polar Molecules

Polar molecules are formed either as a result of electronegative atoms or due to asymmetric arrangement of nonpolar bonds and lone pairs of electrons on the same molecule. The following examples will explain the two phenomena in a more elaborated manner.

Water Molecule:

The electronegativities of H and O are 2.20 and 3.44 respectively. The difference of values is 1.24, and it fulfils the main criteria of forming a polar bond. Electrons are attracted more towards the O atom which has a comparatively larger electronegativity. There is a net dipole on the molecule as a result. O is said to be slightly negative (δ-), whereas H atoms are slightly positive (δ+).

In determining the polarity of a molecule, the shape of the molecule also plays a major role. Let’s understand this scenario better by considering the carbon dioxide molecule. 

C is a less electronegative atom than O (2.55 and 3.44) and fulfills the requirement of 0.4 electronegativity difference. However, due to the shape of the molecule, the dipole moments on both C-O bonds are in opposite directions, cancelling each other out. Hence, the net dipole moment is zero.

Polar molecules, when together, get attracted to each other via the opposite charges on their atoms. These forces are stronger than forces between nonpolar molecules but less strong than ionic forces.

Positively charged H atoms make hydrogen bonds with negatively charged O atoms. If H atoms are involved in forming such attractions they are called hydrogen bonds, Intermolecular forces formed without any involvement of Hydrogen atoms are called dipole-dipole forces. Polar molecules dissolve only in polar solvents as they can’t form any attractions with non-polar solvents.

Polar compounds show higher melting points and boiling points comparative to non-polar compounds with similar molecular masses. Energy should be supplied to break intermolecular bonds. Hence, melting points and boiling points are high. This results in low vapour pressure, and rate of evaporation is lower than that of non-polar molecules. Furthermore, polar molecules show greater surface tension.

What are Nonpolar Molecules

Unlike on a polar molecule, there is no negative or positive charge on non-polar molecules. This is because the two atoms have similar attractions toward the electrons they share. The electronegativity difference between the two atoms is <0.4. Therefore, the electron pair is evenly distributed between the atoms. Mostly diatomic gases of the same element are nonpolar molecules. Ex: – O₂, N₂, Cl₂ etc. Hydrocarbon molecules such as Methane, Pentane and Hexane are nonpolar molecules.

It should be noted that nonpolar molecules may exhibit London dispersion forces which are induced by asymmetrical distribution of electrons. This is a spontaneous and temporary force and is the weakest of all intermolecular forces. These London forces are enough for dissolving nonpolar molecules in nonpolar solvents. However, since these forces are weaker than polar dipole forces, if nonpolar molecules are dissolved in polar solvents they don’t mix up. Instead, a heterogeneous system will be formed. The dissolving process is not energetically favoured in this case.

In comparison with polar molecules of the same molecular mass, non-polar molecules have lower melting points and boiling points due to the lack of strong intermolecular forces. Furthermore, as the molecules can be readily evaporated off, the non-polar compounds show high vapour pressures. Therefore, most non-polar molecules form volatile compounds.

Ex: – Pentane, Hexane

Difference Between Polar and Nonpolar Molecules

Net Dipole

Polar Molecules: Net dipole is present due to electronegativity differences of participating atoms or asymmetrical arrangement of the molecule.

Nonpolar Molecules: Net dipole is not present as atoms with similar electronegativity are involved or due to the symmetric arrangement.

Electronegativity Difference 

Polar Molecules: Electronegativity difference between atoms is <0.4.

Nonpolar Molecules: Electronegativity difference between atoms is >0.4.

Molecular Forces

Polar Molecules: Molecular forces are rather strong and form H-bonds or dipole-dipole bonds. 

Nonpolar Molecules: Molecular forces are the weakest available; forms London dispersed forces.

Physical Properties

Polar Molecules: Polar molecules have a high boiling point, melting point, low vapour pressure and high surface tension.

Nonpolar Molecules: Nonpolar molecules have a low boiling point, melting point, high vapour pressure and low surface tension.

Examples

Polar Molecules: Examples include Water, HF and CHF_3.

Nonpolar Molecules: Examples include Pentane, Hexane, and Carbon Dioxide.

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