Polar Molecules: What They Are, How to Identify Them, and Why They Matter

What are Polar Molecules and Why are They Important?

Polar molecules are molecules that have a slight electric charge on one end, while the other end is slightly opposite. This happens when the atoms in the molecule have different attractions for the shared electrons, creating an uneven distribution of charge. Polar molecules are very common in nature and play a vital role in many chemical and biological processes. In this blog, we will explore what polar molecules are, how to identify them, and why they are important.

How to Identify Polar Molecules

To identify if a molecule is polar or not, we need to look at two factors: the polarity of the bonds and the shape of the molecule. 

The polarity of the bonds depends on the difference in electronegativity between the atoms that form the bond. Electronegativity is a measure of how strongly an atom attracts electrons. The higher the electronegativity, the more the atom pulls the electrons towards itself. The table below shows the electronegativity values of some common elements.

If the electronegativity difference between two atoms is greater than 0.4, the bond is considered polar. This means that the electrons are not shared equally and one atom has a partial negative charge, while the other has a partial positive charge. For example, the bond between hydrogen and fluorine is polar, because the electronegativity difference is 4.0 - 2.1 = 1.9. The fluorine atom has a stronger pull on the electrons, so it has a partial negative charge, while the hydrogen atom has a partial positive charge.

If the electronegativity difference between two atoms is less than 0.4, the bond is considered nonpolar. This means that the electrons are shared equally and there is no charge separation. For example, the bond between carbon and hydrogen is nonpolar, because the electronegativity difference is 2.5 - 2.1 = 0.4. The carbon and hydrogen atoms have similar attractions for the electrons, so there is no charge difference.

The shape of the molecule depends on the arrangement of the atoms and the presence of lone pairs of electrons. Lone pairs are electrons that are not involved in bonding and occupy space around the central atom. The shape of the molecule determines if the bond dipoles cancel out or add up. A bond dipole is a vector that points from the positive end of the bond to the negative end. If the bond dipoles point in opposite directions, they cancel out and the molecule is nonpolar. If the bond dipoles point in the same direction, they add up and the molecule is polar.

To determine the shape of the molecule, we can use the VSEPR (valence shell electron pair repulsion) theory. This theory states that the electron groups around the central atom repel each other and try to minimize the repulsion by adopting a certain geometry. The electron groups can be either bonded atoms or lone pairs. The table below shows some common molecular geometries and their polarity.

For example, water has four electron groups around the central oxygen atom: two bonded hydrogen atoms and two lone pairs. The molecular geometry is bent, with a bond angle of 104.5°. The O-H bonds are polar, and the bond dipoles do not cancel out, because the molecule is not symmetrical. Therefore, water is a polar molecule.

Carbon dioxide has three electron groups around the central carbon atom: two bonded oxygen atoms and no lone pairs. The molecular geometry is linear, with a bond angle of 180°. The C-O bonds are polar, but the bond dipoles cancel out, because the molecule is symmetrical. Therefore, carbon dioxide is a nonpolar molecule.

Why are Polar Molecules Important?

Polar molecules are important because they have many properties and functions that depend on their polarity. Some of these properties and functions are:

• Solubility: Polar molecules are more likely to dissolve in polar solvents, such as water, than in nonpolar solvents, such as oil. This is because polar molecules can form intermolecular forces, such as hydrogen bonds, with polar solvents, which lower the energy of the system and increase the solubility. Nonpolar molecules do not form strong intermolecular forces with polar solvents, so they are less soluble. For example, sugar is a polar molecule that dissolves well in water, but oil is a nonpolar molecule that does not dissolve in water.
• Boiling Point and Melting Point: Polar molecules have higher boiling points and melting points than nonpolar molecules of similar size and shape. This is because polar molecules have stronger intermolecular forces than nonpolar molecules, which require more energy to overcome and change the state of matter. For example, water has a boiling point of 100°C and a melting point of 0°C, while methane has a boiling point of -161°C and a melting point of -182°C.
• Surface Tension and Capillary Action: Polar molecules have higher surface tension and capillary action than nonpolar molecules. This is because polar molecules have stronger cohesion, which is the attraction between molecules of the same substance, and adhesion, which is the attraction between molecules of different substances. For example, water has a high surface tension, which allows it to form droplets and resist external forces. Water also has a high capillary action, which allows it to rise in narrow tubes and transport nutrients in plants.
• Biological Functions: Polar molecules are essential for many biological functions, such as transport, communication, and regulation. For example, water is a polar molecule that acts as a universal solvent, a medium for chemical reactions, a temperature regulator, and a transport agent in living organisms. Another example is DNA, which is a polar molecule that stores and transmits genetic information in cells. DNA has a double helix structure, which is stabilized by hydrogen bonds between the polar bases.

Conclusion

Polar molecules are molecules that have a slight electric charge on one end, while the other end is slightly opposite. This happens when the atoms in the molecule have different attractions for the shared electrons, creating an uneven distribution of charge. Polar molecules can be identified by looking at the polarity of the bonds and the shape of the molecule. Polar molecules are important because they have many properties and functions that depend on their polarity, such as solubility, boiling point, surface tension, and biological functions. Polar molecules are very common in nature and play a vital role in many chemical and biological processes.