What is a polar molecule?

A polar molecule is a molecule that contains polar bonds, that is, with positive and negative charges . It can also be defined as a molecule with a permanent dipole moment.

Polar molecules: dipoles and definition

what are dipoles

To better understand what a polar molecule is, it is necessary to understand what a dipole is. The dipole is also known as the chemical dipole moment, and is a measure of the intensity of the attractive force that exists between two atoms. Likewise, a dipole can be defined as the expression of the difference in electrical charge that exists in a chemical bond . Dipole moments are measured in debye units , after the chemist and physicist Peter Debye, who first studied dipoles in molecules.

Molecular dipoles can be divided into:

• Permanent dipoles: occur when two atoms in a molecule have different electronegativity. For example, if one atom has a greater attraction of electrons than another, it becomes more negative, and at the same time, the other atom is left with a more positive charge.
• Instantaneous dipoles: these usually occur randomly when in a molecule the electrons are more concentrated in one place than in another, giving rise to a temporary dipole. These instantaneous dipoles have a smaller magnitude than the permanent ones.
• Induced dipoles: These can occur when a molecule with a permanent dipole repels electrons from another molecule, inducing a dipole moment in that molecule. A molecule is polarized when it has an induced dipole.

What is a polar molecule

Polar molecules are those with polar bonds whose dipoles do not cancel. That is, they are the molecules that present permanent dipole moments, resulting in a difference in the electric charge in the bonds of the molecule.

The most common example of a polar molecule is water (H ₂ O). In the water molecule, the oxygen atom and the hydrogen atoms share electrons in covalent bonds. However, the oxygen atom attracts electrons more strongly than the hydrogen atoms. The hydrogen atoms are located at one end due to this attraction, leaving one part of the molecule with a slight positive charge and the other part with a slight negative charge.

It is important to mention that for a molecule to be polar it is necessary that it have polar bonds, but not all molecules with polar bonds are actually polar, and in a polar molecule there is separation between the positive and negative charges. An example of this is carbon dioxide, which although it is formed by polar bonds, the dipole moments cancel each other, and therefore, it is not a polar molecule.

How to know if a molecule is polar or nonpolar?

To know if a molecule is polar or nonpolar, it is necessary to observe its poles. If one end of the molecule is positively charged and the other end is negatively charged, then it is a polar molecule. On the other hand, if a charge, positive or negative, is distributed uniformly around a central atom, the molecule is nonpolar.

Other examples of polar molecules

In addition to the water molecule, there are other polar molecules. Other examples of them are:

• Ethanol is polar because oxygen atoms attract electrons due to their higher electronegativity, unlike other atoms in the molecule. Therefore, the -OH group in ethanol has a slight negative charge.
• Ammonia (NH3 ₎ .
• Sulfur dioxide (SO ₂ ).
• Hydrogen sulfide (H ₂ S).

Bibliography

• Gómez Aspe, R. Theory and solved problems of organic chemistry . (2013). Spain. Synthesis.
• Rodríguez Morales, M. Formulation and nomenclature. organic chemistry . (2014). Spain. Oxford University Press Spain.
• pearson. Chemistry : a molecular approach. (2013, eText). Spain. pearson.