In chemistry, covalent bonds between atoms can be classified into polar or non-polar depending on how their shared electrons are distributed between the two elements they are holding together.
Polar vs Non-Polar
The main difference is that a polar bond will have an uneven distribution of electrons between the two bonded atoms, whereas a non-polar bond will have electrons being shared equally. Whether a bond between two atoms will be polar or non-polar is dictated by the degree of difference in the elements’ electronegativity (known as X), which can be explained as the intensity of the shared electron’s attraction to a given element.
Polar and non-polar covalent bonds, which refer to how individual atoms are connected, should not be confused with polar and non-polar molecules, which can potentially be composed of many different atoms and bonds, both polar and non-polar.
Comparison Table Between Polar and Non-Polar Bonds (in Tabular Form)
| Parameter of Comparison | Polar Bonds | Non-Polar Bonds |
|---|---|---|
| Bonds two of the same element | No | Yes |
| Electronegativity difference | 0.5-1.7 | 0-0.4 |
| Resulting molecule charge | Slightly negative at one end and positive at the other | Neutrally charged molecule |
| Equal electron sharing | No | Yes |
| Boiling and melting points | Varied | Generally very low |
What is Polar Bond?
A polar bond is a chemical joining together of two elements with a difference in electronegativity of greater than 0.4 and less than 1.8.
Negatively charged electrons are shared between the two connected atoms (known as a molecule), however, the difference in electronegativity causes the electrons circulating around the nuclei of each atom to be unevenly distributed between the two.
Electrons will be preferentially distributed around the nuclei of the atom with a higher electronegativity, for example in a water molecule, which is made up of two hydrogen atoms (X = 2.2) attached to the side of one oxygen atom (X = 3.44), the shared electrons that make up the water molecule will spend a longer period of overall time circulating the nuclei of the oxygen atoms.
In this instance the higher number of electrons attracted by the oxygen atom will result in the oxygen end of the water molecule to be slightly negatively charged (known as a dipole moment), and the hydrogen ends slightly positively charged. This is known as a polar molecule.
It is interesting to note that polar covalent bonds do not always result in a polar molecule being formed.
In many cases this is true, however depending on the geometric arrangement of the atoms within a given molecule, the difference in electronegativity of certain polar bonds may end up cancelling each other out.
One example of this is the carbon dioxide molecule (CO2), which contains an oxygen molecule attached to both ends of a carbon molecule.
The linear arrangement results in the negative charges on either end of the molecule being equal, and even though the carbon atom in the middle of the molecule is positively charged, we will end up with two polar bonds within one non-polar molecule.
What is Non-Polar Bond?
If a polar bond is one where shared electrons are unevenly distributed between two atoms, then it stands to reason that when electrons are shared evenly across the two atoms, the resulting bond is known as non-polar.
The difference in electronegativity in non-polar covalent bonds must be between 0-0.4 so that the negatively charged electrons are shared evenly (or very close to evenly) between the two atoms due to a lack of electromagnetic “pull” from either atom.
As such, non-polar covalent bonds generally occur when an atom of a given element joins with another identical element, for example, O2 (oxygen), H2 (dihydrogen) and O3 (ozone).
These bonds are considered to be very strong bonds and require large amounts of energy to sever.
However, this is not always the case. Carbon (X= 2.55) and hydrogen (X= 2.2) can be found bonded together in a huge amount of organic compounds, and due to the small difference in electronegativity (0.35) is still considered a non-polar bond.
These types of bonds are very important in biology. The life-giving molecules of oxygen and ozone are both made possible by non-polar bonds. Even inside our own bodies, we have strings of amino acid peptides that join together via non-polar bonds and help form our DNA.
Main Differences Between Polar and Non-Polar Bonds
- In polar bonds, the electrons will preferentially associate with the element which has the higher electronegativity, however, non-polar bonds are between elements with the same or very similar X values and therefore present with an equal distribution of electrons on either side of the bond.
- Once the electrons are distributed unevenly across a polar bond, what’s known as a dipole moment will ensue, which results in a slight difference in electrical charge between the two ends of the bonded atoms. However, the dipole moment does not occur in non-polar bonds.
- After the dipole moment in a polar bond, the atom with the higher electronegativity will have a slightly negative charge due to its larger number of electrons, leaving the other end slightly positive in comparison.
- This can be compared to non-polar bonds, where we have zero, or negligible difference in charge due to the even distribution of electrons across the bonded elements.
- Non-polar bonds normally occur between elements that are either identical or very electronegatively similar, which means they are very difficult and require a lot of energy to separate, however, polar bonds between distinct elements are generally easier to break.
Conclusion
The way that shared electrons move between the different atoms is directly influenced by the bond being either polar or non-polar and goes on to contribute to dictating the nature of the electrical charge of the resulting molecule.
Polar bonds result in a net increase of electron presence around the element with higher electronegativity, whereas non-polar bonds join two elements of equal or very similar electronegativity and are therefore characterized by a very even distribution of electrons around the nuclei of the two atoms.
References
- https://pubs.acs.org/doi/abs/10.1021/i160009a001
- https://aip.scitation.org/doi/full/10.1063/1.4772647