Covalent Bonds

Claimed by Carter Gatland, 4/24/2022

The Main Idea

Covalent bonds are a type of interatomic bond through which two atoms share a pair of valence electrons and become bound to each other as a result. These are the bonds which allow for the formation of the majority of matter on our planet, and carbon's wide variety of possible covalent bond configurations is one of the primary reasons why life was able to eventually spring from it. Covalent bonds can even produce a dipole moment within molecules if an electron is attracted more greatly to one of the two atoms. Molecules made from nonpolar covalent bonds are unable to mix with those made from polar covalent bonds, which is what causes some substances to be hydrophobic, as they are unable to mix with the polarized water molecules.

There are five main types of covalent bonds:

Sigma Bond (σ Bond): Formed by the direct overlapping of electron orbitals on two atoms. This is the strongest type of covalent bond.
Pi Bond (π Bond): Formed by the lateral overlapping of p or d orbitals on two atoms.
Double Bond: A pairing of one sigma bond and one pi bond, both of which connect the same two atoms together.
Triple Bond: A grouping of one sigma and two pi bonds, all of which connect the same two atoms together.
Half Bond: A bond formed from either one electron being shared between two atoms, or from when an electron shared between two atoms actually disrupts a regular covalent bond, slightly repelling the two electrons in the bond away from each other. These bonds hold half the amount of energy of a single covalent bond between the same atoms.

A Mathematical Model

The total energy present within the covalent bonds of a molecule can be found through simply adding together each bond's unique bond energy. For example, water (H₂O) contains two H-O bonds, each with a bond energy of 464 kJ/mol, so its total bond energy would be 464 + 464 = 928 kJ/mol. Therefore, it would take 928 kilojoules of energy to break apart the interatomic bonds in a mole of water molecules. [table of common covalent bonds and their bond energies, found in https://openstax.org/books/chemistry-atoms-first-2e/pages/9-4-strengths-of-ionic-and-covalent-bonds]

A Computational Model

PhET's "Atomic Interactions" simulation, which depicts the differences in energy between when atoms are bonded and unbonded

Examples

Find the total bond energy of an ozone molecule, which has two O=O bonds:

498 + 498 = 996 kJ/mol

Find the total bond energy of a benzene ring, which has 3 C=C bonds, 3 C-C bonds, and 6 H-C bonds:

3(611) + 3(345) + 6(415)
1983 + 1035 + 2490 = 5508 kJ/mol

Connectedness

How is this topic connected to something that you are interested in?

I personally have always been fascinated with genetics and how it could be altered, and since the different nucleotides are all very similar in their chemical makeup, I hope that our understanding of how to cause the formation or destruction of these bonds can eventually allow us to alter nucleotides, and thus alter someone's genetic code, for example to cure a form of cancer.

How is it connected to your major?

The bonding of different atoms together can greatly alter their chemical and physical properties, and thus is responsible for much of the world around us and the events we witness within it. For example, "hydrogen bonding" between water molecules, that being how they attract and repel each other when at different orientations due to their polarity, greatly affects the pattern into which it solidifies, in this specific case causing ice, the solid form of water, to actually be less dense than liquid water. Not only is this an abnormal physical characteristic, it is also likely a major reason complex life was able to develop on Earth, since only the surface of the oceans would freeze during ice ages rather than their entirety, allowing life to grow under the insulating layer.

Is there an interesting industrial application?

The ability to trigger the formation of covalent bonds has allowed researchers to synthesize artificial DNA.

History

While it was not coined as covalent bonding until 1919 by Irving Langmuir, the theory of the ability for two atoms to share a pair of electrons can be traced back to Gilbert N. Lewis in 1916, who proposed that atoms would bond to each other in order to fill empty energy shells, thus causing them to no longer have any valence electrons. It was also Lewis who created dot notation, commonly used to indicate how many electrons an atom has and how many of those electrons are alone rather than being part of a pair. Much later, in 1927, Walter Heitler and Fritz London proposed the first quantum explanation for covalence, arguing that the bonds are created by the overlapping of electron orbitals.