Of the millions of chemical compounds that make up the universe, the vast majority contain carbon. The Chemistry Explained website estimates that more than 95 percent of all known compounds are carbon compounds. What is it that makes carbon the basis for so many chemical compounds?
To answer this question, a basic understanding of how and why atoms combine to form compounds is needed. As you probably know, an atom consists of a nucleus surrounded by electrons. Each element has a different number of electrons. These electrons group together at specific distances from the nucleus. These groups of electrons are called shells. Each shell holds a certain number of electrons. Electrons beyond that number, if any, are bumped up to the atom’s next outer shell. The total number of electrons an atom has determines how many shells there will be.
For our purposes, all you need to understand is this: many atoms, including carbon, want to have eight electrons in their outermost shell. This creates a stable state for these atoms. If one of these atoms has less than 8 electrons in the outer shell, it will try to get the desired eight electrons by combining, or bonding, with other atoms. In the case of carbon, this bonding takes place by sharing electrons. A bond is created when two atoms each contribute one electron to form an electron pair. That pair of electrons can then serve as two electrons for each atom.
Atoms that are bound in this way are called molecules. A molecule made up of atoms from different elements is called a compound. A molecule that consists of multiple atoms of the same element is not a compound. For example, water (H²O) is a compound, but oxygen gas (O²) is not. Not all elements play well together. An element will have a strong affinity for certain elements, and none at all for other elements.
The carbon atom has four electrons in its outer shell, meaning carbon atoms may have to bond with as many as four other atoms to get the eight outer-shell electrons it wants. In addition, the carbon atom has properties that allow it to very easily combine with many common elements, including hydrogen, oxygen, sulphur, and nitrogen. The unique combination of needing to bond with a relatively large number of atoms to be stable, plus an ability to bond easily with many different elements, make carbon the basis of a vast number of compounds.
In addition, carbon atoms don’t always need four atoms to reach eight outer-shell electrons. More than one electron pair can be shared between two atoms. When two pairs are shared, it is a double bond. Three shared electron pairs create a triple bond. Carbon’s ability to form double and triple bonds means it can potentially become stable by bonding to fewer than four atoms, making the number of potential carbon compounds even larger.
Carbon can also bond to itself in many different configurations, including chains, rings and branched structures of varying size and complexity. Some of the individual atoms of these complex carbon structures still may not have eight electrons in the outer shell. These carbon atoms might then bond with other elements, making the number of potential compounds containing carbon almost limitless.
Carbon is not only a part of nearly everything in our lives, it makes life as we know it possible. Without carbon, the compounds and chemical processes necessary for life would not exist. Obviously, the complex chemistry behind carbon compounds has been simplified for this article, but hopefully it gives you a better understanding of just why carbon is so much a part of our universe.