Your DNA, the fuel in a car, and the plastic bottle on a shelf are all, at their core, the same basic story — carbon atoms linking to one another in almost endless variety, a trick no other element can quite match.
Every one of the five branches of chemistry introduced in the first article of this series covers a wide slice of the subject, but one branch is so vast it deserves its own dedicated introduction: organic chemistry, the study of carbon-based compounds. It quietly touches nearly everything: the food on a plate, the fabric of a shirt, the fuel burning in an engine, and every living cell on Earth.
Organic chemistry is the branch of chemistry that studies compounds built primarily from carbon, almost always bonded to hydrogen and often to oxygen, nitrogen, sulfur, or other elements. In today's broad usage, an organic compound is simply any compound whose framework is built from carbon. Carbon might seem like an unremarkable choice at first, sitting quietly in the middle of the periodic table, but it possesses two unusual properties that no other element combines quite so well.
Tetravalency means a neutral carbon atom always forms four covalent bonds, arranged toward the corners of a tetrahedron, since carbon has four electrons in its outer shell and needs four more to complete its octet, exactly as the bonding article earlier in this series explained. Catenation is carbon's remarkable ability to bond with itself, forming long chains, branched networks, and rings, more strongly than any other element in the periodic table.
Together, tetravalency and catenation explain why carbon alone can build molecules ranging from a simple methane molecule (CH₄, one carbon atom) to a strand of DNA (billions of carbon atoms linked in a precise, repeating framework).
The simplest organic compounds, containing only carbon and hydrogen, are called hydrocarbons. Depending on how their carbon atoms are bonded together, hydrocarbons fall into three main families.
| Family | Bond Type | Example |
|---|---|---|
| Alkanes | Single bonds only (saturated) | Methane (CH₄), Ethane (C₂H₆) |
| Alkenes | At least one carbon-carbon double bond | Ethene (C₂H₄) |
| Alkynes | At least one carbon-carbon triple bond | Ethyne (C₂H₂) |
Alkanes are called saturated because every possible bonding position on their carbon atoms is already filled with hydrogen; alkenes and alkynes are unsaturated, since their double or triple bonds could, under the right conditions, be broken open to bond with additional atoms.
The number of bonds between carbon atoms distinguishes alkanes, alkenes, and alkynes.
Beyond plain hydrocarbons, organic molecules often contain a functional group: a specific cluster of atoms attached to the carbon framework that gives the molecule its characteristic chemical behaviour. Two molecules can share almost the same carbon backbone and behave completely differently depending only on which functional group they carry.
| Functional Group | Compound Class | Example |
|---|---|---|
| −OH (hydroxyl) | Alcohols | Ethanol, the alcohol in beverages |
| −COOH (carboxyl) | Carboxylic acids | Acetic acid, the acid in vinegar |
| −NH₂ (amino) | Amines | Found throughout proteins |
Because of catenation, carbon is exceptionally good at forming polymers: long chains built by joining together thousands of small, repeating molecules called monomers. This same basic pattern, small repeating units linking into a long chain, builds both natural materials and many everyday plastics.
Polyethylene, one of the world's most common plastics, used in everything from shopping bags to milk bottles, is made by joining thousands of small ethene (C₂H₄) monomers into one enormous carbon chain. The same monomer-to-polymer principle also builds natural materials: proteins are polymers of amino acid monomers, and starch is a polymer built from repeating glucose sugar units.
No other element in the periodic table can match carbon's talent for bonding with itself in almost endless variety, which is exactly why organic chemistry needed a branch of its own. From the simplest methane molecule to the polymer chains in a plastic bottle, and the far more intricate chains that make up living cells, the same two properties, tetravalency and catenation, are quietly at work throughout. The next article in this series turns to a different corner of the carbon-free world: the chemistry of metals and non-metals.
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