Hydrocarbons are made of just two elements: carbon and hydrogen. Nothing else. Every hydrocarbon molecule, from the methane in natural gas to the benzene in industrial solvents, contains only carbon atoms bonded to hydrogen atoms through covalent bonds, where atoms share electrons rather than transferring them. What makes hydrocarbons so diverse is how those carbon atoms connect to each other and how many hydrogen atoms attach along the way.
Why Two Elements Create So Many Compounds
Carbon has a unique ability among elements: each carbon atom can form four bonds simultaneously. It can bond to hydrogen atoms, to other carbon atoms, or to some mix of both. This means carbon atoms can link together into chains, branches, and rings of almost any length, with hydrogen atoms filling in the remaining bonding spots. A single carbon atom bonded to four hydrogens gives you methane (CH₄), the simplest hydrocarbon. Two carbons with six hydrogens make ethane (C₂H₆). Three carbons with eight hydrogens make propane (C₃H₈). The pattern continues up to molecules with dozens or even hundreds of carbon atoms.
The type of bond between carbon atoms is what separates hydrocarbons into distinct families, each with different physical properties and chemical behavior.
Alkanes: Single Bonds Only
Alkanes are the simplest family. Every carbon-to-carbon connection is a single bond, and each carbon atom bonds to four other atoms total (either carbon or hydrogen). Because every possible bond is “used up” with single connections, alkanes are called saturated hydrocarbons. There’s no room to add more hydrogen.
You encounter alkanes constantly. Methane is the main component of natural gas. Propane fuels backyard grills. Butane fills disposable lighters. Longer-chain alkanes make up gasoline, diesel fuel, and paraffin wax. As the carbon chain gets longer, the substance shifts from gas to liquid to solid at room temperature.
Alkenes and Alkynes: Double and Triple Bonds
When two carbon atoms share a double bond instead of a single one, the molecule is an alkene. Double bonds create a rigid, flat connection between the two carbon atoms involved, which changes how the molecule behaves. Alkenes are called unsaturated because the double bond could theoretically “open up” to accept more hydrogen atoms.
The simplest alkene is ethylene (C₂H₄), and it’s a workhorse of the chemical industry. The U.S. alone produces about 25 billion kilograms of ethylene annually, more than any other synthetic organic chemical. Over half of it goes into making polyethylene, one of the most common plastics on the planet. Propylene, the next alkene up, gets converted into plastics, isopropyl alcohol, and a range of other products.
Alkynes take things a step further with triple bonds between carbon atoms. A triple bond creates a straight, rod-like connection with a 180-degree bond angle. The simplest alkyne is acetylene (C₂H₂), widely used in oxyacetylene torches for cutting and welding metals. That triple bond stores a lot of energy, which is why acetylene burns with such an intense flame.
Aromatic Hydrocarbons: Carbon Rings
Some hydrocarbons form closed rings rather than open chains. Aromatic hydrocarbons are built around a special ring structure, most commonly a six-carbon ring where electrons are shared across the entire ring rather than sitting in fixed single or double bonds. This gives the ring unusual stability.
Benzene (C₆H₆) is the most fundamental aromatic hydrocarbon. It’s a flat, hexagonal ring of six carbon atoms, each bonded to one hydrogen. Benzene has significant commercial importance as an industrial solvent, used for cleaning printing equipment and in adhesives like those that attach soles to shoes. It was once found in everyday consumer products like paint strippers and rubber cement, though its use has been restricted as its health risks became better understood.
Where Hydrocarbons Come From in Nature
Most of the hydrocarbons we use formed underground over millions of years. When ancient marine organisms died and settled into sediment, their organic remains were buried under layers of rock. As that rock piled up, temperatures and pressures increased. Once temperatures in these organic-rich sedimentary layers exceeded about 120°C (250°F), the buried organic material began to “cook,” transforming into crude oil and natural gas.
By the time burial pushed temperatures above 150°C (300°F), most of the oil that could form had already been produced. Above those temperatures, remaining oil breaks down further into natural gas, which is why the deepest petroleum reserves tend to contain gas rather than liquid oil. Some hydrocarbon gas also forms at much shallower depths through bacterial processes, without needing high temperatures at all. These biogenic gas deposits typically occur at depths of less than 2,000 feet.
What Happens When Hydrocarbons Burn
Because hydrocarbons contain only carbon and hydrogen, their combustion is straightforward. When a hydrocarbon reacts with oxygen and burns completely, it produces just two things: carbon dioxide and water. This is true whether you’re burning methane on a stovetop, gasoline in a car engine, or propane in a grill. The carbon atoms combine with oxygen to form CO₂, and the hydrogen atoms combine with oxygen to form H₂O.
Incomplete combustion, which happens when there isn’t enough oxygen available, produces carbon monoxide or soot (pure carbon particles) instead of carbon dioxide. This is why a well-tuned gas burner burns blue and clean, while a poorly adjusted one produces a yellow flame and black residue.
How Bond Type Affects Everyday Properties
The distinction between single, double, and triple bonds isn’t just academic. It directly determines how a hydrocarbon behaves. Single-bonded alkanes are relatively stable and unreactive, which is why propane can sit in a tank safely until you ignite it. Double-bonded alkenes are more chemically reactive, making them useful as building blocks for plastics and other synthetic materials. Triple-bonded alkynes store the most energy and burn the hottest.
Chain length matters just as much. Short-chain hydrocarbons (one to four carbons) are gases at room temperature. Medium chains (five to about seventeen carbons) are liquids, covering everything from gasoline to kerosene. Long chains (eighteen carbons and up) are waxy solids. All of them are made from the same two atoms, just arranged in different patterns and quantities.