Why life is built on carbon

The sugar in fruit, the fat under your skin, the protein in your muscles, and the DNA in your cells look nothing alike. Underneath, they share the same framework: chains and rings of carbon atoms.

Organic molecules

Atoms are the tiny units that make up elements such as carbon, hydrogen, and oxygen, and a group of atoms held together by chemical bonds is a molecule. The carbon-containing molecules found in every form of life are organic molecules, and the science of carbon-containing molecules is organic chemistry. The name reflects where these molecules were first found: in living things. (It has nothing to do with how food is farmed.)

People once believed that organic molecules could be made only by living bodies, with the help of a special life force. That idea fell apart in the 1800s, when chemists made urea, a carbon-containing waste molecule found in urine, from nonliving starting materials. Organic molecules follow the same chemical rules as every other molecule.

Four bonds per carbon atom

Carbon’s most important trait is that each carbon atom forms four bonds by sharing electrons with other atoms.

An atom has a tiny, dense center surrounded by electrons, which are arranged in layers called electron shells. An atom is most stable when its outermost shell is full. For carbon, whose outer shell is its second one, full means eight electrons, but carbon has only four there.

Carbon makes up the difference by sharing. When two atoms share a pair of electrons, that shared pair holds them together in a covalent bond. Each carbon atom forms four covalent bonds, and the four shared pairs fill its outer shell with eight electrons.

Carbon’s usual partners are other carbon atoms plus hydrogen, oxygen, nitrogen, and sulfur. Two atoms can share one pair of electrons (a single bond), two pairs (a double bond), or three pairs (a triple bond).

Carbon forms single and double bonds with carbon, oxygen, and nitrogen, and triple bonds with another carbon (C≡C) or with nitrogen (C≡N). Whatever the combination, carbon shares four pairs of electrons in all.

Because carbon atoms bond readily to one another, they link into straight chains, branched chains, and rings. That flexibility lets a handful of elements form an enormous variety of molecules, each with its own shape and job.

Polar and nonpolar bonds

What carbon is bonded to decides whether a molecule mixes with water.

Atoms differ in how strongly they pull on shared electrons, a property called electronegativity. Carbon and hydrogen pull about equally, and two carbon atoms pull exactly equally, so C–H and C–C bonds share their electrons evenly and have no charged ends. Such bonds are nonpolar.

Oxygen and nitrogen pull harder than carbon, so the shared electrons spend more time near them. That end of the bond becomes slightly negative and the other end slightly positive, which makes the bond polar.

Water molecules are polar too, and opposite charges attract. That leads to a simple rule:

• Molecules made mostly of carbon-hydrogen bonds, called hydrocarbons, have no slightly charged regions for water to cling to. They’re hydrophobic (“water-fearing”) and dissolve poorly in water.
• Molecules with polar bonds, such as C–O or O–H, attract water molecules and usually dissolve easily. They’re hydrophilic (“water-loving”).

You can see the rule in a kitchen. Cooking oil, built mostly of long carbon-hydrogen chains, floats on water in its own layer. Table sugar, covered with polar oxygen-hydrogen groups, dissolves completely. Carbon can form both kinds of bonds, often in the same molecule, so it can serve as the backbone for molecules with very different properties.

Stable in heat and cold

Carbon’s bonds also hold up in both heat and cold. A carbon atom is small, so two bonded carbon atoms sit close together, and the shorter a bond, the stronger and more stable it tends to be. As a result, carbon’s bonds stay intact across the whole range of temperatures where life exists, from the ice near Earth’s poles to the scalding water around deep-sea vents.

Check yourselfA molecule contains only carbon and hydrogen. Will it dissolve easily in water?

No. Its carbon-carbon and carbon-hydrogen bonds are nonpolar, so this hydrocarbon has no slightly positive or negative regions to attract water molecules. It’s hydrophobic.