What Is a Molecule?

A molecule is a group of two or more atoms joined together by chemical bonds, acting as a single unit. Molecules are the building blocks of most of the substances around you. The water you drink, the air you breathe, the sugar in your tea and the DNA inside your cells are all made of molecules. Just as words are made of letters joined together, substances are made of molecules joined together in countless different shapes and sizes.

What it isTwo or more atoms joinedBy chemical bonds
Smallest moleculeH22 hydrogen atoms joined
Largest natural moleculeDNAUp to billions of atoms long
Famous exampleWater (H2O)2 hydrogen + 1 oxygen
Air moleculesN2 and O2Two atoms of the same element
Sugar (glucose)C6H12O624 atoms in each molecule

How molecules form

Atoms join into molecules by sharing or trading electrons, the small particles that buzz around the outside of every atom. The connection that forms between them is called a chemical bond. Atoms bond because, by joining together, they reach a more stable energy state. It is a bit like two people leaning on each other: alone they are wobbly, together they are steady.

There are several types of chemical bond. The two most common in molecules are covalent bonds, where atoms share electrons evenly, and polar covalent bonds, where atoms share electrons unevenly so one end of the molecule is slightly positive and the other slightly negative.

Famous molecules

Water (H2O): 2 hydrogen + 1 oxygen. The most famous molecule on Earth. Without it, no life as we know it.
Oxygen (O2): 2 oxygen atoms joined together. The form of oxygen we breathe.
Carbon dioxide (CO2): 1 carbon + 2 oxygen. What we breathe out, and what plants take in.
Methane (CH4): 1 carbon + 4 hydrogen. The main gas in natural gas, used for cooking and heating.
Glucose (C6H12O6): a simple sugar. The main fuel your body burns for energy.
DNA: a giant molecule that can contain billions of atoms, carrying the instructions for building every living thing.

Fact Some molecules are made of just one type of atom (like O2, oxygen gas). Others combine different types (like H2O, water). When a molecule combines different elements in a fixed ratio, it is called a compound. So all compounds are molecules, but not all molecules are compounds.

What shape are molecules?

Molecules are not flat. They have real three-dimensional shapes, decided by the way their atoms bond. Some are straight lines, some are bent, some look like little pyramids or rings, and some twist into spirals.

Water is bent like a boomerang, with the two hydrogen atoms angled at 104.5 degrees from the oxygen.
Methane is shaped like a tiny pyramid (tetrahedron), with carbon in the middle and hydrogens at four corners.
Carbon dioxide is a straight line: oxygen, carbon, oxygen.
DNA twists into the famous double helix, like a tiny spiral staircase.

The shape of a molecule decides how it behaves. The bent shape of water is what makes it such an excellent solvent. The double helix of DNA is what lets it be copied perfectly when cells divide. In medicine, drug molecules are designed to fit perfectly into other molecules in the body, like a key in a lock.

Did you know? The biggest molecule in your body is called chromosome 1 of your DNA. It is a single molecule about 8.5 centimetres long when stretched out, but it is so thin you cannot see it without a microscope. Inside each of your cells, there are 46 such chromosome molecules folded up tightly enough to fit inside a tiny nucleus only 0.005 millimetres across.

How big are molecules?

Molecules range from very small to enormous (by molecule standards):

Hydrogen gas (H2): 2 atoms, about 0.074 nanometres long. The smallest molecule possible.
Water (H2O): 3 atoms, about 0.27 nanometres across.
Caffeine: 24 atoms. A typical small molecule used in medicine.
Haemoglobin: 4 chains of around 10,000 atoms each. The protein that carries oxygen in your blood.
Human DNA: about 200 billion atoms in each cell, split across 46 chromosomes.

Try this Build molecule models out of marshmallows and toothpicks. Use different colours for different elements. Make a water molecule (1 red marshmallow for O, 2 white for H, two toothpicks between them at an angle). Make a methane molecule (1 black for C, 4 white for H, with all the hydrogens spaced out evenly). Try carbon dioxide, ammonia, even glucose. You will get a feel for how 3D molecules really are.

Molecules versus compounds

The words molecule and compound sound similar but mean different things.

A molecule is any group of atoms held together by chemical bonds.
A compound is a molecule made of two or more different elements in a fixed ratio.

So H2 is a molecule but not a compound (only one element). H2O is both: a molecule made from 2 different elements (so a compound).

Deeper dive: smelly molecules and the chemistry of perfume

The reason you can smell coffee, cut grass, fresh bread or your friends perfume is that small molecules drift through the air, slip into your nose, and stick to special receptor molecules inside it. Each receptor is shaped to fit certain shapes of molecule, like a tiny lock that accepts certain keys.

For example, the smell of cut grass comes mainly from a molecule called cis-3-hexenal. Vanilla smells like vanilla because of a molecule called vanillin. Even tiny changes to a molecules shape can totally change its smell. The molecules called carvones come in two mirror-image forms (like a left and right hand). One smells like spearmint, the other smells like caraway seeds. Same atoms, same bonds, different 3D arrangement.

Perfume makers spend years learning how molecules behave together. A finished perfume can contain over 100 different fragrant molecules, blended to release their scents in stages. Top notes are small, light molecules that evaporate quickly (the first smell you get). Heart notes are medium-sized molecules that last for hours. Base notes are big, heavy molecules that linger all day. Understanding molecules has turned perfume from magic into chemistry.

For more, see chemical bonds and what is an atom.