Imagine a drop of water. Split it in half, then in half again, and again. For a long time, each tiny drop is still water. But in the end you reach the smallest bit of water there is: one water molecule.
Split that molecule, and it stops being water. You get two hydrogen atoms and one oxygen atom.
What is a molecule?
A molecule is a group of atoms that hold on to each other. The hold between two atoms is called a chemical bond. Water, the air, sugar and the plastic in your toys are all made of molecules. So is much of your own body.
How do you read a chemical formula?
Chemists write molecules as short codes called chemical formulas. Water is H₂O, and you say it “H-two-O”. Here is how to read it:
- Each symbol stands for one kind of atom, called a chemical element. A symbol is a capital letter, sometimes with a small letter after it: H is hydrogen, O is oxygen and Cl is chlorine.
- A small number after a symbol tells how many of those atoms there are. H₂ means two hydrogen atoms.
- No number means one. So the O in H₂O is one oxygen atom.
Some symbols come from Latin names. Gold is Au, from aurum, and sodium is Na, from natrium.
Can a molecule be made of one kind of atom?
Yes! The oxygen you breathe is O₂: two oxygen atoms joined together. Nitrogen, the main gas in the air, is N₂.
Most molecules mix different kinds of atoms, like water (H₂O) or carbon dioxide (CO₂). A substance made of two or more kinds of atoms joined together is called a compound.
The same atoms can make very different molecules. Two oxygen atoms make the oxygen we need to live. Three oxygen atoms make ozone, O₃, a sharp-smelling gas that is harmful to breathe. High up in the sky, though, a layer of ozone shields us from the Sun’s harmful ultraviolet light.
How big is a molecule?
Molecules are tiny. More than three million water molecules, lined up side by side, would fit across one millimetre.
Some molecules are much bigger than others:
| Molecule | How many atoms |
|---|---|
| water, H₂O | 3 |
| table sugar, C₁₂H₂₂O₁₁ | 45 |
| haemoglobin, which carries oxygen in your blood | about 9,000 |
| DNA, which carries the instructions for your body | billions |
DNA molecules are the giants of your body. The longest one in most of your cells would be about as long as a grown-up’s finger if you stretched it out. Yet it is only about 2 millionths of a millimetre thick.
Do molecules keep still?
Never! In a gas, like steam, molecules zoom around and bump into each other. In a liquid, like water, they slide past each other. Even in solid ice, they jiggle in place.
The warmer something is, the faster its molecules move. That is what temperature measures.
When ice melts or water boils, the molecules only change how they move and how close they are. Each one stays whole: it is still H₂O. To make new molecules, you need a chemical reaction.
Is everything made of molecules?
No. Some things are built in other ways:
- Salt is a crystal of ions: plus sodium ions and minus chloride ions, packed in a huge grid. Its formula, NaCl, only tells you that there is one sodium for every chlorine.
- Metals, like iron or copper, are rows of atoms held together by a shared sea of electrons.
- Noble gases, like the helium in party balloons, are single atoms that almost never join with anything.
Go deeper: The exact definition for grown-ups and the extra curious
IUPAC, the international body that sets the rules for chemistry, defines a molecule as an electrically neutral particle made of more than one atom. A group of atoms that carries a charge, like the hydroxide ion OH⁻, is called a polyatomic ion instead.
In the theory of gases, single atoms of noble gases are sometimes called “monatomic molecules”, because they behave like the particles of any other gas. For salts, chemists count formula units instead of molecules: one formula unit of sodium chloride is one Na⁺ and one Cl⁻. For metals, they count atoms.
Go deeper: Counting molecules: the mole for grown-ups and the extra curious
Molecules are so small that chemists count them in huge batches. One mole is exactly 6.022 140 76 × 10²³ particles, a number called the Avogadro constant. A mole of water weighs about 18 grams, a little more than a tablespoonful.
A glass of 250 millilitres holds about 14 moles of water: about 8 × 10²⁴ molecules. All the oceans together hold about 1.3 billion cubic kilometres of water, or about 5 × 10²¹ such glasses. So the glass has more than a thousand times as many molecules as the oceans have glasses.
Human chromosome 1, the longest DNA molecule in our cells, has about 249 million pairs of building blocks. Stretched out, it would be about 8.5 centimetres long and 2 nanometres wide. All the DNA in one cell together would stretch about 2 metres.
Go deeper: Where the letters-and-words picture breaks for grown-ups and the extra curious
- Words are flat rows of letters, but molecules are 3D shapes, and the shape matters as much as the atoms. Read how shapes form in how atoms join into molecules.
- The same atoms can be joined in a different order, a bit like anagrams. Ethanol (the alcohol in drinks) and dimethyl ether (a gas used in some spray cans) are both C₂H₆O. Molecules like these are called isomers. A structural formula, such as H–O–H for water, shows which atom is joined to which.
- One molecule on its own is not wet, and it cannot boil or freeze. Those are things that huge crowds of molecules do together.
Check yourself
What does the formula CO₂ tell you?
Show the answer
One carbon atom and two oxygen atoms — C stands for carbon and O for oxygen. The small 2 belongs to the oxygen. No number after the C means one carbon atom.
C stands for carbon and O for oxygen. The small 2 belongs to the oxygen. No number after the C means one carbon atom.
Check yourself
Which of these is not made of molecules?
Show the answer
Table salt — Salt is a crystal of plus and minus ions, packed in a huge grid. Water and sugar are made of separate molecules.
Salt is a crystal of plus and minus ions, packed in a huge grid. Water and sugar are made of separate molecules.