Molecules & compounds

The water from your tap, the air you breathe out, the gas in a kitchen stove: all of it is made of molecules. Atoms rarely stay alone, they join up in small teams. And as soon as they do, something amazing happens: a substance appears that is completely different from its building blocks.

Atoms become molecules

A molecule is a particle made of two or more atoms firmly bonded together. The most famous example is water: two hydrogen atoms and one oxygen atom together form one water molecule, H₂O for short. The astonishing part: at room temperature hydrogen and oxygen are both gases, yet together they make a liquid that can put out many fires. With burning fat or live electrical devices, though, water would be dangerous.

It is similar with table salt: it forms from sodium, a soft metal, and chlorine, a toxic gas. Together they make the harmless stuff you sprinkle on your breakfast egg. Salt does not form molecules, by the way, but a lattice of charged particles, yet it still counts as a compound.

Molecules you know

You meet many molecules every day: H₂O is water. CO₂ is carbon dioxide, the gas in sparkling water and in the air you breathe out. O₂ is the oxygen you breathe in, a molecule of two oxygen atoms. And CH₄ is methane, the main component of natural gas. So even a handful of atom types can build very different substances.

The small number at the lower right of an element symbol, the subscript, tells you how many atoms of that kind sit in the molecule. Where there is no number, there is exactly one. So CO₂ holds one carbon atom and two oxygen atoms.

Element or compound?

If a substance consists of only one type of atom, it is an element, for example O₂ with its two identical oxygen atoms. If different types of atoms sit in one molecule, we call it a compound, like H₂O or CO₂. A compound is not a mixture: the atoms are firmly stuck together, and the new substance has its own properties that you cannot guess from its building blocks.