Atoms and Molecules

Everything you can see, touch, taste or breathe is made of atoms — particles so unimaginably small that a single full stop on this page contains many billions of them. Atoms are the fundamental building blocks of all matter in the universe, from your body to distant stars.

Although atoms are tiny, they have a structure of their own. At the centre is a dense core called the nucleus. Whizzing around the nucleus are even tinier particles called electrons. Most of an atom is actually empty space between the nucleus and the electrons.

The nucleus is made of two kinds of particle. Protons carry a positive electric charge, and neutrons carry no charge at all. The electrons orbiting outside carry a negative charge. In a normal, balanced atom, the number of protons equals the number of electrons, so the charges cancel out.

When two or more atoms join together, they form a molecule. Atoms join by sharing or swapping their electrons, forming links called bonds. A water molecule is made of two hydrogen atoms bonded to one oxygen atom, which is why water is written as H₂O.

Different combinations of atoms make completely different substances. Carbon dioxide, the gas we breathe out, is one carbon atom joined to two oxygen atoms (CO₂). Ordinary table salt is made of sodium and chlorine atoms joined together. Changing which atoms join, and how many, changes the substance entirely.

The state of matter — solid, liquid or gas — depends on how strongly the atoms or molecules are held together and how much energy they have. In a solid the bonds are strong and the particles only vibrate in place. In a liquid the bonds are looser so particles slide around. In a gas the particles break free and zoom about. So atoms explain the states of matter you met earlier!