Everything around you — the air you breathe, the water you drink, your phone, even you — is built from unimaginably tiny particles called atoms. They are so small that a single glass of water contains more atoms than there are glasses of water in all the world's oceans. An atom is the smallest part of an element.
Elements, compounds and mixtures
These three words are the foundation of chemistry — and mixing them up is the most common early mistake, so it's worth getting them clear:
- Element — a substance made of only one type of atom, listed on the periodic table (e.g. oxygen, carbon, gold). It can't be broken down into anything simpler.
- Compound — two or more different elements chemically bonded together in fixed amounts (e.g. water, H₂O, is hydrogen and oxygen joined). Crucially, a compound has completely different properties from the elements it's made of.
- Mixture — two or more substances simply mixed but not chemically joined (e.g. air, or sand and salt). Each part keeps its own properties.
| Made of | Joined? | Separated by | |
|---|---|---|---|
| Element | one type of atom | — | can't be |
| Compound | 2+ elements | chemically bonded | a chemical reaction |
| Mixture | 2+ substances | not bonded | physically (e.g. filtering) |
Inside the atom
An atom is mostly empty space. At its centre is a tiny, dense nucleus holding two kinds of particle, with a third kind orbiting the outside:
| Particle | Charge | Where it is |
|---|---|---|
| Proton | +1 (positive) | in the nucleus |
| Neutron | 0 (neutral) | in the nucleus |
| Electron | −1 (negative) | in shells around it |
An atom has no overall charge — it is neutral — because it has the same number of protons and electrons, so the positive and negative charges cancel out exactly.