Summary
All matter is made of atoms, the smallest pieces of each chemical element. An atom has a nucleus, with protons (positive charge) and neutrons (no charge), and electrons (negative charge) around it. The number of protons, called the atomic number, is what decides which element it is: 1 proton is hydrogen, 6 is carbon and 8 is oxygen.
Atoms are almost never alone: they join together forming molecules. A molecule can be of a single element, like the oxygen we breathe (O₂, two oxygen atoms), or of several, like water (H₂O, two hydrogen and one oxygen). When many identical atoms or molecules gather they form a pure substance; if different types mix, a mixture.
Key points
- Atoms are the smallest pieces of each element.
- The atom has a nucleus (protons + neutrons) and electrons orbiting around it.
- Protons (+), electrons (−) and neutrons (no charge).
- The atomic number (number of protons) determines the element.
- Atoms join together forming molecules.
- Examples: O₂ (oxygen), H₂O (water), CO₂ (carbon dioxide).
- An element has one kind of atom; a compound, several joined.