1. Atoms
Everything is made of atoms. An atom has a nucleus containing protons and neutrons, with electrons moving around it.
| Particle | Charge | Location |
|---|---|---|
| Proton | +1 | Nucleus |
| Neutron | 0 | Nucleus |
| Electron | −1 | Around the nucleus |
2. The periodic table
Elements are arranged by atomic number (the number of protons). Elements in the same column, called a group, behave in similar ways.
- Alkali metals (Group 1): soft, very reactive metals such as sodium and potassium.
- Halogens (Group 17): reactive non-metals such as chlorine and fluorine.
- Noble gases (Group 18): very unreactive gases such as helium and neon.
3. States of matter
Matter is commonly found as a solid, liquid or gas. Changes between states include melting, freezing, boiling, evaporation, condensation and sublimation (solid straight to gas).
4. Chemical reactions
In a chemical reaction, atoms are rearranged to form new substances. Mass is conserved, so equations must be balanced. For example, making water:
2H₂ + O₂ → 2H₂O
- Combination: two or more substances join to form one.
- Decomposition: one substance breaks into simpler ones.
- Displacement: a more reactive element replaces a less reactive one.
- Combustion: a substance reacts with oxygen, releasing heat and light.
- Neutralisation: an acid reacts with a base to form a salt and water.
5. Acids, bases and pH
The pH scale runs from 0 to 14. Below 7 is acidic, 7 is neutral (pure water) and above 7 is alkaline. Lemon juice is acidic, while soap and bleach are alkaline.