Elements, compounds and molecules
An element is the simplest pure form of matter: it cannot be split into simpler substances by physical or chemical methods. A compound has two or more elements chemically combined in fixed mass ratios. It can be split only by chemical methods: in a teacher’s demonstration, electricity splits water into hydrogen and oxygen. A compound has properties different from its elements. Molecules can have one atom (monatomic: He, Ne), two atoms (diatomic: H₂, O₂, N₂, Cl₂) or many (polyatomic: O₃, CH₄, HNO₃). A molecule of an element has only one kind of atom; a molecule of a compound has several kinds. Oxygen (O₂) and ozone (O₃) are both made of oxygen atoms but differ in the number of atoms. A molecular formula shows the kinds and numbers of atoms: CH₄ has 1 carbon and 4 hydrogen atoms, and HNO₃ has 1 hydrogen, 1 nitrogen and 3 oxygen atoms. Compounds that are mainly carbon joined to hydrogen, and sometimes oxygen and nitrogen, are organic. Plastics and haemoglobin are huge molecules with thousands of atoms.
| Molecule | Atoms in all | Kinds of atoms |
|---|---|---|
| He | 1 | 1 |
| O₂ | 2 | 1 |
| O₃ | 3 | 1 |
| CH₄ | 5 | 2 |
| HNO₃ | 5 | 3 |
| Atom | How many |
|---|---|
| calcium (Ca) | 1 |
| copper (Cu) | 1 |
| silicon (Si) | 4 |
| oxygen (O) | 10 |
Model molecules. Use clay or dough balls in different colours as atoms, and short paper strips to join them. Make O₂, O₃ and CH₄. For each, write the formula, count the atoms, and say if the molecule is of an element or of a compound. Use only soft materials and no sharp tools.