Molecules & Compounds
Section: 2. Atoms, Elements & Compounds | Syllabus: Cambridge IGCSE Chemistry (0620)
What is a Molecule?
A molecule is a group of two or more atoms held together by covalent bonds. A molecule can be a molecular element (only one type of atom, e.g. O₂, N₂) or a molecular compound (two or more different elements, e.g. H₂O, CO₂).
Diatomic Elements
Seven elements exist naturally as diatomic molecules (two atoms each), remembered by the mnemonic "HOFBrINCl":
- H₂ (hydrogen), O₂ (oxygen), F₂ (fluorine), Br₂ (bromine), I₂ (iodine), N₂ (nitrogen), Cl₂ (chlorine)
Worked Example: Counting Atoms in a Formula
- Question: how many atoms in total are in one molecule of glucose, C₆H₁₂O₆? [1]
- Answer: 6 carbon + 12 hydrogen + 6 oxygen = 24 atoms [1]
Common Mistakes
- ❌ Assuming any molecule with only one element symbol shown, like O₂, must be a compound because there is a subscript
- ✓ a molecule is only a compound if it contains atoms of two or more different elements; O₂ is a molecular element, not a compound, because it only contains oxygen atoms
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