2.1.4 - Determination of Formulae

• A molecule is 2 or more atoms held together by covalent bonds
• the molecular formula is the number of atoms of each element in a molecule
• many compounds also exist as molecules so the molecular formula is used in equations for this
• empirical formula is the simplest whole number ratio of atoms of each element in a compound
• empirical formula is important for substances that do not exist as molecules e.g. metals, some non-metals (e.g. carbon and silicon), and ionic compounds (e.g. sodium chloride). these substances form giant crystalline structures of atoms or ions so it would be impossible to base a formula on the actual number of atoms or ions, as they are too large
• many coloured crystals are hydrated, so water molecules are part of their crystalline structure. this is known as water of crystallisation and is shown in equations using a large dot (·)