Formulae of Compounds and Ions

• A formula tells you the number and type of elements present in one molecule of a compound (ionic or covalent).
• Ionic molecules have no charge.
• In ionic molecules the number of positive charges cancel out the number of negative charges.
• Oxidation numbers can be used to show how many of each type of atom combine in compounds.
• Oxidation numbers are the number of electrons that need to be added (or taken away) to make an element neutral.