Chemists have devised a useful “accountancy” tool to help keep track of electrons in compounds and reactions. This is particularly important in redox reactions where some atoms lose (are oxidised) and others gain (are reduced) electrons. Lewis structures are another "accountancy" tool for molecules - Lewis structures and oxidation numbers are intimately linked. Each atom in a molecule is assigned an oxidation number (sometimes called oxidation state). This is the positive or negative charge the atom would have if the molecule was ionic.