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
) and others gain
) 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.