The oxidation number of an atom represents how many electrons it has lost, gained or shared. In an uncombined element the oxidation state is always zero because no electrons have been transferred. For a monoatomic ion, the oxidation state is equal to its charge. For an ion with more than one atom the sum of the atoms oxidation states is equal to the overall charge. In a covalently bonded atom, the oxidation number depends on the electronegativity of the atoms it is sharing electrons with. Oxidation number increases when atoms are oxidised, and decreases when they are reduced during redox reactions.