To calculate the oxidation number of an element in a compound, follow these rules:1. The oxidation number of an atom in its elemental form is always 0.2. The oxidation number of a monatomic ion is equal to its charge.3. The sum of the oxidation numbers in a neutral compound is 0, and in a polyatomic ion, it is equal to the charge of the ion.4. The oxidation number of oxygen is usually -2, except in peroxides like H2O2 where it is -1.5. The oxidation number of hydrogen is +1 when bonded to non-metals and -1 when bonded to metals.To determine if a redox reaction has occurred, compare the oxidation numbers of the elements in the reactants and products. If the oxidation numbers have changed, a redox reaction has taken place.Example of a redox reaction:Mg s + 2HCl aq MgCl2 aq + H2 g In this reaction, the oxidation numbers are as follows:- Mg: 0 elemental form - H: +1 bonded to a non-metal - Cl: -1 monatomic ion In the products:- Mg: +2 monatomic ion - Cl: -1 monatomic ion - H: 0 elemental form Comparing the oxidation numbers, we see that Mg has increased from 0 to +2 oxidation and H has decreased from +1 to 0 reduction . This indicates a redox reaction has occurred.The oxidizing agent is the species that causes the oxidation of another species, so in this case, it is HCl, as it causes the oxidation of Mg. The reducing agent is the species that causes the reduction of another species, so in this case, it is Mg, as it causes the reduction of HCl.