Chemical compounds and chemical bonds. These reductions always involve electron transfer between atoms. Oxidation number rules for Elements and Monoatomic Ions. The oxidation number of any element is zero. The oxidation number of any monoatomic ion is the same as the charge on this ion.
All redox reactions involve electron transfer.
They are classified by the direction of the transfer. Oxidation removal of electrons. Reduction gain of electrons. Reducing agents cause reduction. Oxidizing agents cause oxidation.