The term oxidation originally meant a reaction in which oxygen combines chemically with another substance, but its usage has long been broadened to include any reaction in which electrons are transferred.
Oxidation and reduction always occur simultaneously (redox reactions), and the substance which gains electrons is termed the oxidizing agent. For example, cupric ion is the oxidizing agent in the reaction: Fe (metal) + Cu++ --> Fe++ + Cu (metal); here, two electrons (negative charges) are transferred from the iron atom to the copper atom; thus the iron becomes positively charged (is oxidised) by loss of two electrons while the copper receives the two electrons and becomes neutral (is reduced).
Electrons may also be displaced within the molecule without being completely transferred away from it. Such partial loss of electrons likewise constitutes oxidation in its broader sense and leads to the application of the term to a large number of processes which at first sight might not be considered to be oxidations. Reaction of a hydrocarbon with a halogen, for example, CH4 + 2 Cl --> CH3 Cl + HCl, involves partial oxidation of the methane; halogen addition to a double bond is regarded as an oxidation.
Dehydrogenation is also a form of oxidation, when two hydrogen atoms, each having one electron,
a removed from a hydrogen-containing organic compound by a catalytic reaction with air or oxygen, as in oxidation of alcohols to aldehydes.
The number of electrons that must be added to or subtracted from an atom in a combined state to convert it to the elemental form; i.e. in barium chloride ( BaCl2 ) the oxidation number of barium is +2 and of chlorine is -1. Many elements can exist in more than one oxidation state.
Now, let us look at some common ions. An ion is the reactive state of the chemical, and is dependant on its place within the periodic table.
Have a look at the periodic table of the elements. It is arranged in columns of elements, there are 18 columns. You can see column one, H, Li, Na, K etc. These all become ions as H +. L i+. K +. etc. The next column, column 2, Be, Mg, Ca etc. become ions Be 2+. Mg 2+. Ca 2+. etc. Column 18, He, Ne, Ar, Kr are inert gases. Column 17, F, Cl, Br, I, ionise to a negative F -. Cl -. Br -. I -. etc. What you need to memorise is the table of common ions, both positive ions and negative ions.