redox reactions consist of reduction and oxidation reactions
oxidation reactions are the giving out of electrons, the oxidation number of the species giving out the electrons increases during oxidation
reduction reactions are taking in electrons and the oxidation number of the species taking in electrons reduces after reduction
in the reaction Ca + Cl₂ --> CaCl₂
oxidation numbers of Ca in Ca - 0 and CaCl₂ - +2
oxidation numbers of Cl in Cl₂ - 0 and CaCl₂ - -1
oxidation number of Ca has increased from 0 to +2 therefore Ca is oxidised
