let's take a look at the following reaction sodium metal and chlorine gas combine to form sodium chloride or table salt in this process sodium atoms will lose one electron to form sodium ions and chlorine atoms each gain one electron to form chloride ions these processes can be illustrated by the half reactions for these substances from these half reactions we can see that all that is really happening is electron transfer so this will be called an oxidation reduction reaction or a redox reaction in a redox reaction one substance will be oxidized which means that it loses electrons and one substance will be reduced which means that it gains electrons in this example sodium has been oxidized and chlorine has been reduced it may be confusing to associate reduction with a gaining of electrons so think of it as a reduction of charge neutral chlorine atoms become 1 minus chloride ions so the charge has been decreased or reduced in addition we want to understand that the substance that is oxidized is the reducing agent because it facilitated the reduction in the other substance and the substance that is reduced is the oxidizing agent because it facilitated the oxidation in the other substance we should be able to assign oxidation numbers to a substance these are sort of like electrical charges but they aren't actually formal charges here are the rules for assigning oxidation numbers 1 elements have oxidation numbers of zero while monoatomic ions have oxidation numbers equal to their formal charge 2 hydrogen is typically plus 1 except in hydrides where it is minus 1 3 oxygen is typically minus 2 except in peroxides where it is minus 1 for halogens are typically minus 1 and 5 the sum of the oxidation numbers in a neutral molecule must add up to 0 or for a polyatomic ion they must add up to the formal charge on the ion let's see if we can assign the oxidation numbers to the following compounds remember that for a neutral molecule they must add up to 0 and for a polyatomic ion they should add up to the charge on the ion here are the numbers we should get and now we can assign oxidation numbers