we know that an oxidation reduction reaction or redox reaction is typically one in which electrons are transferred between species this will happen if the ionization energies and electron affinities for the substances involved are such that electron transfer is energetically favorable as it is for sodium and chlorine here let's look at the complete chemical equation for this electron transfer chlorine exists as a diatomic molecule so we will need two sodium metal atoms to react with one molecule of chlorine gas in order to get two formula units of sodium chloride however to better depict the redox chemistry that is happening it is best to split this up into half reactions first we have two sodium metal atoms giving up one electron each to yield two sodium ions and two electrons this is the oxidation half reaction the one involving a loss of electrons then we have one chlorine molecule reacting with two electrons the ones given up by the two sodium ions to produce two chloride ions this is the reduction half reaction the one involving a gain of electrons it may seem counterintuitive that a reduction involves a gain of electrons but think of it as a reduction in charge here we went from neutral chlorine to negatively charged chloride ions so the charge went in the negative direction just to get some more terminology we need to understand that sodium is acting as a reducing agent this may also seem counterintuitive as sodium was oxidized but if it was oxidized losing electrons then those electrons go to reduce something else so sodium is a reducing agent because it causes a reduction in some other substance likewise chlorine acted as an oxidizing agent because it is what caused the sodium to be oxidized we must note however that there are redox reactions that do not involve electron transfer such as this reaction involving hydrogen gas and chlorine gas to produce hydrochloric acid for these types of reactions we look at the oxidation number of the substances the rules for assigning these are as follows elements are always zero a monoatomic ion has an oxidation number equal to its formal charge hydrogen is usually plus one unless bound to a metal where it will be negative one oxygen is usually minus two but can be less if bound to itself halogens are usually minus one and lastly the sum of all the numbers in a compound must equal zero or if a polyatomic ion the charge on the ion so for something like h2s hydrogen will be plus one as we would expect and sulfur will therefore be minus two and that's what we need to know about redox reactions