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Oxidation Number Method - Concept

Teacher/Instructor Jacqueline Spivey
Jacqueline Spivey

Ph.D.,U.C.Santa Cruz
Teaching at a top-ranked high school in SF

She teaches general and chemistry at a top-ranked high school in San Francisco. Prior to that, she lead and published a number of research studies and lectured at SF State University.

The oxidation number method, also called oxidation states, keeps track of electrons gained when a substance is reduced and the electrons lost when a substance is oxidized. Each atom in a neutral molecule or charged species is assigned an oxidation number. When there is an increase in the oxidation number, oxidation occurs. When there is a decrease in the oxidation number, reduction occurs. The sum of all the oxidation numbers in a compound is equal to its overall charge.

This segment lets go ahead and discuss the oxidation number method, so this will be really important when you're first learning how to do oxidation reduction reactions and as you go on through Chemistry so that you understand how the charges on different species kind of work. So oxidation number method is also referred to as oxidation states so you'll hear that come up repeatedly probably in your adventures with this topic. So what is the oxidation number method? Well it helps us keep track of the electrons that are gained when a substance is reduced and the electrons that are lost when the substance is oxidized. Remember that oxidation and reduction happen together they're oxidation reduction pairs if you will and I put OILRIG here as a reminder that oxidation is losing of electrons and reduction is gaining of electrons so this is always a handy tool to kind of write down and hope you keep track of what's happening.

So each atom in a neutral molecule or charged species is assigned what we call an oxidation number and that's the actual charge for a monatomic ion, remember a monatomic ion mono meaning just one so a singular ionic species. So the oxidation number of sodium atoms will change in a redox reaction alright that kind of make sense because there are electrons being transferred from one species to another so when oxidation occurs there's going to be an increase in the oxidation number. However, when reduction occurs there's going to be a decrease in the oxidation number again remember oxidation is losing of electrons so an increase in positive charge reduction is a gain of electrons so a decrease in positive charge.

So how do we actually assign oxidation numbers? So there are a few rules there's always exceptions to rules all of them that will have a chance to say here but these are the basics of what you need to note to get you started and increase your proficiency with this topic, so let's go ahead and see what they are. Understand that it will take you quite a bit of time before you become familiar and this feels like second nature for you so hopefully you won't get lost and bogged down in the details and just kind of be open to learning how to do it.

So the first one, is for an atom that's in it's elemental form, so basically the way you find it on the periodic table, it's going to have an oxidation number of 0 so for instance if we're talking about the diatomic chlorine gas Cl2 each chlorine atom in chlorine gas has an oxidation number of 0.

So for the second rule, for a monatomic ion the oxidation number is going to equal to charge on the ion, so for instance group I, your alkaline metals, when they, they're oxidized and so they're going to loose an electron to get a plus 1 charge. Group II they're going to loose 2 electrons to have a plus 2 charge. Your halogens, group VII, they're going to gain an electron to have a minus 1 charge.

So our third rule, is that nonmetals usually have negative oxidation numbers although they can sometimes be positive so overwhelming so they're going to have negative oxidation numbers. Remember your nonmetals are over to the right hand side of the periodic table, so oxygen for instance; usually it's minus 2 in both ionic and molecular compounds. The exception for that is a peroxide and peroxides are O2 2- so then hydrogen usually is a +1 when it's in cahoots with an nonmetal so if you have HCl for instance Cl is going to be -1 hydrogen is going to be +1. However, if you have hydrogen in cahoots with a metal say sodium, sodium is going be +1 hydrogen is going to be -1 we call that a hydride and then fluorine is -1.

And the last rule is that the sum of the oxidation numbers of all atoms in a neutral compound is 0 and that the sum of oxidation numbers in a polyatomic ion equals the charge of the ion so those, this last rule is probably going to be the most difficult to remember, so I put up a fairly complicated example of that just so that we can see how it kind of works so Cr2O7 chromium and oxygen with a 2 minus charge. Okay so we want to know what the charge on the chromium is so we know that the oxygens are going to have a 2- each right oxygen is 2- so it means we have the oxygen is contributing a charge of negative 14 but we know the overall charge on the polyatomic is 2 minus which means we have a charge of negative 12 that's unaccounted for so that means that if we have two chromiums that each of those chromiums must be a positive 6 charge in order to counter balance the minus 12 charge okay? So that's kind of how we learn how to work with the charges on polyatomics and to understand oxidation numbers.