Oxidation Numbers and How to Find Them: Complete Guide

in this video we're going to go over oxidation numbers and how to find them so let's say

if we're given the element zinc what is the oxidation number

of zinc now the first rule that you need to know is that the oxidation state of any pure

element is always zero so the oxidation state of oxygen gas as

a pure element is zero fluorine gas as the pure element is zero even uh phosphorus

as a pure element is zero so if there's no charges and it's only one pure element it's not a compound the

oxidation state will always be zero so that's the first rule we need to keep in mind

now the second thing is the oxidation state of ions

the oxidation state of the zinc 2 plus ion is basically the charge

of what you see there it's positive 2. the oxidation state of

the fe plus 3 ion is simply positive 3. now sometimes you might have diatomic

ions for example the mercury 2 plus ion individually

each mercury ion has an oxidation state of one because there's two of them so you need to write

an equation two mercury atoms has a net charge of positive two so if you divide both sides by two

you can get the individual oxidation state of each mercury particle which is plus one

so here's another example this is the peroxide ion to find the oxidation state of each

oxygen atom in this ion you could write an equation as two oxygen atoms with a total charge of

negative two so individually each oxygen atom has a charge of minus

one so that's the oxidation state of oxygen individually in the peroxide ion this is the superoxide ion so if you

want to find the oxidation state you need to divide the total charge by two

so each oxygen atom has a net charge of negative one half so two of them combined will have a net

charge of negative one so keep this in mind anytime you have a pure element

the oxidation state will always be zero and if you have an ion

let's say if it's a monoatomic ion the oxidation state is the same as that ion

now let's talk about compounds whenever you have fluorine inside a compound when it's not a pure element

fluorine is always going to have a negative one oxidation state fluorine is the most

electronegative element when oxygen is in a compound it's going to have a negative 2

oxidation state unless it's bonded to fluorine or unless you

hear the name peroxide or superoxide whenever you hear the name peroxide oxygen has

a negative one oxidation state if you hear the word superoxide it has a negative one-half oxidation state but if

you hear the word oxide then the oxidation state is negative two which is

99 of the time now hydrogen will have an oxidation state of plus one

when bonded to a non-metal when bonded to a metal hydrogen will have an oxidation state of

negative one and really the key is electronegativity hydrogen is more electronegative than

most metals that's why it bears a negative charge but hydrogen is usually less

electronegative than most non-metals and so that's why it bears a positive

charge so typically the element that's more electronegative is the one that usually

carries the negative charge now let's work on some examples

what is the oxidation state of magnesium and chlorine in this compound by the way most halogens

are usually negative one chlorine typically has a negative one charge like fluorine

if we write an equation mg plus 2cl this whole compound is neutral so therefore the total charge is 0.

now if chlorine has a negative one oxidation state that means magnesium

has to have a positive two oxidation state you can literally solve it and it makes sense magnesium

is an alkaline earth metal which typically has a positive two charge go ahead and find the oxidation states

of aluminum and fluorine in this example well we know that fluorine is negative one in a compound always

and aluminum based on where it's located in a periodic table

it's typically positive three within an ionic compound and you could solve it too a out plus three f

should add up to zero because the net charge is zero so each fluorine atom has an oxidation

state of negative one so now we gotta add three to both sides so aluminum has an oxidation state of

positive 3. here's another example find the oxidation state of vanadium and

oxygen in this compound so this is called vanadium oxide so whenever you hear the word oxide

oxygen has a negative two charge so we got two vanadium atoms plus five

oxygen atoms with a net charge of zero so each oxygen atom has an oxidation state of negative two

five times negative two is negative ten and then add ten to both sides so 2v is equal to 10.

next divide both sides by 2. so 10 divided by 2 is 5.

and so the oxidation state of vanadium is positive 5. now let's go over some examples

containing polyatomic ions consider sulfate what is the oxidation state of sulfur

and sulfate we know oxygen is usually negative two so let's write an equation sulfur

plus four oxygen atoms has a net charge of negative two

so each oxygen atom has an oxidation state of minus two and four times negative two that's

negative eight next we need to add eight to both sides negative two plus eight is positive six

so this is the oxidation state of sulfur and sulfate let's look at another example

phosphate go ahead and find the oxidation state of phosphorus and phosphate

so once again oxygen is still negative two so we got a phosphorus atom plus four

oxygen atoms and the net charge is negative three based on what we see here

so it's going to be p plus four times negative two and four times negative two is negative

eight and then add eight to both sides so negative three plus eight

that's going to be positive five and that's the oxidation state of phosphorus

let's look at another example let's try nitrate

and also chlorate as perchlorate go ahead and find the oxidation state of

nitrogen and chlorine in these two polyatomic ions so we have a nitrogen three oxygen atoms

and that's going to equal a net charge of negative one so o is negative two three times

negative two is going to be negative six and negative one plus six if we add six to both sides

that's going to be positive 5. so that's the oxidation state of nitrogen

and for chlorine in perchlorate it's going to be cl plus 4 oxygens

equals a net charge of negative 1. so this is gonna be four times negative two

which is a negative eight and then negative one plus eight that's going to give us an oxidation

state of positive seven so now you know how to find the oxidation states

of elements within compounds and polyatomic ions now i want you to understand the concept

of electronegativity and how it relates to oxidation numbers

electronegativity increases towards fluorine on a periodic table

so as you go up and to the right the electronegativity increases so let me give you some values of common

elements so let's say hydrogen is somewhere in the corner over there and then we have

boron carbon nitrogen oxygen fluorine

chlorine bromine iodine

phosphorus and sulfur hydrogen has an electronegativity value of 2.1

for boron is 2.0 carbon is 2.5 and then 3.0 3.5 is the highest it's 4.0

phosphorus it's 2.1 it's the same as hydrogen so first 2.5

chlorine is 3.0 and this is 2.8 iodine is 2.5

so keep these values in mind so here's a question for you what is the oxidation state

of oxygen and fluorine in oxygen difluoride

now oxygen has an electronegativity value of 3.5 fluorine is 4.0 so which one is more

electronegative electronegativity is the ability of an atom to attract electrons to itself

so fluorine is going to pull on the electrons in this molecule it's going to have a stronger pull

than oxygen so fluorine is going to acquire a partial negative charge

whereas oxygen is therefore going to acquire a partial positive

charge because fluorine pulls on the electron stronger than oxygen can so in this example

oxygen will not have its typical charge of negative two

the only time oxygen will have its oxidation state of negative two is if it's the most electronegative

element in that compound if it's not then it's going to have a positive oxidation state

keep in mind any time fluorine is in a compound it has

an oxidation state of negative one and the reason for that is because fluorine is the most electronegative element on a

periodic table so now we can solve for oxygen so o plus two f

should have a net charge of zero because there's no number here so fluorine

is negative one two times negative one is negative two so if we add two to both sides

oxygen is going to equal positive two which makes sense because it's partially positive

in this particular example now let's look at two other examples hydrochloric acid

and sodium hydride chlorine has an electronegativity value of 3.0

hydrogen is 2.1 and sodium

it's like one point something i'm not sure what the exact number is it could be like

1.7 but i know it's less than two so in this example hydrogen bears a partial positive charge

chlorine bears a negative charge because chlorine is more electronegative than hydrogen

so therefore chlorine is going to have its oxidation state of negative one

which is typical of most halogens hydrogen is going to have an oxidation state of plus one

as you mentioned before whenever hydrogen is bonded to a non-metal the oxidation state is usually

positive one now what about in sodium hydride well we know that sodium is an alkali

metal which always have a positive one charge so therefore sodium is going to have an

oxidation state of plus one but hydrogen has an oxidation state of negative one typically when hydrogen is

bonded to a metal it usually has a negative one oxidation state and it

makes sense because hydrogen is more electronegative than most metals so it usually bears the

partial negative charge that's why it has a negative oxidation state sodium

has the positive charge so it has a positive oxidation state and so you could use electronegativity

to help you determine what the oxidation state will be

so let me give you another example bh3 what is the oxidation state of boron and

hydrogen feel free to try that one now hydrogen has an electronegativity

value of 2.1 and boron is 2.0 now is boron a metal or non-metal

in this example hydrogen is more electronegative so hydrogen bears

the partial negative charge or boron bears the partial positive charge so therefore hydrogen it's going to have

its oxidation state of negative one because it's more electronegative than boron

so then this is gonna be b plus three h which is equal to zero so three times negative one is negative

three so boron is going to have an oxidation state of positive three in this example

now let's consider these two examples sulfuric acid or rather hydrosulfuric acid and also sulfur dioxide

now hydrogen has an en value of 2.1 sulfur is 2.5

and oxygen is 3.5 so in sulfur dioxide oxygen has the partial negative charge

sulfur has the partial positive charge now in h2s hydrogen has the partial positive charge

sulfur has the partial negative charge now in a periodic table when you have elements like nitrogen oxygen fluorine

typically nitrogen has a negative three charge oxygen minus two fluorine negative one

sulfur two sulfur usually has a negative two charge if if sulfur is the more electronegative

element so looking at h2s hydrogen is bonded to a non-metal that

is more electronegative than itself so hydrogen is going to have the positive one oxidation state and there's

two of them so sulfur in this example has its normal oxidation state of negative two

so you can base your answer on a periodic table if sulfur is the more electronegative

element now in so2 you can't do that because sulfur doesn't have the partial negative

charge so you can't base the charge on a periodic table you can do so however for oxygen

because oxygen is the electronegative element in that compound so you can use the negative two

charge for oxygen so oxygen is going to have an oxidation state of minus two

and to find it for sulfur it's going to be s plus two oxygen atoms equals zero

so that's two times negative two which is negative 4 so sulfur is going to have

a positive oxidation state of 4 due to the positive partial charge so elements that are less

electronegative typically those are the ones you got to solve for the ones that are more electronegative

you can find a charge based on a periodic table if they carry a negative charge

now let's look at some other examples nh3 and no2

go ahead and find the oxidation state of each element now in ammonia

hydrogen has an en value of 2.1 but nitrogen is more electronegative it's 3.0

so therefore nitrogen should have its normal charge of negative three if we write an equation n plus three h

is equal to zero hydrogen is going to have a positive one charge

it's partially positive whereas nitrogen is partially negative so typically when hydrogen's bonded to a

non-metal it's usually plus one which means n has to be negative three so as you can see nitrogen is the

electronegative element in this example and it has its periodic charge of negative three which you could find in a

periodic table now in this case o is more electronegative so

nitrogen is going to have a different oxidation state it's not going to be its natural oxidation state of negative 3.

so in this example it's going to be positive 4.

typically when you have elements like nitrogen sulfur phosphorus

if they carry an element that's more electronegative than itself those are the elements you gotta solve

for the one that usually has a partial positive charge

now try these two examples methane and carbon dioxide

in methane hydrogen has a positive one charge hydrogen is less electronegative than

carbon so it's going to be partially positive

carbon is going to be partially negative so solving for carbon we have c plus four h is equal to zero

so that's four times one so c is negative four so when carbon is bonded to hydrogen carbon has

a negative oxidation state but when carbon is bonded to oxygen it's going to have a positive oxidation

state when it's bonded to hydrogen it has a negative oxidation state so oxygen is negative two and there's

two of them so carbon is going to have to be positive four in this example now sometimes

you might have elements that have an average oxidation state that's not a whole number

let's try these two c3h8 and fe3o4 in this example hydrogen is less

electronegative than carbon so it's going to be positive 1. so if we write the formula 3c plus 8h

is equal to 0. so that's going to be 8 times 1 and if we subtract 8 from both sides 3c

is equal to negative 8. so carbon on average has an oxidation state of negative 8 over 3.

now let's do the same thing for fe3o4 so we got three iron atoms and four oxygen atoms with a net charge of zero

so oxygen has an oxidation state of negative two so four times negative two that's

negative eight and if we add eight to both sides

we get this so f e has an oxidation state of eight over

three so eight over three is about two point sixty seven

now keep in mind an individual iron atom cannot have a charge of 2.67

it's usually a whole number like positive 2 or positive 3. because electrons and protons

they're they basically have numerical charges an electron has a

charge of negative one a proton has a charge of positive one so a typical ion won't have a decimal

charge so what does it mean that the average oxidation state is 2.67

so what is meant by that in this compound there are three

iron ions and four oxygen ions

each oxygen has a charge of negative two so the total negative charge is negative eight

in order for the compound to be electrically neutral the total positive charge

has to be positive eight iron metal has two common oxidation states

positive two and positive three now they all can't be positive three

because 3 plus 3 plus 3 is not and they can't all be positive 2 because 2 plus 2 plus 2 is 6.

so some of them is positive 2 and some are positive 3. so the question is how many

iron ions have a plus two charge and how many have a plus three charge in order to get up to eight

two of them has to have a positive three charge and one of them has to have a positive two

charge if you average the numbers 2

3 and 3 and divided by 3 that's going to be 8 over 3 which averages out to 2.67

so whenever you get a decimal value what it really means is that that's the average oxidation state individually

some were positive three and some or positive two so the individual

ions should have a numerical oxidation state but when you have multiple of them the average could be a decimal value

because these they don't all have to be the same they can be different

so hopefully this makes sense in terms of why some oxidation states have a decimal value

now let's try polyatomic ions that have three different elements in it go ahead and find the oxidation state

of every element in that polyatomic ion so oxygen has an oxidation state of negative two

hydrogen is positive one when it's bonded to non-metals usually so all we gotta do is

find sulfur so h plus s plus three oxygen atoms

has a net charge of negative one so hydrogen is one oxygen is negative two

and so three times negative two that's negative six

and then one plus negative six is negative five so now let's add five

to both sides negative one plus five is positive four so in this example

sulfur has an oxidation state of positive 4. go ahead and try this one k2

cro4 find the oxidation state of every element in that example

so we have two potassium atoms a chromium atom and four oxygen atoms now we know oxygen is going to have

an oxidation number of negative two potassium is an alkali metal which all of them

have a positive one charge chromium is the transition metal and it has a variable charge so that's the one we got

to solve for so this is gonna be two times one plus cr

plus four times negative two and all of that is equal to zero so four times negative two that's

negative eight and two plus negative eight is negative six

so therefore in this example chromium has an oxidation state of positive six try this one potassium bicarbonate

find the oxidation state of carbon in this example so we know oxygen is going to be

negative two potassium and alkaline metal is plus one and hydrogen

hydrogen is actually bonded to the oxygen in bicarbonate if you were to draw the lewis structure

so therefore hydrogen's bonded to a non-metal do a covalent bond

and so it's going to be plus one so we have k plus h

plus c plus three o and that's equal to zero

so k is positive one hydrogen is one an oxygen is going to be three

well oxygen is negative two but we gotta multiply that by three so one plus one is two

three times negative two is negative six and then two plus negative six that's negative four

so in this example carbon is positive for potassium bicarbonate

you could break it up into two ions k plus and hco3 minus

so just by looking at k plus that tells you that k has an oxidation state of positive one

now bicarbonate is basically the sum of the hydrogen ion

and the carbonate ion so therefore you can see that hydrogen in this example

also has a positive one charge and then from this you could find the oxidation state of carbon you could say

c plus three o has a net charge of negative two and then you have to add six to both

sides so negative two plus six is positive four so if you understand

the ions and all the polyatomic ions you could break it down individually to

see that hydrogen has a positive one charge in this example

and the same is true for king so that's why it's good to know the polyatomic ion sheet

now i have two more examples for you br cl3

and ibr5 find the oxidation state of every element in this example

so most halogens like fluorine chlorine bromine iodine they typically have a negative one

charge but both bromine and chlorine can't be negative so which one is negative and

which one is positive keep in mind bromine has an electronegativity value of 2.8

chlorine is 3.0 iodine is 2.5 so in this example

chlorine bears the partial negative charge bromine is partially positive

so therefore chlorine is going to have its natural oxidation state of negative one

bromine we need to calculate it so it's going to be br plus three cl

and that's equal to zero so this is going to be three times negative one and so we can see that

bromine has an oxidation state of positive three now in the second example

bromine is going to carry the partial negative charge iodine carries the partial positive

charge if it's incorrectly usually

the electro positive element is written first the electronegative element is written second

so the one that you see on the right side is usually the one that carries the

natural charge that can be found on the periodic table so in this case

bromine is going to have its natural oxidation state of negative one so iodine is going to have an oxidation

state of positive 5 in this example so hopefully you understand the

relationship between electronegativity and oxidation numbers so that's it for this video thanks for

watching and have a good day you