hello and welcome to the chemistry solution this tutorial is on assigning oxidation numbers oxidation numbers are a bookkeeping system used to keep track of electrons in a compound all bonded electrons are assigned to the more electronegative atom remember that oxidation numbers are not real charges on the atoms let's go over some rules for assigning oxidation numbers first the oxidation number for any lone element is zero some examples would be potassium or aluminum but the oxidation number is also zero even for molecules composed of all atoms of the same element like o 2 or s 8 oxidation numbers are usually assigned to the most electronegative elements first which means that the oxidation number assigned to fluorine is always negative 1 the oxidation number assigned to oxygen is always negative 2 except when in the compound of fluorine oxidation numbers are assigned based on the ion they would form using the group number on the periodic table so elements in group 1a and 2a would have oxidation numbers of plus 1 and plus 2 respectively and aluminum is assigned an oxidation number of plus 3 and generally the oxidation number for hydrogen is plus 1 if combined with nonmetals and minus 1 if combined with only metals and lastly the sum of the oxidation numbers of all the atoms must equal the overall charge on the compound let's look at some examples assign oxidation numbers for all of the elements in the following compounds looking at a first compound Oh F 2 we know that we need to assign the oxidation number for the most electronegative element first and that would be fluorine fluorine has an oxidation number of minus 1 but there are two fluorine atoms and so when assigning oxidation numbers I like to keep track by writing the oxidation state right below the atom in the compound and then multiplying it by the subscript on that element for a total of negative two now remember the some of the oxidation numbers must equal the overall charge on the compound so for of2 the sum of the oxidation numbers must be zero this means that oxygen must have an oxidation number of plus two because plus 2 and minus 2 equals zero so in this compound fluorine has an oxidation state of minus 1 and oxygen has an oxidation state of positive 2 moving to the next example again we assign the oxidation number for the most electronegative element first which means that oxygen has an oxidation number of minus 2 you could also determine this by looking at where oxygen is on the periodic table and remembering that oxygen normally forms ions with a charge of negative 2 again I write the oxidation number right below the atom in the compound and then I multiply it by the subscript on that element here we have three oxygen atoms each with an oxidation number of negative 2 giving us a total of negative 6 remember that the sum of the oxidation numbers for all of the atoms in the compound must equal the overall charge on that compound and in this case the overall charge on the compound is 0 which means that the sum of the oxidation numbers for iron must be equal to positive 6 now looking at this compound we have two iron atoms this means that the oxidation state on each iron atom must be +3 so plus 3 times 2 which is the subscript on iron gives us positive 6 and positive 6 plus the negative 6 which is the sum of the oxidation numbers on the oxygen equals 0 so in this case oxygen has an oxidation state of negative 2 and iron has an oxidation state of plus 3 looking at our third example remember that hydrogen normally has an oxidation number of plus 1 when combined with nonmetals when we multiply this oxidation number by the total number of atoms of hydrogen in the molecule this gives us a total of positive 4 remembering that the sum of the oxidation numbers must be equal to 0 for a neutral compound we know that the oxidation state for carbon must be negative 4 so each hydrogen atom has an oxidation state of plus 1 and the carbon atom has an oxidation state of minus 4 looking at our last example first we assign the oxidation number to our most electronegative element first this means oxygen has an oxidation state of negative 2 this makes the sum of the oxidation numbers on the three oxygen atoms minus 6 remember that the elements in Group 1 have an oxidation state of plus 1 this is equal to the charge of the ions that these elements typically form and remember that hydrogen has an oxidation state a positive 1 when bonded with nonmetals this means that the oxidation state for carbon must be positive 4 in order for the sum of all of the oxidation numbers in this neutral compound to be equal to 0 so oxygen has a max addition state of minus 2 sodium has an oxidation state of plus 1 hydrogen is also plus 1 and carbon is plus 4 thanks for watching the chemistry solution we hope you enjoyed this tutorial you