How do oxidation numbers work

Recognising this simple pattern is the single most important thing about the concept of oxidation states. If you know how the oxidation state of an element changes during a reaction, you can instantly tell whether it is being oxidised or reduced without having to work in terms of electron-half-equations and electron transfers.

You don't work out oxidation states by counting the numbers of electrons transferred. It would take far too long. Instead you learn some simple rules, and do some very simple sums! The oxidation state of an uncombined element is zero. That's obviously so, because it hasn't been either oxidised or reduced yet!

This applies whatever the structure of the element - whether it is, for example, Xe or Cl 2 or S 8 , or whether it has a giant structure like carbon or silicon. The more electronegative element in a substance is given a negative oxidation state. The less electronegative one is given a positive oxidation state.

Remember that fluorine is the most electronegative element with oxygen second. Note: It has been pointed out to me that there are a handful of obscure compounds of the elements sodium to caesium where the metal forms a negative ion - for example, Na -.

That would give an oxidation state of You can ignore these if you are doing chemistry at A level or its equivalent. If you are interested in these odd compounds, do an internet search for alkalides. Metal hydrides include compounds like sodium hydride, NaH.

In this, the hydrogen is present as a hydride ion, H -. The oxidation state of a simple ion like hydride is equal to the charge on the ion - in this case, Alternatively, you can think of it that the sum of the oxidation states in a neutral compound is zero.

Peroxides include hydrogen peroxide, H 2 O 2. This is an electrically neutral compound and so the sum of the oxidation states of the hydrogen and oxygen must be zero.

The problem here is that oxygen isn't the most electronegative element. The fluorine is more electronegative and has an oxidation state of There are so many different oxidation states that chlorine can have in these, that it is safer to simply remember that the chlorine doesn't have an oxidation state of -1 in them, and work out its actual oxidation state when you need it.

You will find an example of this below. That's easy! What is the oxidation state of chromium in CrCl 3? This is a neutral compound so the sum of the oxidation states is zero. Chlorine has an oxidation state of If the oxidation state of chromium is n :. This is an ion and so the sum of the oxidation states is equal to the charge on the ion. There is a short-cut for working out oxidation states in complex ions like this where the metal atom is surrounded by electrically neutral molecules like water or ammonia.

The sum of the oxidation states in the attached neutral molecule must be zero. That means that you can ignore them when you do the sum. What is the oxidation state of chromium in the dichromate ion, Cr 2 O 7 2-? The oxidation state of the oxygen is -2, and the sum of the oxidation states is equal to the charge on the ion.

Don't forget that there are 2 chromium atoms present. Warning: Because these are simple sums it is tempting to try to do them in your head. Using this scheme in a consistent manner one can balance equations quite nicely. On the other hand, there are times when the formal charge is more important. In this scheme one always splits the electrons in any bond between the two atoms. Thus in water all the atoms have a formal charge of zero.

One can think of these two schemes as follows: oxidation numbers pretend that all bonds are in effect totally ionic with the electrons going to the more electronegative atom. Formal charge pretends that all bonds are covalent and the electrons in a bond are shared equally. Of course neither one is totally correct, but either one can be used as an "electron accounting" scheme. Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group.

Create a free Team What is Teams? Learn more. Why do oxidation numbers work? Ask Question. This is a good candidate for this method of algebraic oxidation number determination.

Find the known oxidation number for the other elements in the compound. Using the rules for oxidation number assignment, assign oxidation numbers to the other atoms in the compound. Be on the lookout for any exceptional cases for O, H, etc. Multiply the number of each atom by its oxidation number. Now that we know the oxidation number of all of our atoms except for the unknown one, we need to account for the fact that some of these atoms may appear more than once.

Multiply each atom's numeric coefficient written in subscript after the atom's chemical symbol in the compound by its oxidation number. Add the results together. Adding the results of your multiplications together gives the compound's current oxidation number without taking into account the oxidation number of your unknown atom. Calculate the unknown oxidation number based on the compound's charge.

You now have everything you need to find your unknown oxidation number using simple algebra. Set an equation that has your answer from the previous step plus the unknown oxidation number equal to the compound's overall charge.

S has an oxidation number of 6 in Na 2 SO 4. We know Oxygen generally shows a oxidation number of Also Cl has an oxidation number of Let the oxidation number of S be X. Now the overall charge is 0. Not Helpful 44 Helpful In normal cases, O has a oxidation number of But in OF2 , F is more electronegative than O. Not Helpful 20 Helpful What is the relation between the oxidation number and valency in case of s-block metals? Oxidation is the ionic number of an element, while valency is the number that shows the relationship of chemical properties of elements in the same group, so the oxidation number of an element is also the number of valence electrons.

Not Helpful 8 Helpful In the case of a molecule, you have to see how many electrons each element needs to fill its shell. For example, NaCl. Sodium has one valence electron that it wants to give away to drop down to its complete the 8 electron shell. Chlorine wants to get one electron to complete its shell since it has 7 Ve- and needs one to get the complete 8.

Not Helpful 38 Helpful There is no formula -- it's a technique. You just need to find the unknown value. It's the same idea as solving equations with one variable. Not Helpful 29 Helpful A decrease in oxidation number means reduction, and an increase means oxidation. Not Helpful 25 Helpful Normally, O2 when it combines with other element it will acquire the -2 oxidation state but in the case of H2O2, oxygen is having an oxidation number of Why is this different?

Because the overall charge of the compound is zero. When you solve that you get -1 as an answer for X, the oxidation number of oxygen for this compound. How do oxidation numbers relate to valence electrons? How do oxidation numbers vary with the periodic table? What is the oxidation number for sulfur? What is the oxidation number for carbon? What is the oxidation number for copper? What is the oxidation number for nitrogen? What is the oxidation number for oxygen?