We are going to look at some examples from vanadium chemistry. If you don't know anything about vanadium, it doesn't matter in the slightest.
Vanadium forms a number of different ions - for example, V2+and V3+. If you think about how these might be produced from vanadium metal, the 2+ ion will be formed by oxidising the metal by removing two electrons:
The vanadium is now said to be in an oxidation state of +2.
Removal of another electron gives the V3+ ion:
The vanadium now has an oxidation state of +3.
Removal of another electron gives a more unusual looking ion, VO2+.
The vanadium is now in an oxidation state of +4. Notice that the oxidation state isn't simply counting the charge on the ion (that was true for the first two cases but not for this one).
The positive oxidation state is counting the total number of electrons which have had to be removed - starting from the element.
It is also possible to remove a fifth electron to give another ion (easily confused with the one before!). The oxidation state of the vanadium is now +5.
Every time you oxidise the vanadium by removing another electron from it, its oxidation state increases by 1.
Fairly obviously, if you start adding electrons again the oxidation state will fall. You could eventually get back to the element vanadium which would have an oxidation state of zero.
What if you kept on adding electrons to the element? You can't actually do that with vanadium, but you can with an element like sulphur.
The sulphur has an oxidation state of -2.
Summary
Oxidation state shows the total number of electrons which have been removed from an element (a positive oxidation state) or added to an element (a negative oxidation state) to get to its present state.
Oxidation involves an increase in oxidation state
Reduction involves a decrease in oxidation state
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.
Working out oxidation states
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 Cl2 or S8, or whether it has a giant structure like carbon or silicon.
The sum of the oxidation states of all the atoms or ions in a neutral compound is zero.
The sum of the oxidation states of all the atoms in an ion is equal to the charge on the ion.
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.
Some elements almost always have the same oxidation states in their compounds:
element usual oxidation state exceptions
Group 1 metals always +1
Group 2 metals always +2
Oxygen usually -2 except in peroxides and F2O (see below)
Hydrogen usually +1 except in metal hydrides where it is -1 (see below)
Fluorine always -1
Chlorine usually -1 except in compounds with O or F (see below)
The reasons for the exceptions
Hydrogen in the metal hydrides
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, -1.
Alternatively, you can think of it that the sum of the oxidation states in a neutral compound is zero. Since Group 1 metals always have an oxidation state of +1 in their compounds, it follows that the hydrogen must have an oxidation state of -1 (+1 -1 = 0).
Oxygen in peroxides
Peroxides include hydrogen peroxide, H2O2. This is an electrically neutral compound and so the sum of the oxidation states of the hydrogen and oxygen must be zero.
Since each hydrogen has an oxidation state of +1, each oxygen must have an oxidation state of -1 to balance it.
Oxygen in F2O
The problem here is that oxygen isn't the most electronegative element. The fluorine is more electronegative and has an oxidation state of -1. In this case, the oxygen has an oxidation state of +2.
Chlorine in compounds with fluorine or oxygen
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.
Warning!
Don't get too bogged down in these exceptions. In most of the cases you will come across, they don
We are going to look at some examples from vanadium chemistry. If you don't know anything about vanadium, it doesn't matter in the slightest.Vanadium forms a number of different ions - for example, V2+and V3+. If you think about how these might be produced from vanadium metal, the 2+ ion will be formed by oxidising the metal by removing two electrons: The vanadium is now said to be in an oxidation state of +2.Removal of another electron gives the V3+ ion: The vanadium now has an oxidation state of +3.Removal of another electron gives a more unusual looking ion, VO2+. The vanadium is now in an oxidation state of +4. Notice that the oxidation state isn't simply counting the charge on the ion (that was true for the first two cases but not for this one).The positive oxidation state is counting the total number of electrons which have had to be removed - starting from the element.It is also possible to remove a fifth electron to give another ion (easily confused with the one before!). The oxidation state of the vanadium is now +5. Every time you oxidise the vanadium by removing another electron from it, its oxidation state increases by 1.Fairly obviously, if you start adding electrons again the oxidation state will fall. You could eventually get back to the element vanadium which would have an oxidation state of zero.What if you kept on adding electrons to the element? You can't actually do that with vanadium, but you can with an element like sulphur. The sulphur has an oxidation state of -2.SummaryOxidation state shows the total number of electrons which have been removed from an element (a positive oxidation state) or added to an element (a negative oxidation state) to get to its present state.Oxidation involves an increase in oxidation stateReduction involves a decrease in oxidation stateRecognising 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.Working out oxidation statesYou 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 Cl2 or S8, or whether it has a giant structure like carbon or silicon.The sum of the oxidation states of all the atoms or ions in a neutral compound is zero.The sum of the oxidation states of all the atoms in an ion is equal to the charge on the ion.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.Some elements almost always have the same oxidation states in their compounds:element usual oxidation state exceptionsGroup 1 metals always +1 Group 2 metals always +2 Oxygen usually -2 except in peroxides and F2O (see below)Hydrogen usually +1 except in metal hydrides where it is -1 (see below)Fluorine always -1 Chlorine usually -1 except in compounds with O or F (see below)The reasons for the exceptionsHydrogen in the metal hydridesMetal 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, -1.Alternatively, you can think of it that the sum of the oxidation states in a neutral compound is zero. Since Group 1 metals always have an oxidation state of +1 in their compounds, it follows that the hydrogen must have an oxidation state of -1 (+1 -1 = 0).Oxygen in peroxidesPeroxides include hydrogen peroxide, H2O2. This is an electrically neutral compound and so the sum of the oxidation states of the hydrogen and oxygen must be zero.Since each hydrogen has an oxidation state of +1, each oxygen must have an oxidation state of -1 to balance it.Oxygen in F2OThe problem here is that oxygen isn't the most electronegative element. The fluorine is more electronegative and has an oxidation state of -1. In this case, the oxygen has an oxidation state of +2.Chlorine in compounds with fluorine or oxygenThere 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.Warning!Don't get too bogged down in these exceptions. In most of the cases you will come across, they don
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