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What is an Oxidation Number?
Before we consider what an oxidation number is, it is worth recalling what an ion is, and what a charge number is.
Consider an atom of oxygen, O, which has 6 valence electrons.
It has a strong tendency to pull 2 electrons towards itself in order to complete its octet of electrons (it is said to be electronegative).
If the oxygen atom gains 2 electrons it forms an oxide ion, O2-, with a charge of 2- (charge number is 2-).
This happens when our oxygen atom ecounters an atom that is much, much less electronegative (that is, an atom that is more electropositive), such as an atom of magnesium, Mg.
If we react oxygen gas and magnesium metal together in an oxidation reaction then the oxygen atom pulls off 2 of the magnesium atom's electrons forming the magnesium ion, Mg2+, and the oxide ion, O2-.
Together they make up the compound magnesium oxide in which the ratio of Mg2+ to O2- is 1:1 so the compound can be represented as Mg2+O2- but is usually given as a "molecular" formula, MgO.
Note that the compound, Mg2+O2- or MgO, has no overall net charge because the number of positive charges is equal to the number of negative charges in the compound.
The charge number of the compound is 0.
But what happens if our oxygen atom encounters another atom that is not capable of pulling the electrons off the oxygen atom?
An atom like nitrogen for example.
Imagine that we could react nitrogen gas and oxygen gas together in an oxidation reaction to produce new compounds which are oxides of nitrogen.
These new compounds are all covalent, no ions are present.
These new compounds are all electrically neutral, there is no net overall charge, the charge number of each compound is 0.
Consider these possible compounds:
- NO (nitrogen monooxide)
- NO2 (nitrogen dioxide)
Nitrogen monooxide (NO) is the least oxidised form of nitrogen, it contains the lowest number of oxygen atoms. We can say that the nitrogen atom in NO is in a lower state of oxidation, a lower oxidation state.
Nitrogen dioxide (NO2) is the most oxidised form of nitrogen, it contains the greatest number of oxygen atoms. We can say that the nitrogen atom in NO2 is in the highest state of oxidation, the highest oxidation state.
But wait, what about N2O (dinitrogen oxide)? Is the nitrogen atom in N2O less oxidised than NO or more oxidised?
In order to answer that question we will use the concept of an oxidation number to indicate the oxidation state (state of oxidation) of each atom of nitrogen in each molecule above.
An oxidation number is kind of "account keeping", helping us to understand how electrons (negative charges) are distributed within a molecule.
We start by saying, "Let's pretend that each of these covalent compounds with no net overall charge is actually ionic, that is, in each compound we will pretend that the oxide ion, O2-, is present.
Then we can work out what the charge on each of these pretend 'nitrogen ions' would be".
In this scenario, NO is an electrically neutral molecule, it has zero overall charge, charge number = 0
An oxide ion, O2- has a charge number of 2-
So what "charge" must there be on a "nitrogen ion" to balance the charge on the oxide ion?
Let's call the unknown charge x.
For the molecule to have a net charge of 0 the sum of the charges on all the "anions" and "cations" must be 0
0 = charge on oxide ion + charge on "nitrogen ion"
Substitute 2- for the charge on the oxide ion:
0 = 2- + x
Add 2+ to both sides of the equation:
0 + 2+ = (
2+ + 2-) + x
2+ = x
If the nitrogen atom carried a charge then the charge would be 2+.
But this is NOT an ionic compound, so the nitrogen atom DOES NOT carry a charge.
Instead we refer to the nitrogen atom as being in an oxidised state (NOT a charged state), and we refer to the "apparent charge" on the atom as its oxidation number.
Now clearly we can't use 2+ to indicate the oxidation number because this would indicate the charge number on an ion, so, we use either:
- a Roman numeral (no sign required for a positive oxidation state, minus sign, -, required before the Roman numeral for a negative oxidation number)
- an Arabic number (+ or - sign located before the number)
Some Roman numerals and their corresponding Arabic numbers are given in the table below:
| Arabic number
| Roman numeral
We need to convert the "charge" on the nitrogen atom calculated above (2+) to an oxidation number:
- Roman numeral II (no sign is required for a positive oxidation number)
- Arabic number: + sign precedes the number, that is, +2
The oxidation number of nitrogen in NO is II (or, alternatively, +2)
Since the compound NO is NOT ionic, we should never have referred to the oxygen atom in the molecule as an "oxide ion" with a charge number of 2-
But we CAN refer to each oxygen atom in the compound as having an oxidation number of -II (or, alternatively, -2).
Let's calculate the oxidation number of nitrogen in the most oxidised form, NO2.
NO2 is a covalent molecule with no overall net charge, charge number = 0
Let each oxygen atom have an oxidation number of -II (we will use -2 in the calculations)
Let x = oxidation number of the nitrogen atom
Write an expression for calculating the oxidation number of the nitrogen atom:
charge number of molecule = total of the oxidation numbers for oxygen + total of the oxidation numbers for nitrogen
Since there are 2 oxygen atoms and 1 nitrogen atom in the molecular formula:
charge number of molecule = 2 × oxygen's oxidation number + nitrogen's oxidation number
Substitue the oxidation numbers into the equation:
0 = (2 × -2) + x
0 = -4 + x
Add +4 to both sides of the equation:
+4 + 0 =
+4 +-4 + x
+4 = x
The oxidation number of nitrogen in NO2 is IV (or, alternatively, +4)
Note that NO is a less oxidised form of nitrogen than NO2, because :
- it contains less oxygen per atom of nitrogen
(so nitrogen's valence electrons experience less pull from the oxygen atoms)
- nitrogen has a lower oxidation number than in NO2
Now, what about N2O?
The oxidation number of nitrogen in N2O is I (or, alternatively, +1), which is less than the oxidation number of nitrogen in NO (which is II, or, +2), so N2O is a less oxidised form of nitrogen than NO.
You will see two different methods used to indicate the oxidation state of an atom in a compound as set out below:
- IUPAC recommended oxidation numbers use Roman numerals.
(a) If the oxidation state is positive, it is indicated by an oxidation number using the Roman numeral ONLY (no + sign), for example, I, II
(b) If the oxidation state is negative, it is indicated by an oxidation number using a minus sign (-) immediately before the Roman numeral, for example, -I, -II
(c) If the oxidation state is zero, it is indicated by an oxidation number using the arabic number 0
- Alternative method for indicating the oxidation state of an atom uses signs and arabic numbers for the oxidation number (it is easier to use these Arabic numbers in calculations):
(a) If the oxidation state is positive it is indicated by an oxidation number using a plus sign (+) immediately before the arabic number, eg, +1, +2
(b) If the oxidation state is negative it is indicated by an oxidation number using a minus sign (-) immediately before the arabic number, for example, -1, -2
(c) If the oxidation state is zero it is indicated by an oxidation number using the arabic number 0
We can use the concept of an oxidation number even when there is no oxygen present in a molecule!
We can apply the concept of oxidation numbers to the atoms in hydrides, fluorides, chlorides, bromides, iodides, nitrides, sulfides, phosphides, etc
But before we can do this, we will need to learn the rules for assigning oxidation numbers ...