What are oxidation numbers?
An oxidation number or oxidation state is a number assigned to each atom or ion in a compound to show its degree of oxidation.
Oxidation numbers can be positive, negative or zero. It is important to note that the + or – sign should always be included. The higher the oxidation number of an atom or ion the more oxidised it is and the lower the oxidation number, the more that an atom or ion is reduced.
How oxidation numbers are deduced
To deduce the oxidation number of a chemical species we follow the following rules:
- The oxidation number of any free element is always zero. For example, the oxidation number of each atom in H2, S8, P4, Cl2 and Fe is zero.
- usually in many compounds, atoms or ions have fixed oxidation numbers that corresponds to their groups on the periodic table. Such as an oxidation state of :
- +1 for all Group 1 elements
- +2 for all Group 2 elements
- -1 for all Group 7 elements
- hydrogen always has an oxidation state of +1 (except in metal hydrides such as NaH, where it is –1)
- oxygen always has an oxidation state of –2 (except in peroxides, where it is –1, and in F2O, where it is +2).
- The oxidation number of an ion is the same as its charge. For example, Cl– is –1, Al3+ is +3 and Fe2+ is +2.
- The sum of the oxidation numbers in a compound is zero and the sum of the oxidation numbers in an ion is equal to the charge on the ion.
- In either a compound or an ion, a more electronegative element is given the negative oxidation number.
Examples on determining oxidation numbers
Compounds of a metal with a non-metal
When dealing with compounds of a metal and a non-metal, you just have to know that the
- metal always has the positive oxidation number because it forms positively charged ions. Oxidation number is equal to the charge on the ion.
- non-metal always has the negative oxidation number.
For example in sodium oxide, Na2O, the oxidation numbers corresponds to the charges on their respective ions. This means that the oxidation state of Na is +1 and of O is –2.
Compounds of a non-metal with a non-metal
When dealing with compounds containing two non-metals, the most electronegative element is given the negative oxidation number.
In sulphur dioxide, SO2:
- we know that the oxidation number of each O atom is –2. The oxidation number of oxygen is allowed to be negative as it is more electronegative than sulphur.
- The sum of the oxidation numbers in a compound is 0.
- Now if we take the oxidation number of sulphur to be x we can form an algebraic equation for SO2 as follows:
- x + 2(-2) = 0
- x – 4 = 0
- x = 4
- Therefore the oxidation state of sulphur in SO2 is +4.
In iodine trichloride, ICl3:
- chlorine is more electronegative than iodine, so the oxidation state of chlorine is –1 as expected, and the oxidation state of iodine is positive.
- Forming an equation for ICl3, and taking the oxidation number of iodine to be x we get:
- x + 3(-1) = 0
- x – 3 = 0
- x = 3
- Therefore the oxidation state of iodine in ICl3 is +3.
Compound ions are ions with two or more different atoms. For example are the sulphate ion, SO42–.
In the sulphate ion SO42–
- the sum of the oxidation states is -2. The sum of the oxidation in an ion is equal to the charge on the ion.
- oxygen is more electronegative, so it has a negative oxidation number, which is -2 as expected. The oxidation state of sulphur is x.
- Therefore, for SO42–
- x + 4(-2) = -2
- x – 8 = -2
- x = 8 – 2
- x = 6
- Therefore the oxidation number of sulphur in SO42– is +6.
In the nitrate ion, NO3–
- total oxidation number = -1
- oxygen is more electronegative and has an oxidation number of -2
- the oxidation number of nitrogen is x.
- Therefore, for NO3–
- x + 3(-2) = -1
- x – 6 = -1
- x = 6 – 1
- x = 5
- Therefore the oxidation number of nitrogen in NO3– is +5.