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An oxidation number, also known as the oxidation state, is a simple record-keeping concept. Its predecessor was the much used valence number, but as chemical knowledge progressed, it became difficult to accurately pin down a system to help in remembering of formulas and chemical phenomena. This is now the purpose oxidation numbers serve. If a compound were purely ionic then the oxidation state is equal to the charge that the ions making up that compound would carry. Though this is inaccurate (since no compound can be classified as purely ionic or purely covalent, but somewhere between these extremes) the concept of the oxidation state still allows rationalisation of chemical trends and behaviour. Oxidation numbers also allow for easy classification and naming of new compounds and chemical formulae. This entry serves as a brief introduction to the concept of oxidation states and numbers, and how this can be related to the naming of chemical compounds.
There are several basic rules to the finding of oxidation numbers. They are as follows.
The oxidation number of an uncombined atom is always zero.
The oxidation numbers of all the atoms in a neutral compound add up to zero.
The oxidation number of a monatomic ion is equal to the charge of the ion.
The oxidation number of oxygen is always -2.
The oxidation number of hydrogen is +1 when it is in a compound with a non-metal. It is -1 when it is in a compound with a metal.
The Periodic Table
The oxidation numbers of any given elements can be found by their location on the periodic table. Each row of elements have common oxidation numbers, save in a few choice cases.
Main group elements:
- Group 1, the alkali metals: +1
- Group 2, alkaline earth metals: +2
- Group 12: +2 (occasionally +1)
- Group 13: +3 (+1 state more stable for heavier elements of this group)
- Group 14: +/- 4 for carbon, can be +2 and +4 for heavier elements
- Group 15: +3, +5
- Group 16, the chalcogens: -2 (+2, +4, +6 available for sulphur and below)
- Group 17, the halogens: -1 (also +3, +5, +7 available for heavier halides)
- Group 18: the noble gases have no oxidation numbers.
Transition metal elements:
- Group 3: +3
- Group 4: +2, +4
- Group 5: +2, +3, +4, +5
- Group 6: +2, +3, +4, +6
- Group 7: +2, +3, +4, +5, +6, +7
- Group 8: +2, +3, +4
- Group 9: +1, +3 (sometimes also +5 for iridium)
- Group 10: 0, +2 (also +4 for platinum)
- Group 11: +1, +2
The lanthanides and actinides generally have +3 oxidation states though some +2, +4 and +6 states of some elements are known.
In the chemistry of transition metal complexes, the oxidation number of the metal is defined as being the overall charge on the complex minus the charges of the ligand bonded to the metal. For example, by definition, halide ligands, hydride, hydroxide (OH) and cyanide (CN) always have a charge of -1. Oxygen and sulphur as monatomic ligands have charges of -2, nitrogen -3, while ligands bearing no charge include water, ammonia (NH3) and other amines and carbon monoxide (CO). In the complex [PtCl4]2-, the platinum has an oxidation state of +2 since the overall charge on the complex is -2 and there is a total ligand charge of -4, and -2 - (-4) = +2.
The Naming Of Compounds
If a given compound is multinuclear, that is, it consists of a single metal ion and a number of ions of another element then we use prefixes in front of the different parts of the name to denote how many of these atoms, ions or groups are present. Some of these prefixes are presented in the table below.
|Number of atoms,
ions or groups
In ionic compounds containing metal ions, the name is formed with the name of the metal (with the positive oxidation number), followed by the name of the non-metal (with the negative oxidation number) with the added suffix '-ide'.
For example: HCl - Hydrogen Chloride,
NaCl - Sodium Chloride,
MgO - Magnesium oxide,
AlCl3 - Aluminium trichloride,
NaH - Sodium hydride
We can also have more complex cations (positively charged) and anions (negatively charged) which are molecules themselves, but carry an overall charge. There are a range of molecular anions called 'oxo ions' which contain the atoms of an element bonded to several oxygen atoms. The overall charge on these ions is the sum of the oxidation number for the central atom and those for the oxygens present. If the central atom is in its highest oxidation state, the names for these ions tend to end with the suffix '-ate'.
For example: CO32- - Carbonate,
SO42- - Sulphate (containing sulphur(VI)),
NO3- - Nitrate,
PO43- - Phosphate.
Other oxidation states may be available for the central atom, for example the sulphur(IV) ion SO32- is called sulphite. There are also differences for the halogens, for example chlorine forms oxo anions in the +1 (ClO- - hypochlorite),+3 (ClO2- - chlorite), +5 (ClO3- - chlorate) and +7 (ClO4- - perchlorate) oxidation states. Other elements also form neutral compounds with the same partners, but in different oxidation states and sometimes it is sufficient to write the name of the metal with its oxidation in brackets, for example, tin(II) chloride (SnCl2) and tin(IV) chloride (SnCl4). These are also known as stannous chloride and stannic chloride. Here the prefix stann- is derived for the greek word for tin with the suffix -ous denoting the lower common oxidation state and -ic denoting the higher oxidation state. This is also seen for other metallic elements, for example, ferrous (iron(II)) and ferric (iron(III)), cuprous (copper(I)) and cupric (copper(II)).
Another example of the strange world of chemical naming is due to historical accidents. There two carbonates of sodium, sodium carbonate (Na2CO3) and sodium bicarbonate (NaHCO3 containing the ion HCO3-). In the first days of elemental analysis (a technique that characterises an compound based on the percentage by mass of its constituent elements) the accuracy was not good enough to distinguish the mass of hydrogen, the lightest of the elements, and so sodium bicarbonate appeared to have the formula NaCO3 and hence have a carbonate:sodium ratio double that of Na2CO3, hence the word bicarbonate. It is now more accurately named sodium hydrogen carbonate. This is also seen in the sulphates, sodium sulphate (Na2SO4) and sodium bisulphate (NaHSO4)
So, there are several ways of naming compounds, by specifically denoting the numbers of each element or by inferring these relative quantities through the prefixes and suffixes and on known oxidation states of the remaining groups.