Oxidation numbers, also known as the oxidation state, is a simple record keeping concept. It's 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 stae 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 rationalistaion of chemical trends and behaviour. Oxidation numbers also allow for easy classification and naming of new compounds and chemical formulas. This entry serves as a brief introduction to the concept of oxidation states and numbers and how this can be related the naming of chemical compounds.

The Classification.

There are several basic rules to the finding of oxidation numbers. They are as follows.

1. The oxidation number of an uncombined atom is always zero.
2. The oxidation numbers of all the atoms in a neutral compound add up to zero.
3. The oxidation number of a monatomic ion is equal to the charge of the ion.
4. In binary compounds¹ the element with greater electronegativity² is assigned a negative oxidation number. The more electropositive element receives a positive oxidation number.
5. The oxidation number of oxygen is always -2.
6. The oxidation number of hydrogen is +1 when it is in a compound with a nonmetal. 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 a halide ligands, hydride, hydroxide (OH) and cyanide (CN) always have a charge of -1. Oxygen and suphlur as monatomic ligand 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, ie, 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 donate how many of these atoms, ions or groups are present, some of these prefixes are presented in the table below.

In ionic compounds containing metal ions, the name is formed of the name of the metal (with the positive oxidation number), followed by the name of the nonmetal (with the negative oxidation number) with the added suffix "-ide".

For example: HCl - Hydrogen Chloride,
NaCl - Sodium Chloride,
MgO - Mangnesium 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 with 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 it's highest oxidation state, the names for these ions tend to end with the suffux "-ate".

For example: CO₃^2- - Carbonate,
SO₄^2- - Sulphate (containing sulphur(VI) ),
NO₃^- - Nitrate,
PO₄^3- - Phosphate.

Other oxidation states may be available for the central atom, for example the sulphur(IV) ion SO₃^2- 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 same of the metal with it's 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" donating the lower common oxidation state and "-ic" donating 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 it's consitutent elements) the accuracy was not good enough to distinguish the mass of hydrogen, the lightest of the element, and so sodium bicarbonate appeared to be NaCO₃ have a carbonate:sodium ratio double that of Na₂CO₃, hence the prefix bi-. It is now more accurately named sodium hydrogen carbonate. This is also seen in the sulpahtes, sodium sulphate (Na₂SO₄) and sodium bisulphate (NaHSO₄)

So, there are several ways of naming compounds, by specifically donating the numbers of each element or by infering these relative quantities through the prefixes and suffixes and on known oxidation states of the remaining groups.

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