For certain purposes (e.g. nomenclature), chemists like to work out the charges that the atoms in a compound would have if all the bonds were completely polarized and the bonding was ionic. This can be done as follows:
Use electronegativities to decide which atoms will be positive and which negative, and see whether either element is one that is known to form ions of the appropriate sign, in melts or in solution, of only a single type. Consider, for example, carbon tetrachloride, CCl4. Chlorine is more electronegative than carbon. The only stable anion formed by chlorine is Cl-. The completely polarized form of CCl4 must therefore be C4+(Cl-)4.
Some elements having only one stable cation or anion are as follows:
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Element |
Cation |
Anion |
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Hydrogen |
H+ |
H– |
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Sodium |
Na+ |
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Potassium |
K+ |
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Magnesium |
Mg2+ |
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Calcium |
Ca2+ |
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Aluminium |
Al3+ |
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Fluorine |
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F– |
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Chlorine |
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Cl– |
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Bromine |
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Br– |
Elements forming more than one stable cation or anion can be used if the correct ion can be identified. For example, oxygen forms several anions. These are O2- (if oxides like Na2O are completely ionic) and anions containing O-O bonds (e.g. O22-). Compounds containing the latter give H2O2 with acid. If no H2O2 is formed, the appropriate ion is O2-. Similarly, sulfur forms S2- and anions containing S-S bonds (e.g. S22-). Compounds containing the latter give sulfur with acid. Iodine forms I- and I3-. Compounds containing I3- give brown solutions in water.
The charges on the ions when a compound is written in its hypothetical ionic form are called the “polar numbers” or “oxidation numbers” of the elements concerned. Thus the polar number of carbon in CCl4 is +4 and of chlorine –1. “Polar number” is the original term and somewhat more expressive than “oxidation number”: but the latter is now used almost universally.
What are the oxidation numbers of the elements in each of the following compounds?
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(i) |
Methane, CH4. |
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(ii) |
Iron dichloride, FeCl2. |
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(iii) |
Sodium hydride, NaH. |
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(iv) |
Silicon dioxide, SiO2. |
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(v) |
Carbonyl chloride, COCl2. |
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(vi) |
Hydroxylamine, NH2OH. |
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(vii) |
Hydrogen peroxide, H2O2. |
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(viii) |
Silicon carbide, SiC. |
Binary compounds are named by citing the name of the electropositive constituent first, unmodified, followed by the name of the electronegative constituent, modified to end in -ide. For this purpose, an agreed series is used, based on the electrochemical series given above.
Examples: sodium chloride
hydrogen sulfide
In a small number of cases, when the name of the element has to be modified, the Latin name is used.
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Copper |
Cuprum |
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Gold |
Aurum |
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Iron |
Ferrum |
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Lead |
Plumbum |
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Silver |
Argentum |
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Tin |
Stannum |
Example: sodium plumbide
The proportions of the constituents may be indicated in one of the following ways:
(i) By means of Greek numerical prefixes (mono, di, tri, tetra, penta, hexa, etc.). The prefix mono is omitted except where confusion would arise.
Example: iron dichloride
(ii) By means of the oxidation numbers of the elements where these are not obvious, indicated by a Roman numeral placed immediately following the name of the element (without a space).
Example: iron(II) chloride
(Note that when an oxidation number is used with the symbol for an atom of an element, it is placed as a superscript without brackets, e.g. FeII.)
(iii) In the case of genuinely ionic compounds, the proportions may be indicated by means of the charges on the ions where these are not obvious, indicated by an Arabic numeral followed by the sign of the charge placed in parentheses immediately following the name of the ion (without a space).
Example: iron(2+) chloride
An older method that was used to indicate the proportions when a metal forms two series of compounds, based on two different cations (e.g. Fe2+ and Fe3+), was to alter the name of the metal to end in -ous for the lower series and -ic for the higher, e.g.
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FeCl2 |
ferrous chloride |
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FeCl3 |
ferric chloride |
This method has the merit of elegance, but is now disfavoured.
The nomenclature given above is that recommended by the Commission on the Nomenclature of Inorganic Chemistry of the International Union of Pure and Applied Chemistry (IUPAC). This is set out in:
G.J. Leigh (editor), Nomenclature of Inorganic Chemistry: Recommendations 1990, Blackwell, Oxford, 1990.