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A nomenclature, or naming system, is important when the oxidation number of elements within compounds may vary, causing the formula to be different. |
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Roman numerals
As we have seen, some chemical elements have a variable oxidation number. The only way to know an element's oxidation number is to work it out from known oxidation numbers. To avoid confusion in the name, elements with variable oxidation numbers have the number included.
The oxidation state or number of any element within a compound is shown by a Roman numeral immediately after the element in question. Roman numerals are used to avoid confusion. The numerals are only used in the names of the compounds, NOT the formulae.
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Number
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Roman Numeral
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| 1 | I |
| 2 | II |
| 3 | III |
| 4 | IV |
| 5 | V |
| 6 | VI |
| 7 | VII |
| 8 | VIII |
Examples
- copper(II) oxide
- copper(I) oxide
- Sodium sulfate(VI)
- Sodium sulfate(IV)
Naming compounds
In each case the valency or oxidation state of the element immediately prior to the Roman numeral is defined.
- copper(II) oxide has the formula CuO - the oxidation state of the copper is +2
- copper(I) oxide has the formula Cu2O - the oxidation state of the copper is +1 in this case.
- Sodium sulfate(IV) has the formula Na2SO3 - the sulfur is the element with a variable oxidation number; in this case +4.
- Sodium sulfate(VI) has the formula Na2SO4 - the sulfur is the element with a variable oxidation number; in this case +6.
In some situations there may even be two Roman numerals required to prevent any kind of ambiguity.
Example: copper(I) sulfate(IV) - Cu2SO3
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Examples: Give systematic names of the following compounds from the formula. K2MnO4 and KMnO4 K2MnO4 contains a transition element with a variable oxidation number. In terms of valencies (oxidation numbers) (2 x K) + (4 x O) - Mn = 0; therefore Mn = +6 The name of K2MnO4 is potassium manganate(VI) KMnO4 contains a transition element with a variable oxidation number. In terms of valencies (oxidation numbers) (1 x K) + (4 x O) - Mn = 0; therefore Mn = +7 The name of KMnO4 is potassium manganate(VII) |
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Example: Name the following compound - FeSO4 Oxidation state of the oxygen = -2; Oxidation state of the sulfur = +6 (in the sulfate ion) Therefore oxidation state of the iron = - (+6 - 8) = +2 The name of the compound FeSO4 is iron(II) sulfate |
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Example: Name the following compound - TiCl4 Oxidation state of the chloride = -1 Therefore oxidation state of the titanium = - (- 4) = +4 The name of the compound TiCl4 is titanium(IV) chloride |
Naming ions
Exactly the same convention applies to ions. If there is an element within the ion that has a variable oxidation state then the oxidation state is included in the ion name as a Roman numeral. Radical negative ions are named differently from the elements that are present, so a certain amount of flexibility of thought is needed; they end with -ATE.
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Example: Name the following ions. ClO4- and ClO3- ClO4- contains an element with a variable oxidation number (chlorine). In terms of valencies (oxidation numbers) Cl + (4 x O) = -1; therefore Cl = +7 The name of the ClO4- ion is the chlorate(VII) ion ClO3- contains an element with a variable oxidation number (chlorine). In terms of valencies (oxidation numbers) Cl + (3 x O) = -1; therefore Cl = +5 The name of the ClO4- ion is the chlorate(V) ion |
Complex ions
The names of positive complex ions are fairly straightforward. The ligands (species that are bonded to the central metal ion) are named first followed by the metal ion with its oxidation state.
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Example: Name the following complex ion. [Cu(H2O)6]2+ Copper is an element with a variable oxidation state. We know that the water molecules have already cancelled out the oxidation numbers of oxygen and hydrogen (as the water molecule is neutral) Therefore copper is in the +2 oxidation state The name of the complex ion is the hexaaqua copper(II) ion |
If the complex ion is negative (an anion) then the metal changes its name:
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Metal
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Metal in negative ion
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Titanium
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Titanate
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Chromium
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Chromate
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Manganese
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Manganate
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Iron
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Ferrate
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Copper
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Cuprate
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Cobalt
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Cobaltate
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Zinc
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Zincate
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Aluminium
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Aluminate
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Lead
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Plumbate
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Example: Name the following complex ion. [NiCl4]2- Nickel is an element with a variable oxidation state. We know that the each chloride ion has a -1 oxidation state. Therefore Nickel is in the +2 oxidation state The name of the complex ion is the tetrachloro nickelate(II) ion |
