Oxidation numbers of ions
Definition
Oxidation number
A number given to each atom in a substance that shows how many electrons it has lost, as a positive value, or gained, as a negative value, compared with the uncombined element.
Definition
Variable oxidation number
The ability of an element to form ions with different charges, so that it has more than one oxidation number in its compounds.
- For a simple ion, the oxidation number is equal to the charge on the ion.
- A sodium ion, Na+\text{Na}^{+}Na+, has an oxidation number of +1+1+1, and a chloride ion, Cl−\text{Cl}^{-}Cl−, has an oxidation number of −1-1−1.
- The metals in Groups I and II, and aluminium in Group III, form only one type of ion, so each of these metals has a single oxidation number in its compounds.
- A transition element can lose different numbers of electrons, so one transition element can form ions with different charges.
- Transition elements are therefore described as having ions with variable oxidation numbers.
- The oxidation number shows how many electrons each atom of the metal has lost to form the ion.
Iron(II) and iron(III)
- Iron is the most important example of a transition element with variable oxidation numbers.
- Iron(II): an iron atom loses two electrons to form the Fe2+\text{Fe}^{2+}Fe2+ ion, which has an oxidation number of +2+2+2.
- Iron(III): an iron atom loses three electrons to form the Fe3+\text{Fe}^{3+}Fe3+ ion, which has an oxidation number of +3+3+3.
- Both ions contain the same number of protons, because both come from iron atoms, but Fe3+\text{Fe}^{3+}Fe3+ has one fewer electron than Fe2+\text{Fe}^{2+}Fe2+.
- Because the two ions have different charges, they combine with other ions in different ratios and form compounds with different formulae.
- Iron(II) chloride is FeCl2\text{FeCl}_2FeCl2 and iron(III) chloride is FeCl3\text{FeCl}_3FeCl3.
- Iron(II) oxide is FeO\text{FeO}FeO and iron(III) oxide is Fe2O3\text{Fe}_2\text{O}_3Fe2O3.
- Iron(II) sulfate is FeSO4\text{FeSO}_4FeSO4 and iron(III) sulfate is Fe2(SO4)3\text{Fe}_2(\text{SO}_4)_3Fe2(SO4)3.
- Iron(II) hydroxide is Fe(OH)2\text{Fe(OH)}_2Fe(OH)2 and iron(III) hydroxide is Fe(OH)3\text{Fe(OH)}_3Fe(OH)3.
- Solutions containing iron(II) ions are pale green, while solutions containing iron(III) ions are yellow-brown.
- An iron(II) ion can lose one more electron to become an iron(III) ion, which is an example of oxidation: Fe2+→Fe3++e−\text{Fe}^{2+} \rightarrow \text{Fe}^{3+} + e^{-}Fe2+→Fe3++e−.
- This is why a pale green iron(II) solution left open to the air slowly turns yellow-brown.
- Other transition elements show the same behaviour, so copper forms copper(I) oxide, Cu2O\text{Cu}_2\text{O}Cu2O, and copper(II) oxide, CuO\text{CuO}CuO.
Roman numerals in names
- The oxidation number of a transition element is shown by a Roman numeral in brackets written straight after the name of the element, with no space.
- The name iron(II) chloride tells you that the compound contains Fe2+\text{Fe}^{2+}Fe2+ ions.
- The Roman numeral is needed because the name "iron chloride" on its own could mean either FeCl2\text{FeCl}_2FeCl2 or FeCl3\text{FeCl}_3FeCl3.
- Metals with only one possible ion, such as sodium and magnesium, do not need a numeral, so sodium chloride is never written as sodium(I) chloride.
- The numeral gives the charge on each metal ion, not the number of metal ions in the formula.
- To find the oxidation number from a formula, use the known charges of the other ions and the rule that the total charge in a compound is zero.
Example
Finding the oxidation number of iron
- Iron oxide has the formula Fe2O3\text{Fe}_2\text{O}_3Fe2O3.
- Each oxide ion has a charge of −2-2−2, so three oxide ions give a total of 3×(−2)=−63 \times (-2) = -63×(−2)=−6.
- The two iron ions must give a total of +6+6+6 to balance this.
- Each iron ion is Fe3+\text{Fe}^{3+}Fe3+, so the compound is iron(III) oxide.
Example
Writing a formula from a name
- The compound is iron(III) chloride.
- The numeral (III) shows that each iron ion is Fe3+\text{Fe}^{3+}Fe3+.
- Each chloride ion is Cl−\text{Cl}^{-}Cl−, so three chloride ions are needed to balance one iron ion.
- The formula is FeCl3\text{FeCl}_3FeCl3.
Common Mistake
- Do not read iron(III) oxide as a compound with three iron atoms, because the numeral gives the charge on each iron ion and Fe2O3\text{Fe}_2\text{O}_3Fe2O3 contains two.
- Do not write iron(3) or iron III, because the oxidation number is always a Roman numeral inside brackets.
- Do not swap the charges, because iron(II) is Fe2+\text{Fe}^{2+}Fe2+ and iron(III) is Fe3+\text{Fe}^{3+}Fe3+.
Exam technique
Describing variable oxidation numbers
- State that transition elements form ions with different charges, or have variable oxidation numbers.
- Support the statement with iron, giving both Fe2+\text{Fe}^{2+}Fe2+ and Fe3+\text{Fe}^{3+}Fe3+, because one ion alone does not show that the oxidation number varies.
- When naming a compound of iron, always include the numeral, because "iron chloride" does not identify which compound you mean.
Self review
- What is meant by a variable oxidation number?
- Give the formulae of the two ions formed by iron.
- Write the formulae of iron(II) oxide and iron(III) oxide.
- Explain how you would work out the oxidation number of iron in FeCl3\text{FeCl}_3FeCl3.
- Why does a sodium compound not need a Roman numeral in its name?
