Chemguide: Support for CIE A level Chemistry
Learning outcomes 12.1(e) and 12.1(f)
These statements are an introduction to the variable oxidation states of transition metals.
Before you go on, you should find and read the statements in your copy of the syllabus.
Transition metals have variable oxidation states
One of the key features of transition metal chemistry is the wide range of oxidation states (oxidation numbers) that the metals can show.
Note: If you aren't sure about oxidation states, you really need to follow this link before you go on.
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It would be wrong, though, to give the impression that only transition metals can have variable oxidation states. For example, elements like sulphur or nitrogen or chlorine have a very wide range of oxidation states in their compounds - and these obviously aren't transition metals.
However, this variability is less common in metals apart from the transition elements. Of the familiar metals from the main groups of the Periodic Table, only lead and tin show variable oxidation state to any extent.
Examples of variable oxidation states in the transition metals
Iron has two common oxidation states (+2 and +3) in, for example, Fe2+ and Fe3+. It also has a less common +6 oxidation state in the ferrate(VI) ion, FeO42-.
Manganese has a very wide range of oxidation states in its compounds. For example:
You will meet some of these, and lots more examples, in later statements in this section. You don't need to learn them for now.
Predicting what oxidation states an transition metal is likely to have
The table shows the 3d and 4s electrons for the transition elements we are talking about. You can make sensible predictions about the oxidation states from these.
The +2 oxidation state
All of these elements have a +2 oxidation state (although there aren't many titanium compounds in the +2 state). The +2 state is where the two 4s electrons have been lost - or the single 4s electron and one of the 3d electrons in the cases of chromium and copper.
You can think of the +2 oxidation state is being ionic, containing M2+ ions.
Other oxidation states
Maximum oxidation states
There is a simple pattern to work out the maximum oxidation state for any of these elements. The pattern is broken only by copper.
The maximum oxidation state is the number of 4s electrons plus the number of unpaired 3d electrons.
So the maximum oxidation state for chromium is +6 (count them!); the maximum oxidation state for manganese is +7; the maximum for nickel is +4.
The exception is copper whose maximum oxidation state is +3 - although the +3 state is very rare. The commonest oxidation state for copper is the familiar +2.
That still doesn't fit the rule, of course, and copper also has a relatively common +1 oxidation state as its minimum one.
Other oxidation states
Other oxidation states between +2 and the maximum usually exist, but are sometimes very rare. If you are very wide-awake, you might have noticed that the +5 oxidation state was missing in the list of manganese oxidation states further up the page.
That doesn't mean that there isn't one - just that you would never come across it.
In terms of bonding, the higher oxidation states will tend to involve covalent bonding, usually in complex ions. The MnO4- ion is a common example.
The bonds between the manganese and the oxygen are covalent, although the ion as a whole carries a single negative charge.
Note for teachers: There is an error in the teacher support material for the previous syllabus (Schemes of Work Unit 8B, 9.5(f)). It states "The highest oxidation state corresponds to the "loss" of all the 3d + 4s electrons." This only applies to the elements up to manganese - it doesn't work for the elements from iron onwards.
© Jim Clark 2011 (last modified May 2014)