Transition metals

The transition metals are the block of metals in the middle of the periodic table, between groups 2 and 3. Examples include chromium, manganese, iron, cobalt, nickel and copper.

Compared with the group 1 metals, transition metals:

• have much higher melting points (except mercury, which is a liquid)
• have higher densities
• are stronger and harder
• are much less reactive — they react only slowly, or not at all, with water and oxygen

This makes them useful as structural materials, such as iron (as steel) in buildings and bridges, and copper in water pipes.

Transition metals have three special properties:

• They form ions with different charges, such as Fe²⁺ and Fe³⁺, or Cu⁺ and Cu²⁺.
• They form coloured compounds. Copper(II) sulfate is blue, and iron(II) compounds are usually pale green while iron(III) compounds are orange-brown.
• They and their compounds are useful catalysts. Iron is the catalyst in the Haber process for making ammonia, and nickel is used to add hydrogen to vegetable oils.

The difference in reactivity shows clearly with water:

• Sodium, in group 1, reacts vigorously with cold water, fizzing as it gives off hydrogen and forming sodium hydroxide.
• Iron reacts only very slowly, and only with water and oxygen together, to form rust.
• Copper does not react with water at all, which is why it can be used for water pipes.

Group 1 metals also react quickly with oxygen and are stored under oil. Most transition metals can be left in air, reacting slowly or not at all.

A group 1 metal always forms ions with a 1+ charge. A transition metal can form ions with different charges, so the name gives the charge as a Roman numeral:

• iron(II) oxide, FeO, contains Fe²⁺ ions
• iron(III) oxide, Fe₂O₃, contains Fe³⁺ ions

The colours of their compounds help to identify them:

The same colours appear in the hydroxide precipitate tests for metal ions.