Transition metals are hard, strong and dense

1. Transition metals such as iron, copper, nickel and chromium sit in the central block, between Groups 2 and 3.
2. Like other metals they are shiny, conduct electricity and heat, and can be hammered into shape.
3. Most have high melting points, so iron melts at $1538{}^{\circ }C$ while sodium melts at $98{}^{\circ }C$.
4. Most have high densities, so copper has a density of $8.96{g/cm}^3$ while potassium has a density of $0.86{g/cm}^3$.
5. They are hard and strong, which is why iron, in the form of steel, is used for bridges, girders and car bodies.
   1. The Group 1 metals, by contrast, are soft enough to cut with a knife.

Transition metals are less reactive than Group 1

1. Transition metals react very slowly or not at all with cold water, while every Group 1 metal reacts with cold water and releases hydrogen.
   1. Iron rusts only slowly, over days or weeks, when it is in contact with both water and oxygen.
2. Copper does not react with water or with dilute acids, so it is used for water pipes and hot water tanks.
3. Gold and silver are so unreactive that they stay shiny for centuries, which suits jewellery and coins.
4. Transition metal atoms form positive ions less readily than Group 1 atoms, which is why they react less vigorously.

Many transition metals form ions with different charges

1. A Group 1 metal always forms ions with a $1+$ charge, but many transition metals form more than one positive ion.
   1. Iron forms both iron(II) ions, ${Fe}^{2+}$, and iron(III) ions, ${Fe}^{3+}$.
   2. Copper forms both copper(I) ions, ${Cu}^{+}$, and copper(II) ions, ${Cu}^{2+}$.
2. The Roman numeral in the name of a compound gives the charge on the metal ion.
3. Different charges give different formulae, so iron(II) chloride is ${FeCl}_2$ and iron(III) chloride is ${FeCl}_3$.
   1. Compounds of the same metal with different ion charges have different properties, including different colours.

Many transition metal compounds are coloured

1. Compounds of Group 1 metals, such as sodium chloride, are white solids that dissolve to give colourless solutions.
2. Many transition metal compounds are coloured, both as solids and in solution.
   1. Copper(II) sulfate solution is blue, iron(II) sulfate solution is pale green and iron(III) chloride solution is orange-brown.
   2. Potassium manganate(VII) contains manganese and forms a deep purple solution.
3. The colour depends on the ion charge, which is why iron(II) and iron(III) compounds have different colours.
4. The colours help in identifying ions, because each of these metal ions gives a precipitate of its own colour with sodium hydroxide solution.

Transition metals and their compounds are useful catalysts

1. Many transition metals and their compounds speed up reactions without being used up, so a small amount can be used again and again.
2. Iron is the catalyst in the Haber process, which makes ammonia for fertilisers: $$N_2(g)+3H_2(g)\rightleftharpoons 2{NH}_3(g)$$
3. Nickel catalyses the reaction of vegetable oils with hydrogen to make margarine.
4. Manganese(IV) oxide catalyses the decomposition of hydrogen peroxide: $$2H_2{O}_2(aq)\to 2H_{2}O(l)+O_2(g)$$
5. Platinum and rhodium in car catalytic converters speed up reactions that remove carbon monoxide and nitrogen oxides from exhaust gases.