The elements of groups 3 to 12 of the periodic table are called transition elements, and the elements of groups 1, 2 and 13 to 18 are called main-group elements. Some definitions do not count zinc and the other group 12 elements as transition elements.
All the transition elements are metals, and many of them have high melting points and high densities.
Among the main-group elements, elements in the same column of the periodic table have similar properties. Among the transition elements, neighbouring elements in the same row also have very similar properties.
For main-group elements in the same period, each increase of one in the atomic number adds one electron to the outermost shell.
For transition elements, the outer shell keeps just one or two electrons, and the added electrons go into an inner shell. In period 4, from scandium, which follows calcium, to zinc, the number of electrons in the M shell rises from 9 to 18.
Because the number of outer electrons hardly changes, neighbouring transition elements have similar properties.
In transition elements, not only the electrons in the outer shell but also some in the inner shell take part in reactions. As a result, one element can often take several different oxidation numbers.
For example, iron takes the oxidation numbers +2 and +3, and copper +1 and +2. Manganese takes various oxidation numbers from +2 to +7, and in the permanganate ion, \mathrm{MnO_4^{-}}, it is +7.
Many ions and compounds of transition elements are coloured. In solution, \mathrm{Cu}^{2+} looks blue, \mathrm{Fe}^{2+} pale green, \mathrm{Fe}^{3+} yellowish brown and \mathrm{Ni}^{2+} green.
\mathrm{MnO_4^{-}} is purple and \mathrm{Cr_2O_7^{2-}} is orange, and both are used as strong oxidizing agents.
In contrast, most solutions of ions of main-group elements, such as \mathrm{Na}^{+} and \mathrm{Ca}^{2+}, are colourless.
Many transition elements and their compounds act as catalysts. A catalyst is a substance that speeds up a reaction without being changed itself.
When manganese(IV) oxide is added to hydrogen peroxide solution, oxygen bubbles out vigorously.
2\mathrm{H_2O_2} \longrightarrow 2\mathrm{H_2O} + \mathrm{O_2}
In industry, too, a catalyst based on iron is used to make ammonia, and vanadium(V) oxide is used to make sulfuric acid.