A set of elements in the periodic table in which filling of electrons in an inner d- or f-level occurs. With increasing proton number, electrons fill atomic levels up to argon, which has the electron configuration 1s22s22p63s23p6. In this shell, there are 5 d-orbitals, which can each contain 2 electrons. However, at this point the subshell of lowest energy is not the 3d but the 4s. The next two elements, potassium and calcium, have the configurations [Ar]4s1 and [Ar]4s2 respectively. For the next element, scandium, the 3d level is of lower energy than the 4p level, and scandium has the configuration [Ar]3d14s2. This filling of the inner d-level continues up to zinc [Ar]3d104s2, giving the first transition series. There is a further series of this type in the next period of the table: between yttrium ([Kr]4d5s2) and cadmium ([Kr]4d105s2). This is the second transition series. In the next period of the table the situation is rather more complicated. Lanthanum has the configuration [Xe]5d16s2. The level of lowest energy then becomes the 4f level and the next element, cerium, has the configuration [Xe]4f15d16s2. There are 7 of these f-orbitals, each of which can contain 2 electrons, and filling of the f-levels continues up to lutetium ([Xe]4f145d16s2). Then the filling of the 5d levels continues from hafnium to mercury. The series of 14 elements from cerium to lutetium is a ‘series within a series’ called an inner transition series. This one is the lanthanoid series. In the next period there is a similar inner transition series, the actinoid series, from thorium to lawrencium. Then filling of the d-level continues from element 104 onwards.
In fact, the classification of chemical elements is valuable only in so far as it illustrates chemical behaviour, and it is conventional to use the term `transition elements' in a more restricted sense. The elements in the inner transition series from cerium (58) to lutetium (71) are called the lanthanoids; those in the series from thorium (90) to lawrencium (103) are the actinoids. These two series together make up the f-block in the periodic table. It is also common to include scandium, yttrium, and lanthanum with the lanthanoids (because of chemical similarity) and to include actinium with the actinoids. Of the remaining transition elements, it is usual to speak of three main transition series: from titanium to copper; from zirconium to silver; and from hafnium to gold. All these elements have similar chemical properties that result from the presence of unfilled d-orbitals in the element or (in the case of copper, silver, and gold) in the ions. The elements from 104 to 109 and the undiscovered elements 110 and 111 make up a fourth transition series. The elements zinc, cadmium, and mercury have filled d-orbitals both in the elements and in compounds, and are usually regarded as nontransition elements forming group 12 of the periodic table.
The elements of the three main transition series are all typical metals (in the nonchemical sense), i.e. most are strong hard materials that are good conductors of heat and electricity and have high melting and boiling points. Chemically, their behaviour depends on the existence of unfilled d-orbitals. They exhibit variable valency, have coloured compounds, and form coordination compounds. Many of their compounds are paramagnetic as a result of the presence of unpaired electrons. Many of them are good catalysts. They are less reactive than the s- and p-block metals.