Unit the non-transition elements. Hence, transition elements and their compounds are studied separately. However, the usual theory of valence as applicable to the non- transition elements can be applied successfully to the transition elements also. Various precious metals such as silver, gold and platinum and industrially important metals like iron, copper and titanium belong to the transition metals series.
In this Unit, we shall first deal with the electronic configuration, occurrence and general characteristics of transition elements with special emphasis on the trends in the properties of the first row ( d ) transition metals along with the preparation and properties of some important compounds. This will be followed by consideration of certain general aspects such as electronic configurations, oxidation states and chemical reactivity of the inner transition metals. THE TRANSITION ELEMENTS ( d -BLOCK) The d –block occupies the large middle section of the periodic table flanked between s – and p – blocks in the periodic table. The d –orbitals of the penultimate energy level of atoms receive electrons giving rise to four rows of the transition metals, i.e., d , d , d and d .
All these series of transition elements are shown in Table . . In general the electronic configuration of outer orbitals of these elements is (n- ) d – n s – except for Pd where its electronic configuration is d s . The (n– ) stands for the inner d orbitals which may have one to ten electrons and the outermost n s orbital may have one or two electrons.
However, this generalisation has several exceptions because of very little energy difference between (n- ) d and n s orbitals. Furthermore, half and completely filled sets of orbitals are relatively more stable. A consequence of this factor is reflected in the electronic configurations of Cr and Cu in the d series. For example, consider the case of Cr, which has d s configuration instead of d s ; the energy gap between the two sets ( d and s ) of orbitals is small enough to prevent electron entering the d orbitals.
Similarly in case of Cu, the configuration is d s and not d s . The ground state electronic configurations of the outer orbitals of transition elements are given in Table . .