Groups 3 to 12 elements are called d-block elements or transition elements. These elements are present between the p-block and the s-block elements in the Periodic Table. These elements' properties are intermediate between the properties of s - block and p - block elements, i.e. d - block elements represent a change or transition in properties from most electropositive s - block elements to less electropositive p - block elements. Therefore, these elements are called transition elements.
The members of a given transition series are not as different from each other as the members of the same period's representative elements. In a transition series, there is no change in the number of electrons of the outermost shell and only change occurs in the (n-l)d electrons from member to member in a period. These elements show horizontal and vertical relationships or they show similarities in a period as well as in group. The general trends of some of the important properties are discussed here first in the respective periods and then in groups.
28 elements from atomic number 58 to 71 (14 elements) and from atomic number 90 to 103 (14 elements) have been arranged in two horizontal rows below the Periodic Table. These elements are collectively called the f- block elements as the lastor differentiating electron in the atoms of these elements is accommodated on one of the seven f-orbital of the next penultimate energy shell. These elements have also been called inner transition elements because the ante-penultimate energy shell, i.e., (n -2)f-orbitals, lie comparatively deep within the kernel.