The d and f block elements are referred to as transition metals and inner transition metals, respectively, due to their unique electron configurations.

The d block elements, commonly known as transition metals, occupy a position between the s and p block elements in the periodic table. The term “transition” is applied because these elements exhibit properties that vary or transition between those of the s block elements (primarily metals) and the p block elements (which include non-metals and metalloids). This variation arises from the filling of their d orbitals in the electron configuration. Notably, d block elements possess partially filled d orbitals in their ground state or in one or more oxidation states. This characteristic leads to several defining properties of transition metals, such as variable oxidation states, the formation of colored ions, and catalytic activities.

Conversely, the f block elements are designated as inner transition metals. Their placement in the periodic table occurs between the s and d block elements, and their properties transition similarly between those of the s block and d block elements. The f block elements feature partially filled f orbitals in their electron configurations. The filling of f orbitals occurs in two series: the 4f series and the 5f series, which are located at the bottom of the periodic table. These elements are further categorized into two groups: the lanthanides (elements $58-71$) and the actinides (elements $90-103$). The lanthanides are characterized by the filling of the 4f orbitals, while the actinides are defined by the filling of the 5f orbitals.

The distinct electron configurations of the d and f block elements, along with their resultant properties, are what classify them as transition and inner transition elements, respectively. A thorough understanding of these electron configurations and their consequences is essential for grasping the behaviors and characteristics of these elements.