Inner Transition (f-block) Elements: Lanthanoids and Actinoids

Elements whose f orbitals get filled up by electrons are called f block elements. These elements are placed separately at the bottom of the periodic table. They are a subset of the 6th and 7th periods.

Since the f orbital lies much inside the d orbital, in relation to the transition metals the f block elements are called inner transition elements. These elements have 1 to 14 electrons in their f orbital, 0 or 1 in the penultimate energy level, and 2 electrons in the outermost orbital. The lanthanoids are characterized by the gradual filling up of the 4f, and the actinides by the 5f orbitals (the book's own sentence prints "actinides" here; elsewhere it uses "actinoids"). There are 14 elements filling the f orbital in each series. The relative energies of the nd and (n-1)f orbitals are very similar and are sensitive to the electronic configurations. …