The f-block elements contains of two series of inner transition elements i.e. lanthanoids (the fourteen elements subsequent lanthanum) and actinoids (the fourteen elements subsequent actinium). They are also called rate earth elements. All the lanthanoids have electronic configuration with 6s2 common but with variable occupancy of 4f- and 5d- subshells. All actinoids have electronic configuration of 7s2 and variable occupancy of 5f- and 6d- subshells. These f-orbitals are placed to the shell third from the outer shell. This factor accounts for some characteristic properties of these elements. The members of lanthanoid and actinoids series are listed in the table below. Although lanthanum itself does not possess any 4f-electrons, it is customary to include this element in the discussion of lanthanoid series due to its close resemblance with lanthanoids. For lanthanoids the general symbol, Ln is often used. Similarly, actinium is studied alongwith actinoids. The electronic configuration of the elements with fully filled (f14) and half filled (f7) f-orbitals are relatively more stable. The extra stability of half filled orbitals is seen in the elements europium (4f7 6s2) and gadolinium (4f7 5d1 6s2). The element ytterbium has fully-filed f-orbitals with the configuration 4f14 6s2 and the extra electron in lutetium goes in 5d-orbitals (4f14 5d1 6s2). So except for lanthanum, gadolinium and lutetium which have a single electron in 5d-orbitals the lanthanoids do not have electrons in the 5d-orbitals. Electronic configuration of Lanthanum and lanthanoids Electronic configuration of actinium and actinoids