55. fve One electron cntc (2=39) and ends at cadil filling of 5p-orbilals starts at indium (2 The sixth period corresponds to the fillin filling of sixth energy level, i.e., n eriod corresponas to the fillin O. 4f. five 5d and three 6p) are availabl for filling with electrons, therefore Seen onbitals (one 6s, seven 4 sLth period contain.s thirty-two ele 6s-orbital and ends up with radon (Z = 86) in wh after the filling of 6s-orbital, the next electro and thereafter, the filling of seven , Se ents. It begins with caesium (Z= 55) in which one clectron enters the = 55) in which one electron enters the don (Z = 86) in which the filling of 6p-orbitals is complete. In this period. lectron (i.e., 57th) enters the d- orbital against the aufbau principle 4f-orbitals begins with cerium (Z= 58) and ends up with lutetium hehaviour can be explained on the basis of greater stability of the xenon (inert gas) core. This anomalous behaviour can be should occur in 4f-orbital jn accordance with 56). the addition of the next electron (i.e., 57th) should occur in 4f-orbital in accordance with inle This will, however, tend to destabil1ze the xenon core (Z=54, [Kr] 4d 4f 5s 5psd) since After barium (Z= 56), the additi refore, the 57th electron prefers to enter 5d-orbital which lies outside the the 4-orbitals lie inside the core. There xenon core and whose energy 1s only slightly ince 0se energy is only slightly higher than that of 4f-orbital. In doing so, the stability conferred on the atom due to xenon core more than compensates the slight instability caused by the addition of one electron To the higher energy 5d-orbital instead of the lower energy 4-orbital. Thus, the outer electronic configuration of the lower energy 4f-orbital. Thus, the outer electronic configuration of La (7-57) is Sd' 6s rather than the expected 4" 6s. Once Sd-orbital has one electron, the next electron (Le, 58th enters the inner 4f-orbital (instead of the outer 5d-orbital) due togreater nuclear charge Thus, the outer electronic configuration of Ce (Z= 58) is 4f 5d' 6s. The next electron (i.e., S9thy not only goes to 4f-subsholl but also pulls back the single 5d-clectron to 4f-subshell (making it f) due to greater nuclear charge. Thus, the outer electronic configuration of Pr (Z= 59) is 4f° 5d 6s. Thereafter, the regular filling of 4f-orbitals occurs till they are completely half-filled at Eu (Z = 63, 4f 5d 6s). The next electron (i.e., 64th) as expected does not go to 4f-subshell but goes to 3d-subshell due to greater stability of the exactly half-filled 4f-orbitals. Thus, the outer electronic contiguration 0f gadolinium is 4f 5d6s. The next electron (i.e., 65th) not only goes to 4f-subshell but also pulls back the SIngie Sd-electron to 4fsubshell, thereby making it 4f9 5d0, i.e., the outer electronic configuration of Tb (Z=03) 6. Thereafter, the continuous regular filling of the 4/-subshell occurs till it is complete at TO o). Ihe next electron (i.e., 71) as expected goes to the higher energy 5d-orbital thereby compicng u 1anthanide series at lutetium (Z= 71, 4f 5d 6s).