The lanthanide contraction is a phenomenon that describes the greater-than-expected decrease in atomic radii and ionic radii of the elements in the lanthanide series, from left to right/Descriptive_Chemistry/Elements_Organized_by_Block/4_f-Block_Elements/The_Lanthanides/aLanthanides%3A_Properties_and_Reactions/Lanthanide_Contraction). It is caused by the poor shielding effect of the 4f electrons, which are not very effective at shielding the outer shell electrons (n=5 and n=6) /Descriptive_Chemistry/Elements_Organized_by_Block/4_f-Block_Elements/The_Lanthanides/aLanthanides%3A_Properties_and_Reactions/Lanthanide_Contraction). As a result, the shielding effect is less able to counter the decrease in radius caused by increasing nuclear charge, leading to the lanthanide contraction/Descriptive_Chemistry/Elements_Organized_by_Block/4_f-Block_Elements/The_Lanthanides/aLanthanides%3A_Properties_and_Reactions/Lanthanide_Contraction). The ionic radius drops from 103 pm for lanthanum(III) to 86.1 pm for lutetium(III) . About 10% of the lanthanide contraction has been attributed to relativistic effects. The lanthanide contraction applies to all 14 elements included in the lanthanide series, including cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium/Descriptive_Chemistry/Elements_Organized_by_Block/4_f-Block_Elements/The_Lanthanides/aLanthanides%3A_Properties_and_Reactions/Lanthanide_Contraction). Without the lanthanide contraction, a chemical separation of lanthanides would be extremely difficult. However, this contraction makes the chemical separation of period 5 and period 6 transition metals of the same group rather difficult.