The elements in which the additional electrons enters (n−2)f orbitals are called inner transition elements. 4f-block elements are also called lanthanides or rare earths so lanthanides are 14 elements in the sixth period (atomic no. =58 to 71 ) that are filling 4f-sublevel. Related Theory Lanthanoid contraction effect results from poor shielding of nuclear charge (nuclear attractive force on electrons) by 4 felectrons so effective nuclear charge increases; the 6s electrons are drawn towards the nucleus, thus resulting in a smaller atomic radius.