<Tr> <Td> La and Ac below Y </Td> </Tr> <P> Lanthanum and actinium are commonly depicted as the remaining group 3 members . It has been suggested that this layout originated in the 1940s, with the appearance of periodic tables relying on the electron configurations of the elements and the notion of the differentiating electron . The configurations of caesium, barium and lanthanum are (Xe) 6s, (Xe) 6s and (Xe) 5d 6s . Lanthanum thus has a 5d differentiating electron and this establishes it "in group 3 as the first member of the d - block for period 6". A consistent set of electron configurations is then seen in group 3: scandium (Ar) 3d 4s, yttrium (Kr) 4d 5s and lanthanum (Xe) 5d 6s . Still in period 6, ytterbium was assigned an electron configuration of (Xe) 4f 5d 6s and lutetium (Xe) 4f 5d 6s, "resulting in a 4f differentiating electron for lutetium and firmly establishing it as the last member of the f - block for period 6". Later spectroscopic work found that the electron configuration of ytterbium was in fact (Xe) 4f 6s . This meant that ytterbium and lutetium--the latter with (Xe) 4f 5d 6s--both had 14 f - electrons, "resulting in a d - rather than an f - differentiating electron" for lutetium and making it an "equally valid candidate" with (Xe) 5d 6s lanthanum, for the group 3 periodic table position below yttrium . Lanthanum has the advantage of incumbency since the 5d electron appears for the first time in its structure whereas it appears for the third time in lutetium, having also made a brief second appearance in gadolinium . </P> <P> In terms of chemical behaviour, and trends going down group 3 for properties such as melting point, electronegativity and ionic radius, scandium, yttrium, lanthanum and actinium are similar to their group 1--2 counterparts . In this variant, the number of f electrons in the most common (trivalent) ions of the f - block elements consistently matches their position in the f - block . For example, the f - electron counts for the trivalent ions of the first three f - block elements are Ce 1, Pr 2 and Nd 3 . </P> <Table> <Tr> <Td> Lu and Lr below Y </Td> </Tr> </Table>

Describe characteristics of elements in the periodic table