The elements in group 3 have an "ns"2("n" − 1)"d"1 configuration. The first transition series is present in the 4th period, and starts after Ca ("Z" = 20) of group-2 with the configuration or scandium (Sc), the first element of group 3 with atomic number "Z" = 21 and configuration [Ar4"s"23"d"1, depending on the definition used. As we move from left to right, electrons are added to the same "d" sub-shell till it is complete. The element of group 11 in the first transition series is copper (Cu) with an atypical configuration 4"s"13"d"10. Despite the filled d sub-shell in metallic copper it nevertheless forms a stable ion with an incomplete d sub-shell. Since the electrons added fill the orbitals, the properties of the "d"-block elements are quite different from those of "s" and "p" block elements in which the filling occurs either in "s" or in "p"-orbitals of the valence shell.