However, the electronic configurations of all the tripositive
ions (the most stable oxidation state of all the lanthanoids) are of the
form 4f 
n (n = 1 to 14 with increasing atomic number) The overall decrease in atomic and ionic radii from lanthanum to
lutetium (the lanthanoid contraction) is a unique feature in the
chemistry of the lanthanoids It has far reaching
consequences in the chemistry of the third
transition series of the elements The decrease
in atomic radii (derived from the structures of
metals) is not quite regular as it is regular in
M
3+ ions (Fig