Because lanthanum closely resembles the lanthanoids, it is
usually included in any discussion of the lanthanoids for which the
general symbol Ln is often used Similarly, a discussion of the actinoids
includes actinium besides the fourteen elements constituting the series The lanthanoids resemble one another more closely than do the members
of ordinary transition elements in any series They have only one stable
oxidation state and their chemistry provides an excellent opportunity to
examine the effect of small changes in size and nuclear charge along a
series of otherwise similar elements