The last members of the even series resemble in many respects (in their lower oxides, etc.), the first members of the uneven series; thus chromium and manganese in their basic oxides are analogous to copper and zinc. But there are great differences between the last members of the uneven series (haloids), and the first members of the next even series (alkali metals). Now between the last members of the even series there occur, according to the order of atomic weights, all those elements which cannot be included in the small periods. Thus between Cr and Mn in the one series, and Cu and Zn in the next, there come the elements Fe, Co, Ni, and in a similar way after the sixth series come Ru, Rh, Pd, and after the tenth Os, Ir, Pt. Mendelejeff gives the name of a long period to two such series with three intervening members, forming seventeen in all.

From the difficulty of arranging all the elements in this system, it cannot be regarded as yet perfect, but the fact that Mendelejeff was able so correctly to foretell the properties of gallium, shows that it must contain a large element of truth. At the time that he drew up his table there was a blank in the third group of the fifth series.