Synthetic Inorganic Chemistry

CHAPTER XI

HEAVY METALS OF GROUPS VI, VII, AND VIII

OF THE PERIODIC SYSTEM

An inspection of the Periodic Table of the elements shows that
chromium, manganese, iron, cobalt, and nickel, and following
these copper and zinc, come in the middle portion of the long
period that begins with potassium and ends with bromine. The
seven elements mentioned possess high specific gravities, and all
come under the classification of heavy metals. In certain of their
compounds they are similar to one another; in other of their
properties they are very dissimilar and exhibit the chemical
characteristics of the respective groups to which they belong.
The heavy metals occupying a corresponding position in the
middle of the next long period are molybdenum, an element which
should come below manganese, ruthenium, rhodium, palladium,
silver, and cadmium. In the middle of the next long period come
tungsten, rhenium, osmium, iridium, platinum, gold, and mercury.
In the last long period, of which there is at best only a fragmentary
indication, the only representative of this class of heavy metals
is uranium.
In the sixth group, chromium, molybdenum, tungsten, and
uranium constitute Family A. In their trioxides they show the
characteristic valence of the sixth group and resemble in properties
the non-metals of Family B, of which sulphur is the type. In their
lower oxides they possess none of the characteristics of Group VI
but show the general base-forming properties of the heavy metals.
In Group VII, manganese, the best-known representative of
Family A, resembles the halogens in its heptoxide, MH2O7; in its
lower oxides it shows no resemblance to the halogens, but does
show properties similar to those of other heavy metals when they
are in the same state of oxidation; in its lowest oxide, MnO, it is
distinctly a base-forming element. The most characteristic com-
pound of rhenium which justifies its position in Group VII is
potassium perrhenate, KReC>4, which corresponds to potassium
permanganate, KMnC>4, and is derived from the oxide Re 2 O 7.
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