Chemistry Reference
In-Depth Information
Thus in the case of the elementary group {Ce, La, Di}, one observes the following
sequence of atomic weights: 92
95. In what follows, such a natural group of
elements will be denominated by the term
primary group
and their congeners by the
name
primary elements
. Hence, primary groups are defined and recognised as
follows:
94
➠
➠
1. Primary groups are formed during the primary classification of the elements;
2. There exists a steady, almost constant relationship between the atomic weights
of the congeners.
Other examples of such primary groups are the iron group {Fe
¼
56, Ni
¼
59,
Co
¼
59}, the platinum group {Pt
¼
197.1, Ir
¼
198, Os
¼
199}, the palladium
106.6},
23
group {Rh
¼
104.4, Rn
¼
104.4, Pl
¼
and the erbium group {Er
¼
56,
Yt
60}.
In conclusion, and keeping the above-stated crucial distinction between primary
and secondary groups in mind, it should be relatively easy at this point to under-
stand the
¼
character of the rare-earth groups. That is to say, since the
rare-earth elements constituted two primary groups, {Ce, La, Di} and {Er, Yt},
Mendeleev considered these groups
special
'
'
as their congeners exhibited atomic
weight values which were very close to one another, a fact not to be observed in the
'
special
'
'
case of secondary groups where the congeners have radically different
atomic weights. In his article
On the Correlation between the Properties of the
Elements and Their Atomic Weights
, Mendeleev wrote:
A number of questions arise when all of the elements are arranged into one whole, but the
most interesting problem appears to me to be the arrangement of elements having such
similarity as iron, cerium, palladium, and platinum, since, in this case,
elements close to
each other in their nature also exhibit approximately the same atomic weights
, a circum-
stance not to be observed in other rows, for in the latter
similar elements possess different
atomic weights
.
24
normal
'
One can conclude that Mendeleev was blessed with a deep insight into the nature of
and differences between elementary groups—a fundamental understanding, which
in the eyes of the authors, is often lacking within the chemical community of the
twenty-first century.
At least four important consequences can be drawn from the existence of
primary groups: (1) the problematic depiction of both primary and secondary
groups in the periodic table, (2) the transitional function of primary groups,
(3) the rare earth—transition metal analogy, and (4) the problematic nature of
primary groups, undermining the periodic law and subverting the characterisation
of the elements by their atomic weight.
23
In Mendeleev
s nomenclature, Rn represented ruthenium and Pl represented palladium.
24
Mendeleev (1869b/t), op. cit., p. 31. (see note 16) Emphasis added.
'
