Chemistry Reference
In-Depth Information
In the same way, Mendeleev recognised the similarity in chemical and physical
properties of the rare-earth elements, and considered them to be members of a
natural group
as he proclaimed in his 1869 article. “Only with regard to some
groups of elements
are there no doubts that they form a whole and represent a
natural order of similar manifestations of matter”, Mendeleev explained. “Such
groups are: the halogens, the alkaline earth metals, the nitrogen group, and also—in
part—the sulfur group, the companions of platinum,
the companions of cerium
, and
a few others.”
17
As a consequence, and in complete analogy with the alkali metals and the
halogens, Mendeleev tried to accommodate the companions of cerium in the
periodic table as a
group
. This fact is clearly exemplified in the
Attempted System
where the rare-earth elements (Er, Yt, Ce, La, and Di) are grouped together at the
bottom of the system. Yet, Mendeleev promptly understood that the rare earths
constituted a very
special
group of elements, and it seemed that the whole accom-
modation issue had its root in the puzzling nature of this
elementary group
.
“A number of questions arise when all of the elements are arranged into one
whole,” Mendeleev said, “but the most interesting problem appears to me to be
the
arrangement of elements having such similarity as
[
]
cerium
.”
18
...
11.1.5 Primary Versus Secondary Classification
In order to understand why Mendeleev discriminated this group from the other
natural groups (such as the alkali metals and halogens), it will prove useful to
explore the construction methodology of the periodic table. In short, a two-step
process is needed in order to build a periodic table from scratch. First, all the
elements have to be ordered according to increasing atomic weight. This
primary
classification
results in a long horizontal sequence of elements, and has been called
the
Mendeleev Line
by Henry Bent.
19
It will be noted that certain chemical and
physical properties of the elements recur
periodically
. Therefore, the second step,
termed
secondary classification
, consists of partitioning this Mendeleev Line at
certain well defined loci and placing the different sections (i.e.
periods
) underneath
each other so that elements with similar properties will fall into the same vertical
column, forming
natural groups
and representing the periodic law graphically.
It seems that Mendeleev exploited this construction methodology as well—more
in particular when he created his
first attempt
.
20
As Mendeleev explained:
17
Ibid., p. 22. Emphasis added.
18
Ibid., p. 31. Emphasis added.
19
Bent, H. A.
New Ideas in Chemistry from Fresh Energy for the Periodic Law.
Bloomington,
Indiana: AuthorHouse, 2006.
20
The
first attempt
is not to be confused with the
Attempted System
. Whereas the
Attempted System
(Figs.
11.1
and
11.2
) represents the end-product of the process of discovery, Mendeleev most
probably wrote down the
first attempt
at the very beginning of his quest for a classification of the
elements.
