Chemistry Reference
In-Depth Information
level of
primary
and
secondary
groups. Mendeleev thus concluded his paper of
1869 by noting that while all elementary groups were build from “elements
exhibiting similarities in their chemical behaviour” differences in their nature
would continue to exist, since the atomic weights of their congeners “are either
approximately equal (as with Pt, Ir, Os) or [
...
] increase in a uniform manner
(as with K, Rb, Cs).”
58
The essence of the rare-earth crisis rested on the fact that the septuplet of
homeless elements constituted a primary group. Their problematic nature raised a
number of serious problems. Both the principle of periodicity and the characteri-
sation of primary elements on the basis of their atomic weights got undermined.
Mendeleev was also tempted in drawing an analogy between the rare-earth
elements and the transition metals on the basis of their transitional functions in
the periodic system, and he started questioning the simplicity of these elements on a
closer study of the primary groups. In conclusion, Mendeleev clearly grasped the
causes of the problematic accommodation of the rare-earth elements in 1869.
He nevertheless continued to use the old atomic weights and erroneous valencies
for the rare-earth elements, and his unremitting adherence to the
Attempted System
moreover troubled his views with regard to the different relationships between the
chemical and physical properties of the elements.
11.2 The Period 1870-1871
11.2.1 Natural System of the Elements
Mendeleev had been working on the optimisation of the periodic law for quite some
time now. He had always preferred the
long form
table (i.e.
Attempted System
,
Figs.
11.1
and
11.2
), but in November 1870, Mendeleev created a
short form
table—his
Natural System of the Elements
(Fig.
11.4
). This type of classification
remained the standard format during the next 100 years, and it succeeded in
exhibiting a number of new relationships between the chemical elements (as for
example the close connection between the main-block elements on the one hand
and the transition metals on the other).
The
Natural System
consisted of two rows of
typical elements
(H till F) and five
more
periods
which were further subdivided in an odd and even
series
of seven
elements each. Some elements could not be placed in any series and were therefore
“arranged in order of their properties and atomic weights between the last member
of the even series and the first member of the odd series” in an independent, eighth
group.
59
“In this manner”, Mendeleev continued, “Fe, Co, and Ni are placed
58
Mendeleev (1869b/t), op. cit., p. 33. (note 16)
59
Mendeleev (1871c/t), op. cit., p. 48. (note 55). See also Sect.
11.1
, five for an illustration of the
construction methodology when building a short form table (i.e.
first attempt
).
