Chemistry Reference
In-Depth Information
accommodating all the chemical elements in his system. “In the proposed system
the
atomic weight
of an element serves to
determine its
place”,
41
Mendeleev
explained. He concluded for that reason that “the magnitude of the
atomic weight
determines the character of an element
to the same extent that the molecular weight
determines the properties and many of the reactions of a compound substance.”
42
An important consequence of taking the atomic weight as the characteristic
property of basic substances was the possibility of distinguishing between the
(chemically and physically very similar) congeners of a certain elementary group
in the periodic table. The natural group of alkali metals, for example:
Li
¼
7
Na
¼
23
K
¼
39
Rb
¼
85.4
Cs
¼
133
consisted of five metals which shared a lot of similar properties—their metallic
lustre, their low melting points and densities, their pronounced reactivity with
respect to water, their rapidly oxidising character (tarnishing the metallic surface
in a dull and lustreless grey colour), etc. It thus seemed that the differences in
atomic weights were the only possible way to differentiate between these analogous
elements. “Similar elements [in chemical and physical properties] possess different
atomic weights”,
43
Mendeleev proclaimed.
But in the case of the cerium group {Ce
95}, the difference
in atomic weights was scarcely noticeable. Indeed, one recalls that in sharp contrast
with the
secondary groups
, which were characterised by a saltatory relationship
between the atomic weights of the
secondary elements
,
primary groups
represented
a steady and unchanging relationship between the atomic weights of the
primary
elements
. As Mendeleev explicated in his
Osnovy khimii
: “In spite of the great
similarity existing at present [between the chemical and physical properties of the
congeners of the cerium group], there are no differences, or to speak precisely, there
are
no considerable differences in the values of atomic weights
of [these] similar
elements.”
44
“There are more examples of this kind”,
45
wrote Mendeleev, who
noticed that this observation was not limited to the members of the cerium group.
It turned out that very similar apperceptions could be made with regard to the
transition metal groups:
¼
92, La
¼
94, Di
¼
Such are nickel and cobalt, whose
atomic weights are very close to each other
; rhodium,
ruthenium and palladium on the one hand, iridium, osmium and platinum on the other are
also elements which closely resemble one another, and which have
very similar atomic
weights
. Iron and manganese have similar properties and their
atomic weights are also very
similar
.
46
41
Mendeleev (1869b/t), op. cit., p. 26. Emphasis added. (see note 16)
42
Ibid., p. 27. Emphasis added.
43
Ibid., p. 31.
44
Mendeleev (1868-1871), op. cit. (note 36) English translation by Trifonov (1970), op. cit., p. 38.
(note 36) Emphasis added.
45
Loc. cit. English translation by Trifonov (1966), op. cit., p. 27. (note 29)
46
Loc. cit. Emphasis added.
