Chemistry Reference
In-Depth Information
Properties of Undiscovered Elements
.
65
These valency shifts implied that the
currently used atomic weights would have to be increased by a factor of 1.5.
As Mendeleev wrote in his article
On the Placement of Cerium in the Periodic
System of Elements
: “To confirm the above ideas, I undertook the problem of
determining the heat capacity of the above-mentioned metals.”
66
The experimental
results, obtained in the fall of 1870, were confirmed by the investigations of Bunsen
and spoke in the advantage of the corrected atomic weights.
11.2.3 Accommodating the Rare Earths Individually
When the atomic weight of an element is changed, this logically implies a change in
its position in the periodic table. Thus, due to the atomic weight corrections of
indium, uranium, cerium, lanthanum, didymium, yttrium, erbium, and thorium, all
eight elements had to be removed from their usual place, and they had to be
accommodated differently. As can be seen from Fig.
11.4
, Mendeleev placed the
rare-earth elements throughout the sixth, seventh, and eighth subperiods of his
system in the groups I-VIII, as
homologues
of the other elements, according to a
homologous accommodation methodology
.
The accommodation of cerium went smoothly as Mendeleev had correctly
determined its atomic weight and oxide formulae. According to the dualism of
cerium, this rare-earth element exhibited two oxidation states (+III and +IV),
making its placement in the fourth group very natural. As Mendeleev reasoned in
his article
On the Placement of Cerium in the Periodic System of Elements
:
Cerium will [have to] be located in accordance with the value of its atomic weight
following caesium 133 and barium 137, and in accordance with the formula of its higher
degree of oxidation it should be located in the titanium group, i.e., in the place IV-6.
67
The placement of lanthanum, didymium and the other rare earths proved much
more difficult. Mendeleev finally decided to locate yttrium in the place III-4.
Lanthanum seemed to fit in the place III-6 and didymium was finally given the
65
Mendeleev, D. I. “Concerning the Natural System of the Elements and Its Application in
Determining the Properties of Undiscovered Elements.”
Zhurnal Russkogo Khimicheskogo
Obshchestva
3 (1871a): 7, 25-56. See also a German abstract in Mendeleev, D. I. “
¨
ber Das
Nat
¨
rliche System Der Elemente Und Seine Anwendung Zum Ermitteln Der Eigenschaften
Unentdeckter Elemente.”
Berichte der Deutschen chemischen Gesellschaft
3 (1870b): 990-992.
66
Mendeleev (1870/1871), op. cit. (note 61) English translation by Trifonov (1970), op. cit., p. 40.
(note 36)
67
Loc. cit. English translation by Trifonov (1966), op. cit., p. 25. (note 29) Due to the table
layout—consisting of eighth groups (I-VIII) and ten series (1-10)—each element (
X
) can be
characterised by two coordinates, its group number (
G
) and its series number (
S
), as
G
-
S
. Sodium
for example, is located in the first group and first series, and is therefore given the element
coordinates I-1. Magnesium is located in the place II-1, and titanium is characterised by the
element coordinates IV-2.
