Chemistry Reference
In-Depth Information
11.2.4 Saving the Periodic Law
Let us try to explain this last point somewhat better. One recalls that the periodic
law got undermined by the existence of primary groups due to the fact that the
“regular and gradual changes in the size of atomic weights” were not accompanied
by “the regular and gradual changes” in the distinctive properties of the elements.
69
Thus, in the case of the cerium group:
Ce
¼
92
La
¼
94
Di
¼
95
Valency
2
2
2
the valency number (i.e. oxidation state) remained constant, instead of gradually
increasing. If, however, these elements were accommodated as
homologues
of the
third, fourth and fifth group respectively, their valency also gradually increased
from 3 to 4 and 5, thus resolving the problem entirely:
La
¼
138
Ce
¼
140
Di
¼
144
Valency
3
4
5
Mendeleev was swift at drawing an important conclusion from this: the typically
trivalent rare-earth elements had to exhibit some higher oxidation states as well.
Cerium, for example, was located in the fourth group with the
tetravalent
transition
metals titanium and zirconium, while didymium was taken to be a homologue of
the
pentavalent
metals vanadium and niobium. Neither the tetravalency of cerium
nor the pentavalency of didymium had as yet been established however, and
Mendeleev therefore planned to start his own rare-earth research, in the hope of
revealing these higher oxidation states. This would prove the validity of the
homologous accommodation methodology and would rescue the periodic law
from a painful exception.
11.2.5 Resolving the Characterisation Issues
Previously, the differences in atomic weight values between lanthanum, cerium
and didymium had been too small to differentiate between these three elements.
This undermined Mendeleev
s use of the atomic weight as the characteristic
property of basis substances, and led Mendeleev to believing in physically real
atoms and an internal matter constituting these atoms.
The individuation of the rare-earth elements in the second period turned these
elements from primary into secondary elements. Since the transition metals
'
69
Mendeleev (1868-1871), op. cit. (note 36) English translation by Trifonov (1970), op. cit., p. 38.
(note 36) Emphasis added.
