Chemistry Reference
In-Depth Information
Fig. 11.6 Mendeleev
s Natural System of the elements in 1871
'
element coordinates V-6, although Mendeleev was still playing with the idea of
placing didymium in the same spot as lanthanum in III-6, as can be seen from
Fig.
11.4
.
This homologous placement of the rare-earth elements shows to what extend
Mendeleev
s viewpoints had changed by the end of the first half of 1870. What
catches the eye is that Dmitrii Ivanovich had switched from placing the rare-earth
elements as a
group
in the periodic system to an
individual
placement of each
element separately (Fig.
11.6
). That is, by breaking up the natural group of rare
earths, Mendeleev ended up with a set of distinct elements which he set out to
accommodate on an individual basis in
different
groups of the system.
While this individual accommodation of the rare-earth elements represented an
interesting step forward, it did not remove all problems. Mendeleev had always
used “a web of analogies” in determining the positions of the chemical elements,
but this methodology could no longer be applied in the case of the rare earths.
As Nathan Brooks observed, “the rare-earth elements exposed a serious weakness
in Mendeleev
'
s approach to solving the placement of elements in his periodic
system.”
68
Not surprisingly, Mendeleev remained doubtful as to the new positions
of the rare-earth elements.
Yet, the homologous placement offered a number of advantages as well.
By emphasising the
individuality
of the rare earths, Mendeleev denied the existence
of a
primary group
of rare-earth elements, which implied he could set aside all
problems connected with the existence of such primary groups—in particular, the
danger of undermined periodicity and the seeming impossibility of characterising
the elements, as basic substances, by their atomic weights.
'
68
Brooks (2002), op. cit., pp. 138-139. (note 10)
