Chemistry Reference
In-Depth Information
periodic table that would follow if atomic weight continued to be used to charac-
terise elements, the IUPAC introduced its ruling in 1923 that the elements were to
be characterised by atomic number rather than atomic weight. This was justified in
terms of the increasing understanding of the way the chemical properties of an
element are related to the electronic configuration of the isolated atom, which is in
turn governed by the number of protons in the nucleus, i.e. the atomic number.
The ruling on elements would carry over to their compounds, whose properties play
an important part in characterising positions in the periodic table, so that isotopic
variants of compounds with different isotopes of a given element are counted the
same substance.
5
The 1923 IUPAC ruling followed Paneth
s recommendation, which had been
challenged by Fajans in a debate leading up to the ruling. One of Fajans
'
points built
'
on a thought experiment of Polanyi
s suggesting that the spontaneous mixing of
isotopes is accompanied by a decrease in the Helmholtz free energy because the
process is irreversible, accompanied by an entropy of mixing (van der Vet
1979
,
pp. 294-5). In a comprehensive review and assessment of the Paneth-Fajans debate,
van der Vet (
1987
, Ch. II) argues that the thought experiment is mistaken. The work
is performed by the gravitational field, and in general, the entropy of mixing cannot
lead to observable effects providing a test of difference because any work done in
separation would be due to intermolecular interactions, whereas the entropy of
mixing is independent of any such interactions (pp. 116-8). Polanyi
'
s thought
'
experiment inspired “an incorrect argument
instrumental in the introduction
into the debate of a point accepted as correct today, namely that isotopes are
thermodynamically different” (p. 63). For example, there is an inequality of pro-
portions of isotopes in contiguous phases at equilibrium, which is large enough in
the case of hydrogen isotopes to allow separation by electrolysis.
When the distinction between physical and chemical properties was still a live
issue, it may have seemed that distinctions of
chemical
substance should rely on the
atomic number criterion and overrule thermodynamic distinctions of substances.
This is already a controversial claim in relegating thermodynamics to the study of
physical properties. But since 1923 it has become possible to discern differences in
the kinetics of chemical reactions arising from isotopic differences. The effect is
most marked in the case of hydrogen isotopes, as a result of which heavy water
(unknown in 1923) is poisonous—a classical chemical difference if ever there was
one! Of course, the differences are small and for many purposes may be ignored, as
in other cases (e.g. straight-chain alkanes of high molecular weight) where the
similarity is so great as to be of little consequence. But that doesn
...
t detract from the
fact that there is, in principle, a distinction of substance to be made (Needham
2008a
). This is not to deny that the more fine-grained criterion “was not part of the
intellectual context that informed earlier thinking about the elements” (Hendry
2010
, p. 927) in the pioneering studies of the periodic table. But the question is
'
5
Isotopes of oxygen might all be called oxygen just as isomers of heptane might all be called
heptane. The substantial point is whether isotopes are counted different substances as isomers are.
