Chemistry Reference
In-Depth Information
particle
,
the resulting distribution would be
.
The latter would then reflect
the motion of the particle in the same way in which the density of the image on
along
-
exposure photograph reflects the motion of a macroscopic object
.” (Nelson
1990
, p. 643).
5
However, this picture is not the consequence of an innocent
approximation: it is based on the assumption of individual objects with definite
positions, whose existence is in contradiction with the non-individuality of quan-
tum systems resulting from the principles of the theory. Therefore, the transition
from the concept of orbital as wavefunction to the concept of orbital as a spatial
region of high electronic density is not a continuous transformation within
quantum mechanics, as Mulder claims, but a conceptual breakdown with respect
to the theory that is supposedly “fundamental”.
The concept of orbital is not the only theoretical element where the conceptual
discontinuity between molecular chemistry and quantum mechanics becomes man-
ifest. By separating the wavefunction of the molecule into its electronic and its
nuclear components, the Born-Oppenheimer approximation makes it possible to
compute the energy levels of even very complex molecules. The Born-
Oppenheimer approximation proceeds in two steps. The first step is often referred
to as “
the clamped nuclei approximation
”: the electron-nucleus interactions are
conceived of in terms of electrons in the Coulomb potential produced by nuclei
“clamped” at definite positions. (for a detailed discussion of the implicit assump-
tions underlying the Born-Oppenheimer approximation, see Lombardi and
Castagnino
2010
). But the assumption of particles
ρ
with definite
values of momentum and of position is at odds with the Heisenberg principle, which
prevents a quantum system from simultaneously having definite values of
non-commuting observables. In other words, the clamped nuclei approximation
introduces a conceptual breakdown with respect to quantum mechanics, just as a
speed exceeding the speed of light makes it with respect to special relativity.
In conclusion, the approximations generally used to link molecular chemistry
with quantum mechanics are not confined “within” quantum mechanics; on the
contrary, they bring in a discontinuity between the concepts of the two domains,
and such discontinuity prevents those concepts from being interpreted as referring
to the same ontological item.
Despite having claimed the continuity between the quantum-mechanical and the
chemical meanings of the term
the nuclei
, Mulder expresses his worries about how
electrons are conceived in chemistry: “
Does this mean that chemistry is
orbital
'
'
false
'
because it regards electrons as individuals
?
I think that one has to concede that to
the extent that chemists speak of electrons as individuals
,
what they say is indeed
not literally true
.” (Mulder
2011
, p. 32). According to the author, “
individualism
about electrons in chemistry
” has a merely pragmatic character, whereas
'
5
Nelson
s statement is cited by Mulder in his 2010 paper. However, here the author seems to
“forget” what he says in his 2011 paper, where he admits that the term
'
has two different
meanings. From a more decidedly reductionist position, in his 2010 paper Mulder considers
Nelson
orbital
'
'
s position simply wrong: the chemical view of orbitals is a misconception.
'
