[C] [Wittrs] Re: philosophy, physics, chemistry, and Justintruth's argument
- From: "J D" <ubersicht@xxxxxxxxx>
- To: wittrsamr@xxxxxxxxxxxxx
- Date: Thu, 04 Feb 2010 00:41:41 -0000
Justin,
Just replying to answer some of your specific questions, re: periodocity,
orbitals, and reduction, although these issues don't exhaust the range of
issues just beginning to be considered in the emerging field of philosophy of
chemistry.
From "The Case for Philosophy of Chemistry" by Serri and McIntyre
One very important form of explanation which pervades all areas of
chemistry, from teaching to frontier research, lies in talk of electron shells
or orbitals, as they are often called. The formation of bonds, acid-base
behavior, redox chemistry, photochemistry, reactivity studies, etc., are all
regularly discussed by reference to the interchange of electrons between
various kinds of orbitals.
This approach may at first sight seem to speak in favor of the epistemo-
logical reduction of chemistry to physics, since talk of electron shells is
thought to belong primarily to the level of atomic physics. However, a more
critical examination of the issues involved reveals no such underpinning
from fundamental physics. It emerges that explanations in terms of electron
orbitals, and indeed all talk of orbitals in chemistry, is not sanctioned by
our present understanding of quantum mechanics. The remarkable fact is
that at the most fundamental quantum mechanical level electronic orbitals
become ontologically redundant. Electronic orbitals simply do not exist
according to quantum mechanics, although they remain as a very useful
explanatory device. This result is embodied in the more fundamental ver-
sion of the Pauli Exclusion Principle, which is frequently forgotten at the
expense of the restricted and strictly invalid version of the Principle, which
does uphold the notion of electronic orbitals (Scerri 1991, 1995).
This situation implies that most explanations given in chemistry which
rely on the existence of electrons in particular orbitals are in fact "level
specific" explanations, which cannot be reduced to or underwritten by
quantum mechanics.18 Thus, a case has been demonstrated where the
explanation of what it is that we seek to know when we engage in chemical
explanation would seem to suggest that we eschew reductive explanations,
and support the explanatory autonomy of chemistry.
from "Philosophy of Chemistry?A New Interdisciplinary Field?" by Serri
How many of us
have experienced students' frustration when we give different
chemical explanations depending on the context in which one
and the same phenomenon is being discussed?
If one believes only in fundamental explanations, this
form of activity appears to be seriously mistaken. However,
as chemists we are also aware of the need to operate on many
levels and the fact that explanations can be genuinely level-
specific. Such approaches must be used very carefully. They
should not degenerate into the introduction of ad hoc explana-
tions that are invoked in the explanation of particular
chemical facts but cannot be generalized to other situations.
One example is the wide variety of explanations given for
the apparent orbital paradox concerning the relative occupation
and ionization of the 4s and 3d levels in the first transition
metal series. The paradox I allude to is that the 4s orbital
is preferentially occupied but also preferentially ionized.
Nobody has yet rationalized this situation at a level that might
be appropriate for teaching general chemistry. Most educators
and textbooks continue to argue that the 4s orbital is prefer-
entially occupied because it has a lower energy than 3d, in
spite of several articles published in this Journal that state
that the 4s orbital never has a lower energy than the 3d (9).
Another response, encountered particularly among theo-
eticians, is that this is a futile question because the concept
of orbitals ceases to refer to any objective entities in more
advanced calculations and can only be maintained at the level
of the Hartree?Fock approximation. I suggest that this kind
of response is just another way of expressing Dirac's famous
dictum whereby chemistry has been explained in principle
by quantum mechanics. Such a response amounts to evading
the issue, which is to try to obtain a consistent explanation
within an orbital approximation such as the Hartree?Fock
model, since within this regime the concept of an orbital is
well defined.
JPDeMouy
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