Chemistry Reference
In-Depth Information
mathematical devices: “basis sets, or a form of coordinate system, with which the
wave function of an atom, ion, or molecule can be expanded mathematically to any
degree of accuracy dictated by the available computational power” (ibid). I take it
that most chemists would consider the models constructed on the basis of atomic
orbitals to be highly idealized and of approximate nature, but not causal facts per se.
They are not to be taken literally in all respects. Indeed it is difficult to resist the idea
that the term “orbital” is non-referential and hence defending an ontological com-
mitment to “chemists
orbitals” is a challenging task.
6
Similar concerns apply to molecular orbitals. One constructs molecular orbitals
and populates them with electrons is a manner analogous to an individual atom
by adopting the linear combination of atomic orbitals (LCAO) approximation.
While this might lend the impression that molecular orbitals are merely an exten-
sion of atomic orbitals, they are conceptually distinct. An atomic orbital is a
description of the state of motion of an electron subject to the influence of a single
nucleus plus other electrons. But molecular orbitals describe electron motions in the
field of two or more nuclei plus the other electrons and the use of the LCAO method
is merely a matter of mathematical convenience (Gavroglu and Simoes
2012
,
p. 83). The delocalized character of molecular orbitals is conceptually quite distinct
from the idea of atomic orbitals, and Mulliken - one of the originators of the
molecular orbital approach - was at pains to distinguish his “conceptual scheme”
from the methods employed to compute them (ibid, pp. 84-85).
Although conceptually distinct, molecular orbitals are methodologically under-
written by the idea of electron configurations and depend on atomic orbitals for
their veracity. And the use of molecular orbitals in the study of organic reactions
requires significant idealization. Perhaps they could be treated as idealizations
which might have been intended to be taken literally, but it turns out that they are
not causal difference-makers after all. They would then be what Strevens calls a
“preidealization” (op. cit., p. 300). They are simply errors, but only mild ones
because they do not damage our explanations as much as an error one might make
by misrepresenting a genuine difference-maker. An example of what Strevens
has in mind is Newton
'
s explanation of Kepler
'
s laws. Attributing a force of gravity
to the explanation of Kepler
'
s laws was meant to be taken literally, but in an age of
relativistic physics we know that force is an addition that does not make a differ-
ence to the explanatory target. We no longer take Newton
'
s
laws literally, although most would agree that it still has some explanatory power
(ibid pp. 327-329).
The extent to which preidealizations have explanatory power is again an issue
that lies outside of the kairetic account proper because it turns out that they do not
misrepresent causal difference-makers but they might enjoy a potential extrinsic
explanatory function in the communicative mode of explanation. But molecular
orbitals are different. They are presumably not intended to be taken literally, but
are
s explanation of Kepler
'
'
6
This would require a defense of the ontological autonomy of chemistry. For an example of such a
defense, see Lombardi and Lambarca (
2005
).
