Chemistry Reference
In-Depth Information
the discontinuity between these two meanings, which for us is a result of the
conceptual breakdown between molecular chemistry and quantum mechanics.
4
Accordingtohim,“
if there is indeed any kind of
“
conceptual breakdown
”
(Labarca and Lombardi
2010a
,p.155),
it appears
within
quantum mechanics
,
equally affecting the quantum
-
mechanical and chemical meaning of
orbital
.”
(Mulder
2011
, p. 33), and it is due to the use of approximations, in particular the
suppression of all interactions except those with the nuclei at relatively fixed
nuclear positions: “
Approximations are therefore made within quantum mechan-
ics
,
as a result of which the quantum
-
mechanical concept of an orbital arises
.”
(Mulder
2011
, p. 33). Moreover, “
the meaning of
'
'
asaregionofhigh
probability density follows quite straightforwardly from its meaning as a wave
function
.[
orbital
'
'
]
The two concepts are therefore continuous with one another
.”
(Mulder
2011
,p.33).
It is interesting to analyze this criticism because it brings to light the difference
between two kinds of approximations, those that remain confined in the conceptual
context of a discipline, and those leading to a conceptual breakdown. When, in the
domain of classical mechanics we study the motion of a body on a surface with very
low friction, we are entitled to approximate the situation to another with no friction
which, although non-existent in the empirical world, is an admissible situation in
classical mechanics. On the contrary, if we suppose that a very fast particle moves
at a speed exceeding the speed of light
...
an assumption completely legitimate in
the classical domain
, we introduce a conceptual breakdown with respect to
special relativity: the state of affairs resulting from the assumption contradicts
one of the principles of the theory itself.
Inthecaseoftheapplicationofquantum mechanics to the description of
molecules, the suppression of interactions between electrons is an approximation
that remains within quantum mechanics. But this is not the case when, as is usual
in chemistry, the electronic density
computed by squaring the amplitude of the
wavefunction of an electron
is interpreted as a kind of mean value of the
definite positions occupied by the electron in its motion around the nucleus: “
If
a series of measurements could be made of x without disturbing the motion of the
4
We will not discuss the charge of “
incoherence
” that Mulder directs to our position (Labarca and
Lombardi
2010a
), since the charge is based on the fact that we supposedly “
maintain that in
chemistry as well
,
electrons do not have definite trajectories
,
i.e. no definite positions at all times
.”
(Mulder
2011
, p. 32). Since Mulder does not cite the criticized claim, and fails to indicate the page
number, it is hard to understand where he could have drawn this conclusion from. In fact, we take
just the opposite position: whereas quantum items are not spatially localized individuals, electrons
in chemistry
although ruled by a law that fixes their position only statistically
are individual
objects in a classical sense. Perhaps Mulder
s conclusion is due to the fact that we say that
“
electrons do not follow definite orbits
” (Labarca and Lombardi
2010a
, p. 154). But this does
not mean that “chemical” electrons do not follow definite trajectories, since not every trajectory is
an orbit; an orbit is a closed trajectory which, in general, is governed by a simple law. By contrast
with Mulder
'
s reading, we claim that, although chemistry does not adopt a planetary model of the
atom where electrons follow definite orbits, electrons are still conceived as individual and local
objects with a definite spatial position.
'
