Chemistry Reference
In-Depth Information
twentieth century, after which structural formulas have been interpreted as
depicting molecules (an idea completely foreign to Dalton
s conception of micro-
structure, which included no notion of entities comprising small numbers of
interlinked atoms not so linked with neighbouring atoms). From such beginnings
there arises the
molecular structure thesis
, according to which a quantity of a single
substance is a collection of molecules of a single kind and sameness of substance
is governed by sameness of molecular structure. But this microscopic criterion
faces several challenges.
Although molecular structure thesis is straightforwardly applicable to a range of
organic compounds, many substances are not molecular. The silicon dioxide in a
grain of sand is not divided into SiO
2
molecules, but a covalently bonded structure
extends over the whole grain. Similarly, an ordinary ionic compound like common
salt comprises uniformly distributed ions, not NaCl molecules (a conclusion still
disputed in the late 1920s
6
). However, broadening the general thesis by more
carefully specifying “molecular structure” in different ways to accommodate
many less straightforward cases raises the danger of conflict. Thus, for many
substances which, unlike typical organic compounds, are not molecular in the
sense that quantities of them are collections of molecules of a single kind, it
would appear that the molecular structure thesis would have to count as many
substances as there are different molecular structures, contrary to normal practice
based on other criteria. Other substances apparently call for further specification of
structural features beyond what is needed to distinguish stereoisomers, and this can
lead to conflict with what is said in the case of non-molecular substances. Water and
proteins illustrate these points nicely.
Water is the notorious example of a non-molecular substance which is particu-
larly complicated in the liquid phase. Intramolecular bonds are constantly being
broken and reformed with the creation and recombination of hydrogen and
hydroxyl ions whilst intermolecular hydrogen bonds forming polymeric species
form and break incessantly. Consequently, a given quantity of water cannot be
mereologically partitioned into simple H
2
O molecules for any given time but
comprises entities considerably larger than an H
2
O molecule whose parts are
constantly severed and rejoined in new constellations. Nevertheless, it is a single
substance by the phase-law criterion. But it is difficult to see how the molecular
structure thesis could agree with this ruling, although it is equally unclear how
many substances it would rule that there are in a glass of water at normal temper-
ature and pressure during a given time.
7
'
6
Armstrong (
1927
, p. 478) regarded Bragg
'
s claim that in sodium chloride there are “no molecules
represented by NaCl. The equality in number of sodium and chlorine atoms is arrived at by a chess-
board pattern of these atoms; it is a result of geometry and not of a pairing-off of the atoms” as
“repugnant to common sense” and “not chemical cricket”.
7
I suggest that the macroscopic quantities to which the two-place predicate applies for macro-
scopic intervals of time are structureless mereological sums of all the material bits that partake of
these interconversions at the microlevel (Needham
2010b
).
