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affords them. Nevertheless, the question as to what distinguishes those constituent
structures under which a substance affords an atomic spectrum of such and such a
pattern and a substance which affords a different atomic spectrum is a legitimate
question, the question of the grounding of affordances. According to Bohr
s theory
spectral affordances are grounded in configurations of electrons considered to be
constituents of atoms. When we discover that incandescent sodium affords two
yellow spectrum lines by passing the emitted light through a spectrometer what is
affording this phenomenon - sodium atoms having a certain electronic configura-
tion. If this makes sense, how have we managed to outflank the second
mereological fallacy? We know there are no colours as constituents of atoms.
In the Bohr conception of an atom it is taken for granted that electrons are
constituen
ts of atoms because atoms afford electron phenomena under certain
manipulations. Electrons are nothing like spectral colours, so it seems unpro-
blematic to assign them to the category of constituents of the inner structures
of atoms?
Molecular and atomic spectra play an important part in Mulliken
'
sview
of the nature of chemical beings (Mulliken
1932
).
2
How do we set up an exper-
iment to produce molecular affordances? Why is it illegitimate to project electron
affordances back as constituents of atoms, but legitimate to project atom affor-
dances back as constituents of molecules? The short answer is simply that
there are clear criteria of identity, both numerical and qualitative, that serve to
pick out atoms as material individuals. The metaphysical question was settled
empirically by the development of the technique of the travelling tunnelling
microscope for which Binnig and Rohrer (
1986
)wereawardedtheNobel
Prize. The shape and boundary of an individual atom could be traced out.
Is there a corresponding procedure that would establish electrons as bounded
individuals?
Mulliken
'
'
s invention of the concept of
'
molecular orbital
'
allowed him to
maintain the Boylian style principle that
as
a basic mereological principle while steering clear of committing the second
mereological fallacy of projecting electron-affordances back into molecules as
constituents. If they are not constituents of atoms they cannot be constituents of
molecules. Hence the move from interpreting the quantum mechanical formulation
of the processes of chemical bonding as referring to
'
atoms are constituents of molecules
'
orbitals
rather than
orbits
'
(Llored and Harr´
2011
). As we will see this move also changes the kind of
model
with which molecular chemistry is made intelligible.
'
'
'
2
I am grateful to J.-P. Llored for bringing Mulliken
s way of using the quantum mechanics of
electrons in explaining bonding without requiring the assumption of actual orbits.
'
