Page 248 - Deep Learning

Error Correction: The Specialization Theory 231

each atom strives to have a certain number of electrons in its outer shell, its
so-called noble gas configuration; and that the total number of valence elec-
trons in the molecule cannot differ from the sum of the valence electrons of
the component atoms. Only a few of the logically possible arrangements of the
atoms will satisfy the constraints imposed by this theory.
The cognitive skill of deriving the structure of a molecule from its sum
formula is typically taught at the beginning of an advanced high school or
college course in organic chemistry. Many textbooks teach the derivation
of Lewis structures by giving students (a) an exposition of the theory of
the co-valent bond, (b) a verbal recipe for a weak, radically underspeci-
fied derivation procedure, (c) one or two solved examples, and (d) practice
problems. This learning scenario was simulated with the HS model. The
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principles of the theory of the co-valent bond were encoded as constraints.
The hints about how to proceed typically given to students were encoded
as initial rules. Finally, the model was given a sequence of practice prob-
lems corresponding closely to the problems posed in college textbooks.
The model successfully learned a general strategy for constructing Lewis
structures.
For example, at one point the model had constructed for itself the rule:
If the goal is to balance the Lewis structure,
if there are 2 carbon atoms in the molecule,
and if the 2 carbon atoms have a single bond between them,
then double that bond.
To balance a Lewis structure is to make sure that the number of valence
electrons of the molecule equals the sum of the valence electrons for the com-
ponent atoms. If this is the goal, and if there are 2 carbon atoms that have only
a single bond between them, then consider creating a double bond between
them. This is a standard move in deriving Lewis structures. The hypothetical
rule shown above is representative of the kinds of rules that one would expect
a novice to possess halfway through mastering the target skill: It is sensible but
not completely correct. The context in which doubling a carbon-carbon bond
is appropriate and useful has not been fully specified. Consequently, the rule
creates double carbon-carbon bonds in many situations in which this move
creates an incorrect Lewis structure.
For example, consider a situation in which 2 carbon atoms have a single
bond between them but one of them already has 8 valence electrons, which
is the ideal gas configuration for carbon. If the rule above is executed in this
situation, then the bond is doubled, so the atom with 8 electrons acquires one