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232 Adaptation

more, for a total of 9. Doubling the bonds between two carbon atoms is the
right action in some situations, but it is an error in this situation. The reason is
that carbon atoms strive toward the noble gas configuration, which means that
they strive to have 8 valence electrons. In this case, the carbon atom already
had 8 valence electrons, so creating one more bond overshoots the goal of
achieving the noble gas configuration by giving the atom 1 valence electron
too many.
The HS model can discover this error by inspecting the resulting problem
state and noticing that the number of valence electrons is too large. To do this,
the model has to have some prior knowledge of the fact that there is a maximal
number of valence electrons for each substance and that an individual atom
cannot have more valence electrons than indicated by that number. That is, it
must already possess the following constraint:

If X is an atom in a Lewis structure, X belongs to element M, and atoms of M
have at most N valence electrons, then the current number of valence electrons
for X is equal to or lower than N (or else the Lewis structure is not correct).

The meaning of this constraint is that if the current number of valence elec-
trons for a particular atom is V and the maximum number of valence electrons
for atoms belonging to that substance is N, then it had better be the case that
V is smaller than or equal to N (or else some error has been committed). More
colloquially: An atom cannot have more valence electrons than the maximal
number of valence electrons for its type.
Having detected that the constraint is violated, HS proceeds to revise the
responsible rule. The computations described previously will in this case pro-
duce three possible new conditions:
Atom X belongs to element M.
The maximum number of valence electrons for atoms of element M is N.
The current number of valence electrons for X, V, is less than or equal to
N-1.
The first of these three conditions can be eliminated because it is identical to
an expression that is already part of the rule and hence is redundant. Adding
the remaining two expressions to the existing rule produces the new rule (new
conditions in bold font):

If the goal is to balance the Lewis structure,
if there are 2 carbon atoms in the molecule,
if the 2 carbon atoms have a single bond between them,