9/30/2014
ALEKS Alex H. Yuan - 09/30/2014 11:08:52 PM EDT - Copyright © 2014 UC Regents and ALEKS Corporation
Alex H. Yuan - 09/30/2014 11:08:52 PM EDT
General Chemistry (First Semester) / Chem 110 FA 2014 – CH110 FA14 S25-36 (Dr.
Barber)
1.
Writing Lewis structures for an expanded valence shell central atom
Draw the Lewis structure for the sulfur tetrafluoride
molecule.
What you'll notice as soon as you try to draw the Lewis structure of this molecule is that it
seems to have too many valence electrons to satisfy the octet rule for each atom. That's
because this molecule is one of the exceptions to the octet rule.
First of all, there are
pairs of valence electron pairs in the
(
atom of sulfur with
(
atoms of fluorine, each with
molecule:
valence electrons)
valence electrons)
TOTAL
Secondly, after you have assigned one bonding pair between each pair of bonded atoms,
and enough lone pairs to satisfy the octet rule for every atom in the molecule, you still have
electron pair left over:
F
F
pairs assigned
S
F
total pairs
F
pair left over
Now what?
Since the sulfur tetrafluoride molecule really does exist, and it really does have
pairs of
valence electrons, we are forced to recognize that the octet rule has exceptions. The
exception in this case is called the valence-shell expansion exception:
Valence shell expansion: Atoms of elements in Period
sometimes fit more than
and higher can
pairs of valence electrons around them.
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9/30/2014
ALEKS Alex H. Yuan - 09/30/2014 11:08:52 PM EDT - Copyright © 2014 UC Regents and ALEKS Corporation
Since sulfur is in Period the central atom of sulfur can fit more than pairs of valence
electrons around it. That means we can assign the remaining pair of valence electron as a
lone pair on the central sulfur atom.
Here is the correct Lewis structure for
F
F
S
F
F
Copyright © 2014 UC Regents and ALEKS Corporation
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