Online
Introductory Chemistry
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Dr. Walt
Volland, Bellevue Community College
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Drawing Lewis
Structures
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After you have
studied this lesson you should be able to:
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- state a general method for
drawing Lewis structures
- identify the central atoms in a
molecular formula
- tell the number of valence
electrons on a main-group element
- explain what a central atom is
in a Lewis structure
- tell how many total valence
electrons are in a Lewis structure
- tell how to account for
electrical charges in the electron count in a Lewis
structure
- explain how electron pairs are
represented by "lines"
- identify unshared pairs in a
Lewis structure
- draw Lewis structures for
molecules when given the formula
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Drawing Lewis structures is a trial
and error process. There are some basic steps that can make
the process go better. You are not going to be able to draw
Lewis structures for everything that exists, but you can
successfully draw Lewis structures for many common
molecules.
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Step 1: Look up the group numbers
for all of the atoms in the formula to determine the number
of valence electrons on each.
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Step 2: Calculate the total number
of valence electrons contributed by the atoms. This requires
multiplying the valence electron count by the number of
atoms of the element.
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Step 3: Check to see if there is a
charge on the formula unit. Adjust the total in Step 2 by
the number of electrons added or removed because of any
electrical charge. This gives you the final total of valence
electrons that are available to hold the particle
together.
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Step 4: Identify the central
atom/atoms in the formula unit.
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Step 5: Think of your structure as
a target. Write down the symbol for the central atom in the
"bullseye". Then arrange the symbols for the oxygen atoms
evenly in the next ring around the center atom.
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Step 6: Divide the total number of
valence electrons for the formula unit (Step 3) by 2 so you
know the number of "electron pairs" for the
particle.
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Step 7: Connect the center atom
with a single bond (1 pair of electrons) to each neighboring
atom in the second ring. Then, using a single bond, connect
each atom in the second ring to its neighbor in the third
ring. Count the number of electron pairs used.
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Step 8: Subtract the number of
electron pairs used in Step 7 from the total number of
electron pairs for the particle (Step 6). Use these
remaining electron pairs to fill out the octets for atoms in
the second ring.
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Step 9: The almost last step is
verification that the atoms each have an octet or duet. If
they do you are finished.
If the central atom lacks
an octet when only single bonds are used you need to move a
lone electron pair to make one or more multiple bonds. This
may mean double or triple bonds as appropriate.
NOTE: Any lone pair can
be used here.
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