Lewis Structure Tutorial
To draw a Lewis Structure follow the steps below. At each step
few molecules you can try are listed. Clicking on the
molecular formulas will bring up what you should have for each
at each step. Try it yourself and then check as you go
After you have mastered this there are also some quick
rules which can be used for some of the atoms in the periodic
There are a lot of web sites with descriptions of how to draw Lewis
structures. You might find their explanations or methods
as well. Here are a number of links in no particular order: thinkquest,
Missouri-Rolla. You can find other links by doing a Google search
1) Count the total number of valence electrons using the periodic
table as a guide. Remember that for the main group
the number of electrons contributed by each atom can be determined
the group the element is in. For groups 1 (1A) and 2 (2A) the
of valence electrons equals the group number. For groups 13
- 18 (8A) the number of valence electrons is the number of the group
2) Next draw single bonds from each of the outer atoms to the
in the molecule (in the examples below the central atom is in
type). The central atom is usually the atom with the
electron affinity. However, it is sometimes difficult to tell
additional information. Subtract two electrons from the total
of electrons for each bond you have made. This tells you how
electrons you have left to use elsewhere.
3) Remembering that each bond counts as two electrons, put electrons
the outer atoms to give each atom a total of eight (an octet).
that H (hydrogen) only needs two electrons and B (boron) often only
six electrons. Keep track of how many electrons you are
If you run out of electrons before filling the outer atoms' octets,
4) Any electrons that were not used up in step 3 should be put on
atom. You should now have no unused valence electrons.
5) If any atoms do not have octets, make multiple bonds (double and
by sharing electron pairs from atoms that do have octets.
6) Look for resonance structures. If you have made multiple
or have odd electron species where all the atoms cannot have octets,
may be more than one way to arrange the multiple bonds or place the
electron. If so, the molecule is better modelled as an average
all the possible structures.
7) Use formal charge to pick the resonance structure likely to
contribute the most to the structure. (Formal charge) = (number of
valence electrons atom starts with) - (number of electrons assigned
the atom). (Number electrons assigned to atom) = (number of
non-bonding electrons) + 1/2(number of bonding electrons). Do
this calculation for each atom in each resonance structure.
Resonance structures with formal charges closest to zero are lower
energy. Negative formal charges on the more electronegative
are also preferred.
8) If the central atom is from row 3 of the periodic table or
below, check for additional resonance structures where the central
has more than 8 electrons (expanded octet). Sometimes these
resonance structures produce better formal charges. This is
encountered with compounds of S and P.
****Quick rules which work well for
period two atoms (H, C,
O, F) and the halogens (group 17) most of the time.*******
||Number of Bonds
|F (same for other halogens)
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Last updated: October 11, 2011