Directions for drawing Lewis Structures having a net zero charge.
1) Count the total valence electrons for the molecule: To do this, find the number of valence
electrons for each atom in the molecule, and add them up.
2) Figure out how many octet electrons the molecule should have, using the octet rule: The
octet rule tells us that all atoms want eight valence electrons (except for hydrogen, which
wants only two), so they can be like the nearest noble gas. Use the octet rule to figure out how
many electrons each atom in the molecule should have, and add them up. The only weird
element is boron - it wants six electrons.
3) Subtract the valence electrons from octet electrons: Or, in other words, subtract the
number you found in #1 above from the number you found in #2 above. The answer you get
will be equal to the number of bonding electrons in the molecule.
4) Divide the number of bonding electrons by two: Remember, because every bond has two
electrons, the number of bonds in the molecule will be equal to the number of bonding
electrons divided by two.
5) Draw an arrangement of the atoms for the molecule that contains the number of bonds
you found in #4 above: Some handy rules to remember are these:
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Hydrogen and the halogens bond once.
The family oxygen is in bonds twice.
The family nitrogen is in bonds three times. So does boron.
The family carbon is in bonds four times.
A good thing to do is to bond all the atoms together by single bonds, and then add the multiple
bonds until the rules above are followed.
6) Find the number of lone pair (nonbonding) electrons by subtracting the bonding
electrons (#3 above) from the valence electrons (#1 above). Arrange these around the atoms
until all of them satisfy the octet rule: Remember, ALL elements EXCEPT hydrogen want eight
electrons around them, total. Hydrogen only wants two electrons.
Directions for drawing Lewis Structures having a charge.
1) Count the total valence electrons for the molecule: To do this, find the number of valence
electrons for each atom in the molecule, and add them up. For polyatomic anions, add the
charge of the ion to the number of valence electrons. For polyatomic cations, subtract the
charge of the ion from the number of valence electrons.
2) Figure out how many octet electrons the molecule should have, using the octet rule: The
octet rule tells us that all atoms (including boron) want eight valence electrons (except for
hydrogen, which wants only two), so they can be like the nearest noble gas. Use the octet rule
to figure out how many electrons each atom in the molecule should have, and add them up.
3) Subtract the valence electrons from octet electrons: Or, in other words, subtract the
number you found in #1 above from the number you found in #2 above. The answer you get
will be equal to the number of bonding electrons in the molecule.
4) Divide the number of bonding electrons by two: Remember, because every bond has two
electrons, the number of bonds in the molecule will be equal to the number of bonding
electrons divided by two.
5) Draw an arrangement of the atoms for the molecule that contains the number of bonds
you found in #4 above: Some handy rules to remember are these:
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Hydrogen and the halogens bond once.
The family oxygen is in bonds one, two, or three times.
The family nitrogen is in bonds two, three, or four times
Boron usually bonds four times.
The family carbon is in bonds four times.
A good thing to do is to bond all the atoms together by single bonds, and then add the multiple
bonds until the rules above are followed.
6) Find the number of lone pair (nonbonding) electrons by subtracting the bonding
electrons (#3 above) from the valence electrons (#1 above). Arrange these around the atoms
until all of them satisfy the octet rule: Remember, ALL elements EXCEPT hydrogen want eight
electrons around them, total. Hydrogen only wants two electrons.
7) To find the charge on each atom, compare the number of electrons that each atom has to
the number of valence electrons it usually has. For this purpose, each bond counts as one
electron and each lone pair counts as two electrons. For example, in CO2 above, carbon has four
electrons (because it has four bonds) and oxygen has six (two bonds + 4 lone pair electrons). If
the number of electrons that the atom has is more than the normal number of valence
electrons, the atom has a negative charge. If the number is less than the normal number of
valence electrons, the atom has a positive charge. If it's the same, the atom has no charge at
all.