Lewis Structures: Guidelines and Exercises
GUIDE TO DRAWING LEWIS STRUCTURES
1. If ionic, treat ions separately.
For ionic compounds, including those with polyatomic ions, analyze each of the ions separately.
2. Count valence electrons.
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Sum valence electrons for all atoms. Valence electrons are those in highest ns and np plus
partially filled (n-1)d and (n-2)f orbitals. Use the Periodic Table as a guide.
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Add 1e- for each (-) charge on ion.
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Subtract 1e- for each (+) charge on ion.
3. Assemble bonding framework using single bonds.
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H atoms are always outer atoms.
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Other outer atoms usually have higher electronegativity than inner atoms.
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Multiple inner atoms are sometimes indicated by the molecular formula (e.g., CH3OH or HOCN).
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Experience will help as a guide, but you may have to try several different possibilities, and use
steps 6 & 7 to rule out unlikely arrangements.
4. Complete octets of outer atoms (except H) with lone pairs.
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Place 3 lone (non-bonding) pairs on each outer atom (except H).
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If there are insufficient valence electrons (see step 2) to give all outer atoms octets, complete the
octets of the more electronegative atom first.
5. Assign remaining electrons to inner atoms.
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Place any remaining electrons on the inner atom(s).
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If there are multiple inner atoms, complete octets of more electronegative inner atom first.
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If valence electrons remain after all octets are completed, place remaining electrons on the more
electronegative inner atom that has n>2.
= PROVISIONAL STRUCTURE. Optimize structure(s) below:
6. Optimize inner atom electron configuration.
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If the inner atom lacks an octet, shift lone pairs of outer atoms to create double or triple bonds.
7. Identify equivalent or near-equivalent structures.
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If there are multiple options for shifting outer-atom lone pairs, the most stable structures are
usually those with the lowest magnitude of formal charges (electroneutrality principle).
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Structures that place positive formal charges on the least electronegative atoms, and negative
formal charges on the most electronegative atoms, tend to be favored.
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There may be multiple equivalent or near-equivalent “resonance” structures. Indicate those that
are most favored. The real bonding in the molecule reflects some ‘average’ of these structures.
Formal charge = valence electrons of free atom – (dots + sticks) around atom in structure
Chem 131
Lewis Structures
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LEWIS STRUCTURE EXERCISES
1. Draw Lewis Structures for the following common substances:
NH3 (ammonia)
H2O2 (hydrogen peroxide)
NaOH (sodium hydroxide)
2. Draw the Lewis Structure for the
hydronium ion (H3O+) and
calculate the formal charge on
each atom.
3. Draw the Lewis Structure for formic acid (HCO2H). Try different arrangements of atoms and use
formal charges to determine the best arrangement.
4. Draw the Lewis Structure for sulfur dioxide (SO2).
5. Draw the Lewis Structure for potassium nitrate (KNO3).
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6. Draw the Lewis Structure for nitric acid (HNO3). Try attaching the H to either the N or an O.
Which is the best arrangement? Compare the structure to the nitrate ion in the question above.
7. Draw the Lewis Structure for chlorine trifluoride (ClF3).
8. Draw the Lewis Structure for nitrous oxide (N2O). You should try different arrangements of
atoms and different resonance structures.
Chem 131
Lewis Structures
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