How to draw Lewis Structures for molecular compounds:
Step 1: Arrange atoms and electrons
1. Add up total valence electrons (add more for anions, subtract some for cations).
2. Pick central atom (least electronegative). Always carbon; never hydrogen. Usually the first element
listed.
3. Place noncentral atoms around central atom and connect using lines to simulate bonds. Subtract
electrons used in bonds from total valence electrons.
4. Distribute remaining electrons around the noncentral atoms. Place extra electrons on central atom.
Step 2: Check your structure
5. Modify so that the octet rule is met (double and triple bonds, if possible) Some elements can have an
expanded octet (OK with more than 8 electrons) while some elements like B and Be are okay with less
than an octet (B-6 electrons, Be-4 electrons).
6. Rearrange structure so that all formal charges are as close to (0) as possible. Calculate the formal
charge using: FC= Valence - # of bonds – lone electrons (HELP: If two atoms are attached with opposite
charges (+ and -), move a pair of electrons from the negative atom into a double bond. If you are dealing
with an anion, the negative FC should be on the most electronegative element. If you are dealing with a
cation, the positive RC should be on the least electronegative.)
7. Resonance? Are there two of the same atom that look different? If yes, resonance!
PRACTICE PROBLEMS:
6. IOF5
12. PCl2111. SCl2
7. N3
13. H2SO4 (hint: do sulfate
2. SiF4
8. SOF4
first, then add H)
3. Ethene
9. SO3214. BrO334. CS2
10. OCCl115. Ethanoic acid (acetic
+
5. IO3
11. Pentane
acid/vinegar)
Give: lewis structure, formal charges, resonance, # of sigma and pi bonds,
hybridization, shape name, angles, polar/nonpolar
1. SCl2 (bent)
2. SiF4 (tetrahedral)
3. IO3+(trig planar)
4. CS2 (linear)
5. IOF5(oct)
6. N31- (linear, res)
7. SOF4 (trig bypyr)
8. SO32- (trig pyr, res)
9. OCCl1- (linear)
10. PCl21- (bent, res)
11. BrO33- (t-shape)
12. H2SO4 (hint: do sulfate first and then place H+ in the most logical location)