How To: Drawing Lewis Structures From Condensed Molecular Formulas

 

Drawing Lewis structures from condensed structural formulas is a survival skill for organic chemistry students.  There are three steps you should follow to draw a correct structure.

 

1.     From a condensed molecular formula, you obtain information about which atoms are connected to each other in a molecule.  Connect all of the appropriate atoms with single bonds first (lines).

 

Example: CH3CH2CH2CO2CH3

 

Comment: The difficult part of this structure is deciding how to arrange the two oxygen atoms. Using the arrangement shown will produce a stable structure with filled valences for all of the atoms upon completion of step 3. With practice you will begin to recognize common functional groups such as the carboxylic ester group of this example. If you are unsure, you must draw the different possibilities you are considering, and following completion of the structure, determine which one produces the stable structure with the maximum number of filled valence shells around the atoms.

 

2.     Add all of the additional valence electrons for each atom that does not already have a filled valence shell due to the single bonds.  Remember that for the purpose of counting valence electrons around atoms, assign one electron to each atom taking part in a single bond.  Make sure to keep track of any formal charges that may be present in the condensed structural formula (the present example has none).

 

Comment: Recall that each neutral carbon atom has 4 valence electrons, and each neutral oxygen atom has 6 valence electrons. After taking into account all of the single bonds in the molecule, the  carbon atom connected to both oxygen atoms has a single electron left over (4 total electrons - 3 single bonds = 1 electron left over), the oxygen atom attached only to one carbon atom has five electrons left over (6 total electrons 1 single bond = 5 electrons left over) and the other oxygen atom has four electrons left over (6 total electrons 2 single bonds = 4 electrons left over).

 

3.     Add multiple bonds to eliminate unpaired electrons.  Draw remaining non-bonding electrons as lone pairs.

 

Comment: The only unpaired electrons were on carbon and oxygen, leading to one new bond being formed.

 

 

 

 

The Lewis structure is now complete. The good news is that this will get easier. After you practice so that you are comfortable drawing Lewis structures by counting valence electrons, you will begin to recognize functional groups based on the numbers of bonds and lone pairs on the atoms. For example in molecules with no formal charges (Section 1.2E), hydrogen has one bond and no lone pairs, carbon has four bonds and no lone pairs, nitrogen has three bonds and one lone pair, oxygen has two bonds and two lone pairs, and halogens have one bond and three lone pairs. Remember that double bonds count as two bonds, and triple bonds count as three bonds in this type of analysis.

Atoms with formal charges can also be recognized by their characteristic number of bonds and lone pairs.  Positively charged nitrogen has four bonds, oxygen three bonds and one lone pair, and carbon three bonds (unfilled valence shell). Negatively charged nitrogen has two bonds and two lone pairs, oxygen one bond and three lone pairs, and carbon three bonds and one lone pair.