Question 8.9: Formaldehyde (CH2O), which can be used to preserve biologica...

Formaldehyde (CH₂O), which can be used to preserve biological specimens, is commonly sold as a 37% aqueous solution. Use formal charges to determine which skeletal arrangement of atoms shown here is the best choice for the Lewis structure of CH₂O.

H–C–O–H        H- \overset{\overset{O}{|} }{\underset{ }{C}} – H

Strategy Complete the Lewis structures for each of the CH₂O skeletons shown and determine the formal charges on the atoms in each one.

Setup The completed Lewis structures for the skeletons shown are:

 H- \overset{..}{C}- \overset{..}{O}- H               

In the structure on the left, the formal charges are as follows:

Both H atoms: 1 valence e^– – 1 associated e^– (from single bond) = 0
C atom: 4 valence e^– – 5 associated e^– (two in the lone pair, one from the single bond, and two from the double bond) = -1
O atom: 6 valence e^– – 5 associated e^– (two from the lone pair, one from the single bond, and two from the double bond) = +1

\underset{Formal  charges}{}        \underset{0}{H} – \overset{..}{\underset{-1}{C} } = \overset{..}{\underset{+1}{O} }- \underset{0}{H}

In the structure on the right, the formal charges are as follows:

Both H atoms: 1 valence e^– – 1 associated e^– (from single bond) = 0
C atom: 4 valence e^– – 4 associated e^– (one from each single bond, and two from the double bond) = 0
O atom: 6 valence e^– – 6 associated e^– (four from the two lone pairs and two from the double bond) = 0

Formal charges all zero