Draw an electron-dot structure for formaldehyde, [latex]CH_{2}O[/latex]. STRATEGY First, count the total number of valence electrons. Carbon has 4, each hydrogen has 1, and the oxygen has 6, for a total of 12. Next, decide on the probable connections between atoms, and draw a line to indicate each bond. In the case of formaldehyde, the less electronegative atom (carbon) is the central atom, and both hydrogens and the oxygen are bonded to carbon: [latex]\begin{matrix}\underset{|}{O}\\ H-C-H \end{matrix}[/latex] Six of the 12 valence electrons are used for bonds, leaving 6 for assignment to the ter-minal oxygen atom. [latex]\begin{matrix}:\overset{..}{\underset{|}{O}}:\\ H-C-H \end{matrix}\\ \quad\quad\quad\hookleftarrow \text{Only 6 electrons here} [/latex] At this point, all the valence electrons are assigned, but the central carbon atom still does not have an octet. We therefore move two of the oxygen electrons from a lone pair into a bonding pair, generating a carbon–oxygen double bond and satisfying the octet rule for both oxygen and carbon.

Question 7.7: Draw an electron-dot structure for formaldehyde, CH2O....

Draw an electron-dot structure for formaldehyde, $CH_{2}O$ .

STRATEGY

First, count the total number of valence electrons. Carbon has 4, each hydrogen has 1, and the oxygen has 6, for a total of 12. Next, decide on the probable connections between atoms, and draw a line to indicate each bond. In the case of formaldehyde, the less electronegative atom (carbon) is the central atom, and both hydrogens and the oxygen are bonded to carbon:

\begin{matrix}\underset{|}{O}\\ H-C-H \end{matrix}

Six of the 12 valence electrons are used for bonds, leaving 6 for assignment to the ter-minal oxygen atom.

\begin{matrix}:\overset{..}{\underset{|}{O}}:\\ H-C-H \end{matrix}\\ \quad\quad\quad\hookleftarrow \text{Only 6 electrons here}

At this point, all the valence electrons are assigned, but the central carbon atom still does not have an octet. We therefore move two of the oxygen electrons from a lone pair into a bonding pair, generating a carbon–oxygen double bond and satisfying the octet rule for both oxygen and carbon.

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