Lewis Dot Structures: Multiple Bonds: Videos & Practice Problems

Atoms often form multiple bonds when their valence electrons are insufficient to satisfy the octet rule, which states that atoms prefer to have eight electrons in their outer shell, similar to the electron configuration of noble gases. For instance, consider nitrogen, which can form either a single bond or a triple bond with another nitrogen atom. In the case of a single bond, a total of 10 valence electrons are utilized, as each bond consists of 2 electrons. However, this configuration leaves both nitrogen atoms with only 6 electrons each, failing to meet the octet requirement.

To resolve this, forming a triple bond is more effective. In a triple bond, the total number of valence electrons remains 10, but each nitrogen atom now shares three pairs of electrons, resulting in 8 electrons around each atom. This arrangement satisfies the octet rule, demonstrating that multiple bonds, such as double or triple bonds, may be necessary to achieve a complete octet for certain elements.

It is important to note that hydrogen is an exception to the octet rule; it adheres to the duet rule, desiring only 2 electrons in its outer shell to achieve a stable configuration akin to helium.

To draw the Lewis dot structure for the formaldehyde molecule (CH₂O), we begin by calculating the total number of valence electrons. Carbon, located in group 4A, contributes 4 valence electrons. Each hydrogen, found in group 1A, contributes 1 valence electron, and since there are two hydrogens, that totals 2 valence electrons. Oxygen, from group 6A, adds 6 valence electrons. Summing these gives us:

4 (from C) + 2 (from H) + 6 (from O) = 12 valence electrons.

Next, we place the least electronegative atom in the center, which in this case is carbon, as hydrogen cannot occupy the central position. We connect the carbon atom to the two hydrogen atoms and the oxygen atom using single bonds. This results in a preliminary structure where carbon is bonded to both hydrogens and to oxygen:

H - C - H

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O

Following the octet rule, we add electrons to surrounding atoms until they achieve a full outer shell. Hydrogen, adhering to the duet rule, requires only 2 electrons. We distribute the remaining electrons, ensuring that each hydrogen has 2 electrons and that oxygen has 8 electrons. After placing the electrons, we find that carbon only has 6 electrons around it (2 from each bond with hydrogen and 2 from the bond with oxygen).

To satisfy carbon's bonding preference of forming 4 bonds, we need to create a double bond between carbon and oxygen. We can achieve this by taking one of the lone pairs from oxygen and using it to form a double bond with carbon. This adjustment maintains the total of 12 valence electrons used:

H - C = O

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H

In this final structure, carbon is single bonded to both hydrogens and double bonded to oxygen, which retains two lone pairs. This configuration satisfies the octet rule for carbon and oxygen while adhering to the duet rule for hydrogen, resulting in the correct Lewis dot structure for formaldehyde.