4.3: Multiple Covalent Bonds

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Learning Objectives

Identify when a multiple bond (double or triple) is needed to complete an octet.

The sharing of a pair of electrons represents a single covalent bond, usually just referred to as a single bond. However, in many molecules atoms attain complete octets by sharing more than one pair of electrons between them:

Two electron pairs shared a double bond

Two oxygen molecules double bonded to each other

Three electron pairs shared a triple bond

Because each nitrogen contains 5 valence electrons, they need to share 3 pairs to each achieve a valence octet. N₂ is fairly inert, due to the strong triple bond between the two nitrogen atoms.

In addition to nitrogen and oxygen, carbon will also commonly form multiple bonds to complete valence octets. Additional examples involving these three atoms are shown below.

A double bond is formed between the carbon and oxygen atoms in CH₂O (formaldehyde) and between the two carbon atoms in C₂H₄ (ethylene):

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A triple bond is formed between carbon and oxygen in carbon monoxide (CO) and between carbon and nitrogen in the cyanide ion (CN^–):

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Example \(\PageIndex{1}\): Writing Lewis Structures

NASA’s Cassini-Huygens mission detected a large cloud of toxic hydrogen cyanide (HCN) on Titan, one of Saturn’s moons. Titan also contains ethane (H₃CCH₃), acetylene (HCCH), and ammonia (NH₃). What are the Lewis structures of these molecules?

Solution

Calculate the number of valence electrons.

HCN: (1 × 1) + (4 × 1) + (5 × 1) = 10
H₃CCH₃: (1 × 3) + (2 × 4) + (1 × 3) = 14
HCCH: (1 × 1) + (2 × 4) + (1 × 1) = 10
NH₃: (5 × 1) + (3 × 1) = 8

Draw a skeleton and connect the atoms with single bonds. Remember that H is never a central atom:

Where needed, distribute electrons to the terminal atoms:

HCN: six electrons placed on N
H₃CCH₃: no electrons remain
HCCH: no terminal atoms capable of accepting electrons
NH₃: no terminal atoms capable of accepting electrons

Where needed, place remaining electrons on the central atom:

HCN: no electrons remain
H₃CCH₃: no electrons remain
HCCH: four electrons placed on carbon
NH₃: two electrons placed on nitrogen

Where needed, rearrange electrons to form multiple bonds in order to obtain an octet on each atom:

HCN: form two more C–N bonds
H₃CCH₃: all atoms have the correct number of electrons
HCCH: form a triple bond between the two carbon atoms
NH₃: all atoms have the correct number of electrons

Exercise \(\PageIndex{1}\)

Both carbon monoxide, CO, and carbon dioxide, CO₂, are products of the combustion of fossil fuels. Both of these gases also cause problems: CO is toxic and CO₂ has been implicated in global climate change. What are the Lewis structures of these two molecules?

Answer