# 7.3 Lewis symbols and structures  (Page 3/15)

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## 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 (H 3 CCH 3 ), acetylene (HCCH), and ammonia (NH 3 ). What are the Lewis structures of these molecules?

## Solution

1. Calculate the number of valence electrons.
HCN: (1 $×$ 1) + (4 $×$ 1) + (5 $×$ 1) = 10
H 3 CCH 3 : (1 $×$ 3) + (2 $×$ 4) + (1 $×$ 3) = 14
HCCH: (1 $×$ 1) + (2 $×$ 4) + (1 $×$ 1) = 10
NH 3 : (5 $×$ 1) + (3 $×$ 1) = 8
2. Draw a skeleton and connect the atoms with single bonds. Remember that H is never a central atom:
3. Where needed, distribute electrons to the terminal atoms:

HCN: six electrons placed on N
H 3 CCH 3 : no electrons remain
HCCH: no terminal atoms capable of accepting electrons
NH 3 : no terminal atoms capable of accepting electrons
4. Where needed, place remaining electrons on the central atom:

HCN: no electrons remain
H 3 CCH 3 : no electrons remain
HCCH: four electrons placed on carbon
NH 3 : two electrons placed on nitrogen
5. Where needed, rearrange electrons to form multiple bonds in order to obtain an octet on each atom:
HCN: form two more C–N bonds
H 3 CCH 3 : all atoms have the correct number of electrons
HCCH: form a triple bond between the two carbon atoms
NH 3 : all atoms have the correct number of electrons

Both carbon monoxide, CO, and carbon dioxide, CO 2 , are products of the combustion of fossil fuels. Both of these gases also cause problems: CO is toxic and CO 2 has been implicated in global climate change. What are the Lewis structures of these two molecules?

## Fullerene chemistry

Carbon soot has been known to man since prehistoric times, but it was not until fairly recently that the molecular structure of the main component of soot was discovered. In 1996, the Nobel Prize in Chemistry was awarded to Richard Smalley ( [link] ), Robert Curl, and Harold Kroto for their work in discovering a new form of carbon, the C 60 buckminsterfullerene molecule ( [link] ). An entire class of compounds, including spheres and tubes of various shapes, were discovered based on C 60. This type of molecule, called a fullerene, shows promise in a variety of applications. Because of their size and shape, fullerenes can encapsulate other molecules, so they have shown potential in various applications from hydrogen storage to targeted drug delivery systems. They also possess unique electronic and optical properties that have been put to good use in solar powered devices and chemical sensors.

## Exceptions to the octet rule

Many covalent molecules have central atoms that do not have eight electrons in their Lewis structures. These molecules fall into three categories:

• Odd-electron molecules have an odd number of valence electrons, and therefore have an unpaired electron.
• Electron-deficient molecules have a central atom that has fewer electrons than needed for a noble gas configuration.
• Hypervalent molecules have a central atom that has more electrons than needed for a noble gas configuration.

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