Page 191 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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Encyclopedia of Physical Science and Technology EN011G-539 July 14, 2001 21:48

438 Organic Chemical Systems, Theory

TABLE II Common Building Blocks of Organic Molecules
Number of
electrons in Coordination
Building blocks valence shell number a
In agreement with the rules of valence
H 2 1
B C N 8 4

C N O 8 3

N O F 8 2

O F 8 1
Coordinatively unsaturated

B C N 7 3

C N O 7 2

N O F
7 1

B C 6 3

C N 6 2

N O 6 1
Examples of hypervalent atoms

Si P 10 5

S 10 4

t 10 3

H (4) (2)
a
Parentheses are for hydrogen atom in a hydrogen bond, not always considered hypervalent.
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hand, the presence of a single unpaired electron on an atom The angles of 120 correspond to the center of an equi-
usually has only a minor influence on its valence angles. lateral triangle being connected to its vertices, so that all
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For atoms forming two single or multiple bonds and three bonds are coplanar. The angles of 109 correspond
carrying no lone pairs, the valence angles normally are to the center of a regular tetrahedron being connected to its
in the vicinity of 180 ; for atoms forming three single or vertices. The steric arrangement around an atom carrying
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multiple bonds and carrying no lone pairs they are ∼120 ; a total of five bonds and lone pairs usually corresponds to
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and for atoms forming two such bonds and carrying one a trigonal bipyramid and that around an atom carrying a
lone pair, they are usually a little smaller. For atoms form- total of six bonds and lone pairs to a regular octahedron,
ing four bonds and carrying no lone pairs they are ∼109 , with the atom in the center in each case.
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and for atoms carrying a lone pair plus three bonds or two The valence angles given provide only a rough guide.
lone pairs plus two bonds they are a little smaller still. Their exact values in any real molecule depend on the