The Foundations of Chemistry

A sigma bondis a bond resulting from head-on overlap of atomic orbitals. The

region of electron sharing is along and cylindrically around an imaginary line connecting

the bonded atoms.

All single bonds are sigma bonds. Many kinds of pure atomic orbitals and hybridized
orbitals can be involved in sigma bond formation.

A pi bondis a bond resulting from side-on overlap of atomic orbitals. The regions

of electron sharing are on opposite sides of an imaginary line connecting the bonded atoms

and parallel to this line.

A pi bond can form onlyif there is alsoa sigma bond between the same two atoms. The
sigma and pi bonds together make a double bond (Figure 8-5). The 1sorbitals (with one
e
each) of four hydrogen atoms overlap the remaining four sp^2 orbitals (with one e
each)
on the carbon atoms to form four CXH sigma bonds (Figure 8-6).

Figure 8-5 A schematic

representation of the formation of a

carbon–carbon double bond. Two

sp^2 -hybridized carbon atoms form a

sigma ( ) bond by overlap of two sp^2

orbitals (green, hatched) and a pi ( )

bond by overlap of properly aligned

porbitals (tan). All orbitals are fatter

than shown here.

+

sp^2

sp^2

C

p

C

p

sp^2

sp^2

σ

sp^2

sp^2

C C

sp^2

sp^2

along

with

one π bond

C

p

sp^2

sp^2

sp^2 sp^2 C

p

sp^2

sp

2

one bond

H

H H

H

C σ C

σ

σσ

σ

π

Figure 8-6 Four CXH bonds, one CXC

bond (green, hatched), and one CXC bond

(tan, hatched) in the planar C 2 H 4 molecule.

See the Saunders Interactive

General Chemistry CD-ROM,

Screen 10.5, Sigma Bonds.

See the Saunders Interactive

General Chemistry CD-ROM,

Screen 10.7, Multiple Bonding.