The Foundations of Chemistry

Figure 8-8 The acetylene molecule, C 2 H 2. (a) The overlap diagram of two sp-hybridized
carbon atoms and two sorbitals from two hydrogen atoms. The hybridized sporbitals on
each C are shown in green and the unhybridized porbitals are shown in tan. The dashed
lines, each connecting two lobes, indicate the side-by-side overlap of the four unhybridized p
orbitals to form two bonds. There are two CXH bonds, one CXC bond (green,
hatched), and two CXC bonds (hatched and cross-hatched). This makes the net carbon–
carbon bond a triple bond. (b) The bonding orbitals (tan) are positioned with one above
and below the line of the bonds (green) and the other behind and in front of the line of
the bonds.

8-15 A Summary of Electronic and Molecular Geometries 343

Some other molecules containing triply bonded atoms are nitrogen, SNmNS,
hydrogen cyanide, HXCmNS, and propyne, CH 3 XCmCXH. In each case, both atoms
involved in the triple bonds are sphybridized. In the triple bond, each atom participates
in one sigma and two pi bonds. The C atom in carbon dioxide, /ÅOUCUOv", must partic-
ipate in two pi bonds (to two different O atoms). It also participates in two sigma bonds,
so it is also sphybridized and the molecule is linear.

A SUMMARY OF ELECTRONIC AND

MOLECULAR GEOMETRIES

We have discussed several common types of polyatomic molecules and ions, and provided
a reasonable explanation for the observed structures and polarities of these species. Table
8-4 provides a summation.
Our discussion of covalent bonding illustrates two important points:

1.Molecules and polyatomic ions have definite shapes.

2.The properties of molecules and polyatomic ions are determined to a great extent

by their shapes. Incompletely filled electron shells and unshared pairs of elec-

trons on the central element are very important.

Our ideas about chemical bonding have developed over many years. As experimental tech-
niques for determining the structuresof molecules have improved, our understanding of
chemical bonding has improved also. Experimental observations on molecular geometry
support our ideas about chemical bonding. The ultimate test for any theory is this: Can

8-15

pz one^ bond

(a) (b)

py pz py

a second bond

HC C HH Cσ CH

π π

In propyne, the C atom in the CH 3

group is sp^3 hybridized and at the

center of a tetrahedral arrangement.