http://www.ck12.org Chapter 9. Covalent Bonding
atoms. Additionally, these two pi bonds are perpendicular to one another (Figure9.29); one pi bond is above and
below the line of the molecule, while the other is in front of and behind the page.
FIGURE 9.29
The C 2 H 2 molecule contains a triple bond
between the two carbon atoms, one of
which is a sigma bond, and two of which
are pi bonds. The pi bonds come from
overlap between the pyand the pzorbitals
on each carbon.In general, single bonds between atoms are always sigma bonds. Double bonds are comprised of one sigma and one
pi bond. Triple bonds are comprised of one sigma bond and two pi bonds.
Molecular Orbitals
In most of the above illustrations, bonds are depicted as two overlapping atomic orbitals that still retain their original
shapes. However, a more accurate way to show the electron density after the bond has formed is shown inFigures
9.29 and 9.28. When atomic orbitals combine to make covalent bonds, the results are new orbitals known as
molecular orbitals. Molecular orbital theory plays an important role in our understanding of various molecular
properties.
Lesson Summary
- Valence bond theory describes the formation of covalent bonds in terms of the overlap of singly occupied
atomic orbitals. - The hybridization of nonequivalent atomic orbitals is necessary to correctly predict the bonding and molecular
geometries of many molecules. The types of hybrid orbitals that form on the central atom are dictated by the
electron domain geometry. - Sigma bonds are formed by the end-to-end overlap of bonding orbitals. Pi bonds are formed by the side-to-
side overlap of p orbitals. Single bonds are normally sigma bonds. A double or triple bond consists of one
sigma bond and either one or two pi bonds. - Atomic orbitals from different atoms overlap to form molecular orbitals.
Lesson Review Questions
- Why do atoms "promote" electrons to higher orbitals in order to form bonds?