MOLECULAR SHAPES AND BONDING
We are now ready to study the structures of some simple molecules. We often refer to
generalized chemical formulas in which “A” represents the central atom and “B” repre-
sents an atom bonded to A. We follow the eight steps of analysis outlined in Section 8-1.
We first give the known (experimentally determined) facts about polarity and shape and
write the Lewis formula (part A of each section). Then we explain these facts in terms of
the VSEPR and VB theories. The simpler VSEPR theory will be used to explain (or
predict) first the electronic geometryand then the molecular geometryin the molecule (part
B). We then show how the molecular polarity of a molecule is a result of bond polarities,
lone pairs, and molecular geometry. Finally, we use the VB theory to describe the bonding
in molecules in more detail, usually using hybrid orbitals (part C). As you study each
section, refer frequently to the summaries that appear in Table 8-4.LINEAR ELECTRONIC GEOMETRY: AB 2 SPECIES
(NO LONE PAIRS OF ELECTRONS ON A)A. Experimental Facts and Lewis Formulas
Several linear molecules consist of a central atom plus two atoms of another element,
abbreviated as AB 2. These compounds include gaseous BeCl 2 , BeBr 2 , and BeI 2 , as well
as CdX 2 and HgX 2 , where XCl, Br, or I. All of these are known to be linear (bond
angle180°), nonpolar, covalent compounds.
Let’s focus on gaseousBeCl 2 molecules (mp 405°C). We wrote the Lewis formula for
BeCl 2 in Example 7-5. It shows two single covalent bonds, with Be and Cl each
contributing one electron to each bond.
In many of its compounds, Be does not satisfy the octet rule (see Section 7-7).B. VSEPR Theory
Valence shell electron pair repulsion theory places the two electron pairs on Be 180° apart,
that is, with linearelectronic geometry.Both electron pairs are bonding pairs, so VSEPR
also predicts a linear atomic arrangement, or linear molecular geometry,for BeCl 2.If we examine the bond dipoles, we see that the electronegativity difference (see Table
6-3) is large (1.5 units) and each bond is quite polar:The two bond dipoles are equalin magnitude and oppositein direction. They therefore
cancel to give a nonpolar molecule.Cl Be ClCl Be ClNet dipole 03.0 1.5 3.0
1.5 1.5
(EN) ENCl Be Cl180 Cl Be Cl or Cl Be Cl8-5
See the “Important Note” in Section
8-1; consult your instructor for
guidance on the order in which you
should study Sections 8-5 through
8-12.
314 CHAPTER 8: Molecular Structure and Covalent Bonding Theories
A model of BeCl 2 , a linear AB 2
molecule.
It is important to distinguish between
nonpolar bondsand nonpolar molecules.
The high melting point of BeCl 2 is
due to its polymeric nature in the solid
state.
The VSEPR theory describes the
locations of bonded atoms around the
central atom, as well as where its lone
pairs of valence shell electrons are
directed.