CHAPTER 9: MOLECULAR GEOMETRY & BONDING THEORIES 

The size and shape of a molecule of a particular substance, along with the strength and polarity of its bonds, are primarily responsible for determining the properties of that substance.  Because Lewis structures are drawn with the atoms in the same plane, these structures cannot indicate the shapes of molecules; they simply tell the number and types of bonds between atoms.  Shapes of molecules can be described by using the valence-shell electron-pair repulsion (VSEPR) model. 

• electron domain: regions about a central atom in which electrons are likely to be found 

• bonding pair: a pair of electrons shared by two atoms 

• nonbonding pair (lone pair): a pair of electrons assigned completely to one atom 

• A multiple bond is counted as one electron domain. 

The best arrangement for a given number of electron domains is the one that minimizes the repulsions between the electron domains.  (See Table 9.1, page 345.) 

• The arrangement of electron domains about the central atom is its electronic geometry. 

• The arrangement of atoms around the central atom in a molecule is its molecular geometry. 

• The molecular geometry is the same as the electronic geometry if all electron domains are bonding pairs.  The presence of one or more nonbonding pairs will distort the molecules from the ideal geometries summarized in the above table.  (See Tables 9.2 & 9.3, pages 347 & 350.) 

• Bond angles decrease as the number of nonbonding electron pairs increases.  A nonbonding pair takes up more space than a bonding pair.  In other words, electron domains for lone pairs exert greater repulsive forces on adjacent electron domains, and therefore tend to compress the bond angles. Example: 

• Electron domains for multiple bonds exert a greater repulsive force on adjacent electron domains than do single bonds. 

• In the trigonal bipyramidal arrangement, there are two geometrically distinct positions:  two positions are called axial, and 3 are called equatorial.  Since nonbonding pairs exert greater repulsion than bonding pairs, the lone pairs will always occupy the equatorial positions. 

• In the octahedral arrangement, all six positions are equivalent.  If there are two nonbonding pairs, they will be oriented on opposite sides of the octahedron. 

POLARITY OF MOLECULES: (See figures and sample exercise in text.) 

• Bond polarity is a measure of how equally the electrons in a bond are shared between the two atoms.  As the difference in electronegativity increases, so does the bond polarity. 

• Dipole moment is a quantitative measure of the charge separation in a molecule.  It depends on the polarities of the individual bonds, as well as the geometry of the molecule. 

• Bond dipoles and dipole moments are vector quantities; they have both a magnitude and a direction. 

• The overall dipole moment of a molecule is the sum of its bond dipoles. 

• Molecules in which the central atom is symmetrically surrounded by identical atoms are nonpolar. 

VALENCE-BOND THEORY: a model of chemical bonding in which an electron-pair bond is formed between two atoms by the overlap of orbitals on the two atoms. 

• A sigma (s) bond is an end-to-end overlap of two atomic orbitals.  The electron density is concentrated along the internuclear axis.  Examples (see figure 9.14, page 356): in H₂, the overlap of the two s orbitals; in HCl, the overlap of the s and p orbitals; in Cl₂, the overlap of the two p orbitals. 

• A pi (p) bond is the side-to-side overlap of two unhybridized p orbitals.  The electron density is concentrated above and below the internuclear axis.  (See figure 9.22, page 362.) 

• Single bonds are always sigma bonds; a double bond is composed of one sigma bond and one pi bond; a triple bond is composed of one sigma bond and two pi bonds. 

HYBRID ORBITALS: (See figures and sample exercise in text.) 

• Hybridization is the mixing (merging) of different types of atomic orbitals to form a set of equivalent hybrid orbitals.  These hybrid orbitals are identical in shape, each having 2 lobes (but one lobe is much larger than the other lobe).  The arrangement of these hybrid orbitals in space is dependent on the number and type of orbitals involved in the hybridization.