CHM 413 ADVANCED INORGANIC CHEMISTRY Spring 2004

1
CHM 413 ADVANCED INORGANIC CHEMISTRYSpring 2004

• CHAPTER 3
• Simple Bonding Theory

2
Simple Bonding Theory

• Includes the basic valence-shell electron pair
  repulsion
• theory (VSEPR) through the valence bond to the
• more advanced molecular theory (MO Theory)
• Starting point Lewis electron-dot structures
• - bonds are formed when two atoms share one or
• more pairs of electrons
• There are bonding pairs and lone pairs

3

• Lewis structures are based on the octet rule
• - this supposes that the most stable molecules
  are
• formed when each atom acquires eight electrons in
  its
• valence shell
• Note in drawing Lewis structures only valence
• electrons are used
• Some atoms can also accommodate more than eight
  e-s
• in their valence shell

4

• Typically true for elements with n ? 3
• - the elements are said to be hypervalent
• Take a closer look at the Lewis structure for
  the CO32-
• ion

Resonance hybrid ?
5
Formal Charges

• Compounds that show resonance may not have
• equal contribution from all structures
• Calculating formal charges provide a useful way
  of
• estimating contributions from resonance
  structures
• The formal charge is not a real charge

6

• Notes on formal charges
• Sum of all FC charge on molecule or ion
• Structures minimizing formal charges are most
  stable
• Negative formal charges are likely to be found on
• more electronegative atoms
• 4. Like formal charges on adjacent atoms lead to
• instability
• 5. Big charge separation is unlikely

7
VSEPR Theory

• Useful for predicting molecular shapes
• Built on the primary assumption that electron
  pairs
• repel each other and so occupy positions as far
  apart
• as possible
• A molecule typically described by AXmEn
• - X atom or group of atoms E lone pair
• The steric number, SN, is the number of
  positions
• occupied by atoms or lone pairs around a central
  atom

8

• The idealized geometry for AXn (n 1,2,3,4,6)
  is
• that of the regular polyhedra with the same
• number of vertices
• AX3 trigonal planar AX4 tetrahedral
• AX6 octahedral

• For AX5 the geometry has two
• types of vertices

trigonal bipyramidal
9

• The presence of a lone pair has a profound
  effect
• on molecular shape
• Consider CH4, NH3, H2O

106.6 ?
104.5 ?
109.5 ?
Angular or bent
Trigonal pyramidal
Tetrahedral
10

• The size of the central atom also has some
• effect on bond angles
• e.g. H2O gt H2S gt H2Se
• Electronegativity is also another factor
  affecting
• bond angles
• e.g. OF2
• Consider going from AX5 to AX4E (e.g. SF4)

11
Electronegativity

• Electronegativity, ?, is an empirical measure
  of
• the ability of an atom in a molecule to attract
  electrons
• Consider two atoms A and B with different
• electronegativities
• DAB ½ (DAA DBB) D dissociation energy
• This is not observed experimentally
• DAB gt ½ (DAA DBB)
• So ?AB DAB - ½ (DAA DBB)

12

• Experimentally
• ?A - ?B 0.102 ??AB
• On this Pauling scale fluorine is the most
• electronegative element with a value of 4.0
• Other scales include the Allred-Rochow scale
• ?AR 0.359 (Z/r2) 0.744
• ? typically increases up a group and across a
  period

13
Polar Molecules

• Polar bonds result when the two bonding atoms
• have different electronegativities
• The polarity of a bond can be defined in terms
  of its
• dipole moment, ?
• ? Q?r
• where Q is difference in charge and r is the
• separation of the charges

14

• In general for a molecule with polar bonds to
• be polar, the bonds must not be arranged
• symmetrically
• The polarity of molecules allow for very strong
• interactions between such molecules
• These interactions are called intermolecular
  forces
• One of the strongest type of intermolecular
  forces is
• the hydrogen bond

15

• H-bonds are formed between molecules having
• a hydrogen atom attached to a small highly
• electronegative atom possessing a lone pair of
  electrons
• H-bonds are very strong and have a significant
  impact
• on the physical properties of substances that
  contain
• them
• London or dispersion forces are weak
  intermolecular
• forces arising from polarization of molecules