Chapter 9 Molecular Geometry & Bonding

Overview

• Molecular Shapes
• VSEPR Model
• Predicting Shapes
• Effect of Nonbonding Electrons
• Polarity of Molecules
• Covalent Bonding

• Hybrid Orbitals
• sp, sp², sp³
• hybrids containing d orbitals
• Multiple Bonds
• sigma (s) & pi (p)
• localized & delocalized
• Molecular Orbitals
• electron configurations & bond order
• diamagnetism & paramagnetism

Molecular Shapes & VSEPR

• Shapes defined by bond angles
• linear, 180° angles
• trigonal planar, 120° angles
• tetrahedral, 109.5° angles
  
• VSEPR
• Valence Shell Electron Pair Repulsion theory
• electron pairs are arranged symmetrically with maximum separation
• Two electron pairs
• 180° apart
• linear geometry
• Three electron pairs
• 120° apart
• trigonal planar geometry
• Four electron pairs
• 109.5° apart
• tetrahedral geometry
• Five electron pairs
• angles of 90° and 120°
• trigonal bipyramidal (TBP) geometry
• Six electron pairs
• angles of 90°
• octahedral geometry

VSEPR Tutorial on the internet

• URL is
• http://www.chem.purdue.edu/courses/chem116/test/vsepr/
  or
• go to the MDL Chemscape Chime sample page which is bookmarked
• click on product
• scroll down to chemscape & click on Chime
• scroll down & click on cool examples
• scroll down to Chemistry Tutorials from Purdue Univ. & click on Valence Shell Electron Pair Repulsion Model

Geometries

• Electron pair geometry
• arrangement of electron pairs around a central atom
• Molecular Geometry
• arrangement of atoms around a central atom
• When all electron pairs are bonding pairs
• electron pair geometry = molecular geometry
• When there are unshared electron pairs
• electron pair geometry ¹ molecular geometry
• To determine electron pair geometry
• draw Lewis dot structure
• count shared & unshared electron pairs around central atom
• a multiple bond is counted as only one  bonding pair when predicting geometry
• determine electron pair geometry based on the number of electron pairs
• 2 pair = linear
• 3 pair = trigonal planar
• 4 pair = tetrahedral
• 5 pair = trigonal bipyrimidal
• 6 pair = octahedral

Molecular Geometries with One or More Unshared Pairs

• Two Pairs
  
• electron pair geometry             linear
• bonding pairs                             2
• non-bonding pairs                      0
• molecular geometry                linear
  
  
• Three Pairs
  
• electron pair geometry            trigonal planar
• bonding pairs                                3                          2
• non-bonding pairs                         0                          1
• molecular geometry                   trig. pl.                  bent
• Four Pairs
  
• electron pair geometry          tetrahedral
• bonding pairs                               4                          3                          2
• non-bonding pairs                        0                          1                          2
• molecular geometry                    tet.                   trig. pyr.                  bent
• Five Pairs
  
• electron pair geometry          trigonal bipyramid
• bonding pairs                                  5                     4                   3                     2                  
• non -bonding pairs                          0                     1                   2                     3
• molecular geometry                       tbp                seesaw        T-shp.              linear
• Six Pairs
  
• electron pair geometry            octahedral
• bonding pairs                                6                 5             4           2
• non-bonding pairs                         0                 1             2           4
• molecular geometry                    oct            sq.pyr.     sq. pl.       lin.

Molecular Polarity

• Molecules are always non-polar if all covalent bonds are non-polar
• N2, P4, Cl2
• Molecules with polar bonds can be polar or non-polar
• H - Cl polar bond, polar molecular
• O=C=O two polar bonds but total molecule is non-polar

Single and Multiple Bonds

• s (sigma) bonds
• always the first bond between two atoms
• single bonds are localized between two atoms
• orbitals from two atoms overlap, allowing electrons to be shared
• electron density is on the internuclear axis
• p (pi) bonds
• the second & third bonds between two atoms
• p bond electrons can be delocalized over several atoms to form resonance structures
• electron density is above & below the internuclear axis

Hybridization

• allows for greater number of bonds
• types of hybridization
• sp mixing of one s orbital & one p orbital
• ­¯   ­ ­              ­ ­          ­ ­
  2s       2p                   sp                 p
• sp² mixing of one s orbital & two p orbitals
• ­¯   ­ ­              ­ ­ ­           ­
  2s      2p                     sp²                p
• sp³ mixing of one s orbital & three p orbitals
• ­¯   ­ ­                ­ ­ ­ ­
  2s       2p                        sp³
• in sp hybridization
• the two sp hybrid orbitals form two s bonds with linear geometry
• remaining two p orbitals form p bonds
• in sp² hybridization
• the three hybrid orbitals form three s bonds with trigonal planar geometry
• the remaining one p orbital forms a p bond
• in sp³ hybridization
• the four hybrid orbitals form four s bonds with tetrahedral geometry
• sp3 hybrid atoms can form no p bonds as they have no unhybridized p orbitals

Molecular Orbitals

• mathematical combinations of atomic orbitals
• delocalized over whole molecule
• n atomic orbitals produce n molecular orbitals
• ½ are bonding orbitals and ½ are antibonding orbitals
  
• bond order
• # bonding electrons - # antibonding electron
                                   2

<sup>electron configuration of diatomic, homonuclear molecules

s *      one antibonding orbital from the 2p orbitals
p *      two antibonding orbitals from the 2p orbitals
p         two bonding orbitals from the 2p orbitals
s         one bonding orbital from the 2p orbitals

s *      one antibonding orbital from the 2s orbitals
s         one bonding orbital from the 2s orbitals</sup>

<sup>electron configuration of diatomic, homonuclear molecules with interaction of the 2s and 2p orbitals

s *     one antibonding orbital from the 2p orbitals
p *     two antibonding orbitals from the 2p orbitals
s        one bonding orbital from the 2p orbitals
p        two bonding orbitals from the 2p orbitals

s *     one antibonding orbital from the 2s orbitals
s        one bonding orbital from the 2s orbitals</sup>