VB
theory
Year I
, Structure and Bonding
Core Chemistry 1
Dr Andy Hughes
Lectures 7 & 8
Valence Bond theory
7.1 Valence Bond theory of H2 and F2
7.2 Promotion and hybridisation, sp
7.3 Trigonal planar, sp2
7.4 Tetrahedral, sp3
7.5 Multiple bonds, ethene and ethyne
7.6 Resonance
8.1
Summary of lecture 6
• VSEPR
– Count the number of pairs of electrons in the valence shell of an atom
– Geometry is determined only by electron-electron repulsion
– Adapted for multiple bonds
– Does not work for transition metal compounds
– Takes no account of where orbitals are pointing
8.2
, Summary of lecture 6
• VSEPR
– Count the number of pairs of electrons in the valence shell of an atom
– Geometry is determined only by electron-electron repulsion
– Adapted for multiple bonds
– Does not work for transition metal compounds
– Takes no account of where orbitals are pointing
8.2
Bonding
• Chemists recognise three types of bonding
• Metallic - treated by band theory - 3rd year
• Ionic - treated by the ionic model – Core Chemistry 1B
• Covalent - we are going to consider three models
– Lewis model (what you studied pre-University)
– Valence Bond Theory (VB)
– Molecular Orbital Theory (MO)
• Atoms come together to form more complicated structures through
either the sharing (covalent) or transfer (ionic) of electrons
8.3
, Bonding
• Chemists recognise three types of bonding
• Metallic - treated by band theory - 3rd year
• Ionic - treated by the ionic model – Core Chemistry 1B
• Covalent - we are going to consider three models
– Lewis model (what you studied pre-University)
– Valence Bond Theory (VB)
– Molecular Orbital Theory (MO)
• Atoms come together to form more complicated structures through
either the sharing (covalent) or transfer (ionic) of electrons
8.3
Valence Bond versus Molecular Orbital theories
• Valence Bond (VB)
– Developed by Linus Pauling (Nobel Prize Chemistry 1954, Peace 1962)
– Tied to Lewis model of bonding electron pairs localised between atoms
– Mathematical VB is no longer an active area of research
– Qualitative VB is the method of choice for describing the structure and bonding of
many molecules, especially organic ones, in their ground state
– Fails for some even very simple molecules, e.g., O2
• Molecular Orbital (MO)
– Developed first by Robert Mulliken (Nobel Prize Chemistry 1966)
– Molecular orbitals are spread out or delocalised over the molecule
– More appropriate for a quantitative picture of a molecule, and for describing
excited states of molecules (so spectroscopy)
– A current research chemist would probably calculate their molecules using
Density Functional Theory, DFT, which grows from MO theory
• Each theory is used in its own situation. VB is qualitative, “back of the
envelope”. MO is quantitative and could require a serious computer
8.4
theory
Year I
, Structure and Bonding
Core Chemistry 1
Dr Andy Hughes
Lectures 7 & 8
Valence Bond theory
7.1 Valence Bond theory of H2 and F2
7.2 Promotion and hybridisation, sp
7.3 Trigonal planar, sp2
7.4 Tetrahedral, sp3
7.5 Multiple bonds, ethene and ethyne
7.6 Resonance
8.1
Summary of lecture 6
• VSEPR
– Count the number of pairs of electrons in the valence shell of an atom
– Geometry is determined only by electron-electron repulsion
– Adapted for multiple bonds
– Does not work for transition metal compounds
– Takes no account of where orbitals are pointing
8.2
, Summary of lecture 6
• VSEPR
– Count the number of pairs of electrons in the valence shell of an atom
– Geometry is determined only by electron-electron repulsion
– Adapted for multiple bonds
– Does not work for transition metal compounds
– Takes no account of where orbitals are pointing
8.2
Bonding
• Chemists recognise three types of bonding
• Metallic - treated by band theory - 3rd year
• Ionic - treated by the ionic model – Core Chemistry 1B
• Covalent - we are going to consider three models
– Lewis model (what you studied pre-University)
– Valence Bond Theory (VB)
– Molecular Orbital Theory (MO)
• Atoms come together to form more complicated structures through
either the sharing (covalent) or transfer (ionic) of electrons
8.3
, Bonding
• Chemists recognise three types of bonding
• Metallic - treated by band theory - 3rd year
• Ionic - treated by the ionic model – Core Chemistry 1B
• Covalent - we are going to consider three models
– Lewis model (what you studied pre-University)
– Valence Bond Theory (VB)
– Molecular Orbital Theory (MO)
• Atoms come together to form more complicated structures through
either the sharing (covalent) or transfer (ionic) of electrons
8.3
Valence Bond versus Molecular Orbital theories
• Valence Bond (VB)
– Developed by Linus Pauling (Nobel Prize Chemistry 1954, Peace 1962)
– Tied to Lewis model of bonding electron pairs localised between atoms
– Mathematical VB is no longer an active area of research
– Qualitative VB is the method of choice for describing the structure and bonding of
many molecules, especially organic ones, in their ground state
– Fails for some even very simple molecules, e.g., O2
• Molecular Orbital (MO)
– Developed first by Robert Mulliken (Nobel Prize Chemistry 1966)
– Molecular orbitals are spread out or delocalised over the molecule
– More appropriate for a quantitative picture of a molecule, and for describing
excited states of molecules (so spectroscopy)
– A current research chemist would probably calculate their molecules using
Density Functional Theory, DFT, which grows from MO theory
• Each theory is used in its own situation. VB is qualitative, “back of the
envelope”. MO is quantitative and could require a serious computer
8.4
