Localized and delocalized (non-localized)
approaches to bonding
• There are two main ways of trying to explain how the
electrons of a molecule are involved in bonding:
Localized bond approach (also known as the valence
bond theory)
• It involves regarding all bonds as localized interactions
involving two electrons shared between two atoms.
• In polyatomic molecules this leads to the use of orbital
hybridization as a convenient mathematical (and pictorial)
procedure of manipulating the atomic orbitals to permit
the bonding to be described in terms of a collection of
simple two-centre, two-electron bonds.
, Modern theories of chemical bonding
(Nature of the chemical bond)
• Ionic bonding is basically understandable in terms of
coulombic forces of attraction between oppositely charged
species — cations and anions.
• The description of covalent bonding is limited to writing
Lewis structures.
• According to the Lewis concept a covalent bond is formed
by the sharing of electrons between the combining atoms.
• It serves as a guide for writing structural formulae of
various covalent compounds.
• However, it does not explain:
– How does sharing of electrons lead to the formation of
a bond?
– What is the nature of bonding forces?
, – It tells nothing about the shapes of molecules, bond
energies and bond lengths.
• It was only after the development of wave mechanics that
two alternative theories were put forward to provide
plausible ( )ممکنہanswers to these questions.
• These two approaches are:
– Valence bond theory (VBT)
– Molecular orbital theory (MOT)
Wave mechanics
• A mathematical technique that describes the relationship between the
motion of a particle that exhibits wavelike properties (such as an
electron) and its allowed energies.
Quantum mechanics
• The body of scientific laws that describe the wacky ( آواره- )خبطی
behavior of photons, electrons and the other particles that make up the
universe.
, Valence bond theory
(Electron pair theory or Heitler-London theory)
• The first quantum mechanical treatment to explain the
formation of a covalent bond for the hydrogen molecule
was developed by W. Heitler and F. London in 1927.
• It was later improved and extended by Linus Pauling and
J.C. Slater in 1931.
Postulates of VBT
According to valence bond model:
• A covalent bond is formed between two atoms by the
overlap of half-filled valence atomic orbitals of each atom
containing one unpaired electron.
• The overlap of two atomic orbitals gives rise to a single
bond orbital which is localized orbital and is occupied by
both the electrons.
approaches to bonding
• There are two main ways of trying to explain how the
electrons of a molecule are involved in bonding:
Localized bond approach (also known as the valence
bond theory)
• It involves regarding all bonds as localized interactions
involving two electrons shared between two atoms.
• In polyatomic molecules this leads to the use of orbital
hybridization as a convenient mathematical (and pictorial)
procedure of manipulating the atomic orbitals to permit
the bonding to be described in terms of a collection of
simple two-centre, two-electron bonds.
, Modern theories of chemical bonding
(Nature of the chemical bond)
• Ionic bonding is basically understandable in terms of
coulombic forces of attraction between oppositely charged
species — cations and anions.
• The description of covalent bonding is limited to writing
Lewis structures.
• According to the Lewis concept a covalent bond is formed
by the sharing of electrons between the combining atoms.
• It serves as a guide for writing structural formulae of
various covalent compounds.
• However, it does not explain:
– How does sharing of electrons lead to the formation of
a bond?
– What is the nature of bonding forces?
, – It tells nothing about the shapes of molecules, bond
energies and bond lengths.
• It was only after the development of wave mechanics that
two alternative theories were put forward to provide
plausible ( )ممکنہanswers to these questions.
• These two approaches are:
– Valence bond theory (VBT)
– Molecular orbital theory (MOT)
Wave mechanics
• A mathematical technique that describes the relationship between the
motion of a particle that exhibits wavelike properties (such as an
electron) and its allowed energies.
Quantum mechanics
• The body of scientific laws that describe the wacky ( آواره- )خبطی
behavior of photons, electrons and the other particles that make up the
universe.
, Valence bond theory
(Electron pair theory or Heitler-London theory)
• The first quantum mechanical treatment to explain the
formation of a covalent bond for the hydrogen molecule
was developed by W. Heitler and F. London in 1927.
• It was later improved and extended by Linus Pauling and
J.C. Slater in 1931.
Postulates of VBT
According to valence bond model:
• A covalent bond is formed between two atoms by the
overlap of half-filled valence atomic orbitals of each atom
containing one unpaired electron.
• The overlap of two atomic orbitals gives rise to a single
bond orbital which is localized orbital and is occupied by
both the electrons.
