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Unit 5: Bonding
Valence Bond Theory
Valence Bond Theory (VBT)
• Orbital: a region of space in which
there is a high probability of finding
an electron
• Valence bond theory: a theory that
defines bonds as being formed as a
result of overlapping atomic orbitals
Main Points of Valence
Bond Theory
• Orbitals overlap to form a bond between
two atoms
• Two electrons, of opposite spins can be
accommodated in the overlapping
orbitals.
• Bonding electrons have higher
probability of being found between two
nuclei. Electrons are simultaneously
attracted to both nuclei.
Types of Bonds
• Sigma(σ) bond: bond formed by the
overlap of orbitals head to head, and
with bonding electron density
concentrated along the axis of the bond.
• Pi (π) bond: the second and third bond
in a multiple bond, resulting in overlap
above and below the plane – sideways of
the p orbitals.
Types of Sigma bonds
a)
b)
c)
a) The s – s overlap
b) The s – p overlap
c) The p – p overlap
Sigma Bond Examples
Comparing Pi Bonds and
Sigma Bonds
Bonding in Ethylene (C2H4)
H
H
C
H
C
H
Electron Pair Geometry vs.
Molecular Geometry
• Electron Pair Geometry: the geometry
taken up by all valence electron pairs
around a central atom
– Treats all lone and bonded pairs the
same.
• Molecular Geometry: describes the
geometry of the central atom only
Example
NH3
Molecular Geometry: Pyramid
Electron Pair Geometry: Tetrahedral