Transcript Chapter 10b - Valdosta State University

Chapter 10 –
Molecular Orbitals
Chapter 10 Goals:
1. Understand the difference between valance bond
theory and molecular orbital theory.
2. Identify the hybridization of an atom in a
molecule or ion.
3. Understand the differences between bonding and
antibonding molecular orbitals.
4. Write the molecular orbital configuration for
simple diatomic molecules.
VALENCE BOND THEORY
— Linus Pauling
Valence electrons are localized between
atoms (or are lone pairs).
half-filled atomic orbitals overlap to form
bonds.
— mix available orbitals to form a
new set of orbitals — _______
ORBITALS — that will give the
maximum overlap in the correct
_____________.
MOLECULAR ORBITAL THEORY
— Robert Mulliken (1896-1986)
• Valence electrons are __________.
• Valence electrons are in orbitals (called
__________ orbitals) spread over
entire molecule.
• Essential for higher-energy states.
VALENCE BOND THEORY
• Orbitals overlap to form a bond between two
atoms. Ex. A sigma bond (σ) is form by overlap
of two s orbitals.
Hybridized orbitals
• Theory of orbital hybridization (Pauling): a
hybrid orbital could be created by mixing the
s, p, and d (when required) on an atom.
Theory of Orbital Hybridization
• First Principle
– The number of hybrid orbitals is always the same
as the number of atomic orbitals that are mixed to
create the hybrid orbital set.
• Second Principle
– The hybrid orbitals are more directed from the
central atom toward the terminal atoms, than are
the unhybridized atomic orbitals, leading to better
overlap and a stronger bond between the central
and terminal atoms.
Hybrid Orbitals
• Relate bonded
atoms (structure)
to number of
orbitals.
• This gives the
___________ of
the central atom.
Valance Bond Theory for Methane, CH4
Multiple Bonds: Double Bonds
• Each carbon has a trigonal planar
geometry – sp2 hybridization:
• 3 sp2 hybrid orbitals and one
unhybridized p orbital perpendicular to
the plane of the molecule: it forms a
(π) bond with other carbon.
Cis-Trans Isomerism
• Free rotation can occur around the axis of a single sigma
bond.
• Rotation is severly restricted around double bonds
(requires breaking of π bond).
• Cis-trans isomers (same formula different structures)
do not interconvert at ordinary temperatures.
Double Bonds and Vision
Double Bonds and Vision
Molecular Orbital Theory
• Molecular orbitals – delocalized over entire
molecule.
• First Principle
– The total number of molecular orbitals is always
equal to the total number of atomic orbitals
contributed by the atoms that have combined.
Molecular Orbitals for H2
• Bonding molecular orbital
– addition of two atomic orbitals, leads to the probability of finding an
e- between the atoms.
• Antibonding molecular orbital
– substration of one atomic orbital from the other, leads to reduced
probability of finding an e- between the nuclei, increased in other
regions.
Molecular Orbital Theory
• Second Principle
– The bonding molecular orbital is lower in energy
than the parent orbitals, and the antibonding
orbital is higher in energy.
• Third Principle
– The electrons of the molecule are assigned to
orbitals of successively higher energy (according
to Pauli exclusion principle and the Hund’s rule).
Molecular Orbitals for He2
• He2 has no net stability – two He atoms
have no tendency to combine.
Remember
• OWL end-of-chapter 10 assignment is
required.