MOLECULAR ORBITAL AND VALENCE BOND THEORY EXPLAINED
(HOPEFULLY)
Quantum Mechanics is a very difficult topic, with a great deal of detail that is extremely
complex, yet interesting. However, in this Organic Chemistry Class we only need to
understand certain key aspects of Quantum Mechanics as applied to electronic theory. What
follows is an outline of many of the important concepts, color coded to help you.
The
statements in red are items you need to know
. Items in black are for your information, but it
is not essential that you know them.
Items in purple describe what you need to be able to do,
namely describe organic molecules in terms of overlap of hybridized orbitals
. Keep this in
mind as you go through the following.
QUANTUM OR WAVE MECHANICS
Electrons have certain properties of particles and certain properties of waves.
Electrons have mass and charge like particles.
Because they are so small and are moving so fast, electrons have no defined
position.
Their location is best described by wave mechanics (i.e. a three
dimensional wave) and a wave equation called the Schrödinger equation.
Solutions of the Schrödinger equation are called wave functions and are represented
by the Greek letter psi.
Each wave function describes a different orbital.
There are many solutions to the Schrödinger equation for a given atom.
The sign of the wave function can change from positive (+) to negative () in different parts
of the same orbital. This is analogous to the way that waves can have positive or negative
amplitudes. The sign of the wave function does not indicate anything about charge. [This
can be confusing. Make sure that you understand it before you go on.]
The value of the square of the wave function is proportional to the probability of
finding electron density at a given point in an orbital. Note that the sign of square of
the wave function is always positive, because the square of even a negative value is
still positive.
In a 2p orbital, it is just as probable to find electron density in the negative lobe as it
is to find electron density in the positive lobe. [Make sure you understand this
statement.]
A node is any place in an orbital at which the value of the wave function is zero.
A nodal surface or nodal plane are surfaces or planes where the value of the wave
function is zeor. There is absolutely no electron density at a node, a nodal surface, or
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The Schrödinger equation can in principle describe covalent bonding, but, even with
powerful computers the equation is too complicated to be solved exactly for large
molecules.
MOLECULAR ORBITAL THEORY OF COVALENT BONDING
Molecular orbital theory assumes that individual electron pairs are found in molecular
orbitals that are distributed over the entire molecule.
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 Spring '09
 Iverson
 Organic chemistry, Electron, Pi Bond, Valence, Mole, Chemical bond

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