Chemistry Reference
In-Depth Information
1
Principles
1.1 THE ORBITAL MODEL
The great majority of the applications of molecular quantummechanics to
chemistry are based on what is called the orbital model. The planetary
model of the atom can be traced back to Rutherford (Born, 1962). It
consists of a point-like nucleus carrying the whole mass and the whole
positive charge
þ
Ze surrounded by N electrons each having the elemen-
tary negative charge
e and a mass about 2000 times smaller than that of
the proton and moving in a space which is essentially that of the atom.
1
Electrons are point-like elementary particles whose negative charge is
distributed in space in the form of a charge cloud, with the probability of
finding the electron at point r in space being given by
2
dr
¼
probability of finding in dr the electron in state
jcð
r
Þj
cð
r
Þ
ð
1
:
1
Þ
(r) are called atomic orbitals (AOs, one centre) or
molecular orbitals (MOs, many centres) and describe the quantum states
of the electron. For (1.1) to be true,
The functions
c
(r) must be a regular (or Q-class)
mathematical function (single valued, continuous with its first derivatives,
c
1
The atomic volume has a diameter of the order of 10
2
pm, about 10
5
times larger than that of the
nucleus.
