Environmental Engineering Reference
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fundamental consideration of molecular orbital theories. To illustrate this, several
basic concepts regarding electron configuration in an atom, bonding, and molecular
orbitals are essential and will be presented below at only an introductory level.
Readers who do not have fresh chemistry in mind can refer to general chemistry
textbooks for a thorough review. Other readers can also skip this section for the time
being if only quantitative analysis is of primary concern.
Basic Concepts of Electronic Structure
Imagine electrons are particles as well as waves. They orbit the nucleus (as particles)
and their motions can be described in a volume by solving mathematical wave
equations to tell us where the electron is most likely to be found. When multiple
electrons are present, the electrons in an atom can be thought of as occupying a set of
major shells, subshells, and orbitals that surround the nucleus. Recall from your
general chemistry that
The major shell determines the primary energy level of the electron. The
first major shell is the one closest to the nucleus. Electrons in this first
major shell has the lowest energy. Major shells are designated by a
quantum number ( n ), that is, n ΒΌ 1 , 2, 3, 4 for the 1st, 2nd, 3rd, and 4th
major shells.
The subshells are a substructure within the major shells designated by
letters s, p, d, and f. If you visualize the atom as an onion, then the large
thick peels are the major shells and each onion skin or peel within each
major peel is the subshell. The number of subshell within a major shell is
the same as the quantum number, thus we have one subshell (s) in 1st
major shell, two subshells (s and p) in the 2nd major shell, three subshells
(s, p and d) in the 3rd major shell, and four subshells (s, p, d, and f) in the
4th major shell (Table 8.1).
Each subshell is further structured into orbitals . The numbers of orbitals
are 1 for s subshell, 3 for p subshell, 5 for d subshell, and 7 for f subshell.
Each orbital can accommodate a maximum of two electrons (Pauli Exclu-
sion Principle ). As a result, the maximum numbers of electrons equal
to 2n 2 or 2, 8, 18, and 32 for 1st, 2nd, 3rd, and 4th major shell,
respectively.
Table 8.1 Electron distribution in the first four shells surrounding the nucleus
Major shells (n )
1st shell
2nd shell
3rd shell
4th shell
Subshells
s
s, p
s, p, d
s, p, d, f
Number of atomic orbitals
1
1, 3
1, 3, 5
1, 3, 5, 7
Maximum number of electrons (2n 2 )
2
8
18
32
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