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1
Many Electron Atoms and Periodic Properties
.
I. Electron Structure
A. The Aufbau Principle.
1. We can solve Schrödinger equation only for the hydrogen atom. Therefore, we will
build up the structure of many electron atoms in the following way:
a. Start with hydrogen solutions. Increase the nuclear charge by one and
place another electron in atom. Starting with the hydrogen solutions, use
the rules of quantum mechanics to determine the energy and wave
function of the two electron system; this is the structure of He.
b. To obtain structure of Li, start with the He results, increase nuclear charge
by one and add another electron. This process is repeated and the
structures of the heavier atoms are obtained by " building out " from
lighter atoms. (Aufbau = building out)
2. The results.
a. n,
l,m
l
and m
s
are still good quantum numbers for describing the behavior of the
electrons, that is, the electrons exist in hydrogenlike orbitals.
b. In the absence of strong magnetic fields, the energy depends on both n and
l
.
The order of the subshells encountered in following the aufbau principle is:
1s<2s<2p<3s<3p<4s<3d<4p<5s<4d<5p<6s.
...
c. This sequence can be remembered by the following scheme:
1s
2s 2p
3s 3p 3d Follow the diagonals.
4s 4p 4d 4f
5s 5p 5d 5f 5g
6s 6p 6d 6f 6g 6h
7s 7p 7d 7f 7g 7h 7I
8s 8p 8d…….
.
3. The electron structure of an atom is given by writing the
electron configuration
of the atom.
a. Examples
element:
1
H
2
He
3
Li
5
B
6
C
7
N
electron configuration: 1s
1
1s
2
1s
2
2s
1
1s
2
2s
2
2p
1
1s
2
2s
2
2p
2
1s
2
2s
2
2p
3
32
Ge: 1s
2
2s
2
2p
6
3s
2
3p
6
4s
2
3d
10
4p
2
or : 1s
2
2s
2
2p
6
3s
2
3p
6
3d
10
4s
2
4p
2
Could also group according to n, but fill according to the aufbau principle
Write the subshells occupied and the number of electrons in the particular subshell
as a righthand superscript.
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b. The most important electrons in determining the physical and chemical
properties of an element are the outermost or
valence electrons
.
1) For the main group elements the valence electrons are all those electrons
with the highest value of the principle quantum number n, they will be s and p
electrons.
2) The inner electrons are called the
core electrons
. They are chemically
inert. Their main function is to shield the nuclear charge as seen by the
valence electrons.
3) Example: electron division in Ge:
32
Ge: 1s
2
2s
2
2p
6
3s
2
3p
6
3d
10
4s
2
4p
2
core
valence
4) Some valence configuration are:
3
Li 2s
1
5
B 2s
2
2p
1
6
C 2s
2
2p
2
7
N 2s
2
2p
3
B. Periodic Table and Valence Electron Configuration.
1. Main group elements (Representative elements, s and pblock elements)
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 Spring '08
 Prof.Maguire
 Electron

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