Chapter 5: Orbital filling rules
The rules that you have been applying in order to determine the
electronic configuration of an atom are summarized below.
A. THE AUFBAU PRINCIPLE.
Lowest energy orbitals are filled first.
1s2s2p3s3p4s3d4p5s4d5p6s
4f5d6p7s5f6d7p
B. THE PAULI EXCLUSION PRINCIPLE.
Orbitals can only contain a maximum of two electrons
and when two electrons enter the same orbital they
must have opposite spins (+ ½ or – ½). (In the
“electrons in boxes” diagram they must be drawn 
NOT  OR ).
C. HUNDS RULE.
When orbitals of identical energy are available, electrons enter these
orbitals singly before any spin pairing takes place. In the p, d, and f
sublevels (that contain more than one orbital) no orbital may contain
a pair of electrons before all the orbitals of that sublevel contain one
electron. See detailed explanation below.
D. EXCEPTIONS TO THE AUFBAU PRINCIPLE.
There are some notable exceptions. For example Cr
and Cu achieve extra stability by forming a half-filled
or completely filled d sub-shell by using one of their
4s electrons.
Cr: 1s2 2s2 2p6 3s2 3p6 4s1 3d5
Cu: 1s2 2s2 2p6 3s2 3p6 4s1 3d10
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Detailed Explanation of Hund’s Rule:
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Consider each of the elements listed and the INCORRECT electronic configuration associated with each one. In each case identify
which of the above rules or principles (A, B, C or D) is violated and insert the correct electronic configuration. Then add a possible set
of quantum numbers for the outer most electron. An example is completed for you.
ELEMENT
INCORRECT
CONFIGURATION
VIOLATION
CORRECT
CONFIGURATION
N
1s2 2s2 2px2 2py1
C
1s2 2s2 2px1 2py1 2pz1
Al
1s2
2s2
2p6
3p3
B
1s2 2s3
P
1s2 2s2 2p6 3p5
Cu
[Ar] 4s2 3d9
Mg
[Ne] 
C
1s2 2s1 2px1 2py1 2pz1
C
1s2 2s2 2px2
Ag
[Kr] 5s2 4d9
Mn
[Ar] 4s1 3d6
Ni
[Ar] 4s2 3dxy2 3dxz2 3dyz2 3dz22 3dx2-y20
Cl
[Ne]    
Sc
[Ar] 3d3
B
1s2 2s1 2px1 2py1
Na
1s1 2s2 2p6 3s2
S
[Ne] 3s2 3px2 3py2
V
[Ar] 3d5
P
[Ne] 3s2 3px2 3py1
Kr
[Ar] 4s2 3d16
Cr
[Ar] 4s2 3d4
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POSSIBLE SET OF
QUANTUM NUMBERS FOR
OUTERMOST ELECTRON
2
1
-1
+½