CHAPTER 2 - ATOMIC STRUCTURE

2.3 Electronic Configuration

2.3 Electronic Configuration

At the end of this topic students should be able to:-
a) Explain Aufbau principle, Hund’s rule and

Pauli exclusion principle.
b) Predict the electronic configuration of atoms

or monatomic ions using spdf notation.
c) Justify the anomalous electronic configurations

of chromium and copper.

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What is an electronic configuration?
• An electronic configuration describes the

arrangement of electrons in an atom.
• It shows how electrons are filled in the orbitals of an

atom.
• Electronic configuration can be written as

i) orbital diagram

e.g 9F : 2p

1s 2s

ii) spdf notation

e.g 9F : 1s2 2s2 2p5

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Example:
Draw orbital diagram and write spdf notation
for oxygen atom.

i) orbital diagram magnetic quantum
number , m

8O : 1s 2s 2p

principle quantum angular momentum quantum
number, n number, l

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ii) spdf notation

e.g 8O: 1s2 2s2 2p4

Principal quantum Azimuthal quantum
number, n number, 

8 O: 1s2 2s2 2p4

Number of electrons in
the subshells

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Principles And Rule In Filling The Orbitals

To write an electronic configuration, the following
rule and principles need to be obeyed :

a) The Aufbau Principle
b) The Pauli Exclusion Principle
c) The Hund’s rule

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a) Aufbau Principle

• States that electrons are filled in the orbitals in
order of increasing energy.

• It means that electrons should occupy the orbital
with the lowest energy before occupy the one with
higher energy.

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Relative Energy Level of Atomic Orbitals

4d

n=5 5s

4p

n=4 3d
4s
energy
n=3 3p
3s

n=2 2p
2s

n=1
1s

Orbital energy levels in a many-electrons atom

108

Order of filling electrons in orbitals for a
multi-electrons atom

109

Order of filling orbitals in multi-electrons atom
• The order of filling orbitals is:

1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s

• It starts with the 1s orbital and moves downward,
follows the arrows.
Eg: 4Be

• Electronic configuration : 1s2 2s2

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b) Pauli Exclusion Principle

No two electrons in an atom can have the same
four quantum numbers (n, , m, s)
Eg : Li (3 electrons)

(n, , m, s)
(n=1, l= 0, m= 0, s= +½) - first electron
(n=1, l= 0, m= 0, s= -½) – second electron
(n=2, l= 0, m= 0, s= +½) – third electron

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c) Hund’s Rule
 States that when electrons are added to the

orbital of equivalent energy (degenerate
orbitals), each orbital are filled singly with
electron of the same spin before it is paired.
 The electron in half-filled orbitals have the same
spins, that is, parallel spins.

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Example:
Write the spdf notation for following atoms:
1) Carbon (6 electrons)
2) Nitrogen (7 electrons)
3) Oxygen (8 electrons)

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Answer:

1) Carbon (6 electrons)
1s22s22p2

2) Nitrogen (7 electrons)
1s22s22p3

3) Oxygen (8 electrons)
1s22s22p4

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The Anomalous Electronic Configurations of Cr
and Cu

• Cr and Cu have electronic configurations which are
inconsistent with the Aufbau principle.

• The anomalous are explained on the basis that a
filled or half-filled orbital is more stable.

* a filled orbital IS a completely filled orbital
a half-filled orbital is NOT a partially filled orbital

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Stability of orbital.

 For degenerate orbitals, half- filled orbitals is
more stable than the partially-filled.

Half-filled orbital (stable) nd

or
np

Partially-filled orbital ( less stable)

or nd
np

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Based on the Aufbau principle, the electronic
configuration for 24Cr is 1s22s22p63s23p6 4s2 3d4

24Cr : or 4s 3d
1s 2s 2p 3s 3p

• The 3d orbital is partially-filled, so it is less stable .

• Thus, to achieve stability, one electron from the
4s orbital occupies one of the 3d orbital in order to
have a half -filled orbital arrangement as follow :

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24Cr : 2p 3s 3p 4s 3d
1s 2s

• The actual spdf electronic configuration of 24Cr is
1s22s22p63s23p6 4s1 3d5

• Half-filled 3d orbital arrangement increase the
stability of Cr atom.

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The electronic configuration of 29Cu based on Aufbau
principle is 1s2 2s2 2p6 3s2 3p6 3d9 4s2 or

29Cu :

1s 2s 2p 3s 3p 4s 3d

• To achieve stability 1 electron from 4s orbital
occupies 1 of the 3d orbital in order to have a
filled orbital arrangement.

29Cu : 2p 3s 3p 4s 3d
1s 2s

Or 1s2 2s2 2p6 3s2 3p6 3d10 4s1

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As a conclusion, the electronic configuration of
chromium and copper are :

Element Expected (Aufbau Observed/actual
Principle)
1s22s22p63s23p6
24Cr 1s22s22p63s23p6 4s2 3d4 4s1 3d5

29Cu 1s22s22p63s23p6 3d9 4s2 1s22s22p63s23p6
3d10 4s1

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* To fill electron(s)
- follow the Aufbau principle, Pauli Exclusion
principle and Hund’s rule except for
anamolous cases (Cr and Cu)

* To remove / eliminate electron(s)
- remove electron(s) from the orbital with the
largest n value

e.g :
Co ( Z = 27 ), write the electronic configuration
of Co3+
Co : 1s2 2s2 2p6 3s2 3p6 4s2 3d7
Co3+ : 1s2 2s2 2p6 3s2 3p6 3d6

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z = 21
z = 30

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Exercise:

Write the electronic configuration of the following
atoms and ions:
1) 6C
2) 10Ne
3) 13Al
4) 13Al3+
5) 17Cl
6) 17Cl-

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Answer: 5) 17Cl

1) 6C 1s2 2s2 2p6 3s2 3p5
1s2 2s2 2p2

2) 10Ne 6) 17Cl-
1s2 2s2 2p6 1s2 2s2 2p6 3s2 3p6

3) 13Al
1s2 2s2 2p6 3s2 3p1

4) 13Al3+ 124
1s2 2s2 2p6