1. Section 2: Electron Arrangement in Atoms
Electron configurations
Three rules – the aufbau principle, the Pauli exclusion principle, and Hund’s rule- tell you how to find the electron configurations of atoms.

2. Three Principles about Electrons 3d10… 4s2 Aufbau Principle: 3p6
e– will take lowest-energy
orbital available
3s2
2p6
2s2
Hund’s Rule:
1s2
Friedrich Hund
for equal-energy orbitals,
each must have one e– before
any take a second
Wolfgang Pauli
Pauli Exclusion Principle:
two e– in same orbital
have different spins

3. Section 2: Electron Arrangement in Atoms
Aufbau Principle
According to the aufbau principle, electrons occupy the orbitals of lowest energy first.
The orbitals for any sublevel of a principal energy level are always of equal energy.
Within a principal energy level, the s sublevel is always the lowest-energy sublevel.

4. Section 2: Electron Arrangement in Atoms
Pauli Exclusion Principle
According to the Pauli Exclusion principle, an atomic orbital may describe at most two electrons.
To occupy the same orbital, two electrons must have opposite spins – the electron spins must be paired.
Spin is a quantum mechanical property of electrons and may be thought of as clockwise or counterclockwise.
A vertical arrow indicates an electron and its direction of spin [↑ or ↓]

5. Section 2: Electron Arrangement in Atoms
Hund’s Rule
According to Hund’s rule, electrons occupy orbitals of the same energy in a way that makes the number of electrons with the same spin direction as large as possible.
Electrons then occupy each orbital so that their spins are paired with the first electron in the orbital.

6. Orbital Diagrams
…show spins of e– and which orbital each is in O 1s 2s 2p 3s 3p P 1s 2s 2p 3s 3p

9. Section 2: Electron Arrangement in Atoms
Writing Electron Configurations
A convenient method for showing the electron configuration of an atom involves writing the energy level and the symbol for every sublevel occupied by an electron.
You indicate the number of electrons occupying each sublevel with a superscript.
The sum of the superscripts equals the number of electrons in the atom.

10. Writing Electron Configurations:2p 4s 3p 5s 4p 3d 6s 5p 4d 7s 6p 5d 4f 8s 7p 6d 6f
(2e-)
(6e-)
(10e-)
(14e-)

11. Writing Electron Configurations:
Where are the e–? (probably) H
1s1 He
1s2 Li
1s2
2s1 N
1s2
2s2
2p3 Al
1s2
2s2
2p6
3s2
3p1 Ti
1s2
2s2
2p6
3s2
3p6
4s2
3d2 As
1s2
2s2
2p6
3s2
3p6
4s2
3d10
4p3 Xe
1s2
2s2
2p6
3s2
3p6
4s2
3d10
4p6
5s2
4d10
5p6
1s2
2s2
2p6
3s2
3p6
4s2
3d10
4p6…

12. Sections of Periodic Table to Know
s-block
p-block
d-block
f-block

13. Section 2: Electron Arrangement in Atoms
Exceptional Electron Configurations
You can obtain correct electron configurations for the elements up to vanadium (atomic number 23) by following the aufbau diagram for orbital filling.
If you were to continue in that fashion, however, you would assign chromium and copper the following incorrect configurations.
Cr: 1s22s22p63s23p63d44s2
Cu: 1s22s22p63s23p63d94s2

14. Section 2: Electron Arrangement in Atoms
The correct electron configurations are as follows:
Cr: 1s22s22p63s23p63d54s1
Cu: 1s22s22p63s23p63d104s1
This is because filled energy sublevels are more stable than partially filled sublevels.
Stable electron configurations are likely to contain filled energy sublevels.
At higher principal quantum numbers, energy differences between some sublevels are even smaller creating more exceptions to the aufbau principle.