5.2 Electron Arrangement in Atoms
Electron Configurations
The electron configuration is the way in which electrons are
arranged in various orbitals about the nucleus of an atom.
Electron cloud
Electron orbitals
1. Pauli exclusion principle
2. Aufbau principle
3. Hund’s rule
Pauli Exclusion Principle
An atomic orbital describes at most two electrons. The 2
electrons will have opposite spins.
“Spin-up” electron
Sample
Electron orbital
“Spin-down” electron
Electron pair
Orbital full
Aufbau Principle
Electrons first occupy the orbitals of lowest energy.
Hund’s Rule
Unpaired electrons in orbitals of the same energy all have the
same spin. Electrons will enter each of the sublevels singly, then
add a second if numbers permit.
Electron Configuration of Elements
Element
1s
2s
2px
2py
2pz
3s
Electron
Configuration
H
1s1
O
1s22s22p4
F
1s22s22p5
Na
1s22s22p63s1
Electron Configuration for Oxygen
Why is 1s22s22p4 the electron configuration of oxygen, not
1s22s22p33s1?
1s22s22p33s1
1s22s22p4
Warning: Notation Does Not Give Energy
Consider bromine, which has 35 electrons:
1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p5
Several Possible Notations
Example: Bromine (Br)
Ordered by principle energy level
1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p5
Ordered by energy of sublevel
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p5
Shorthand notation for Br
[Ar] 3d10 4s2 4p5
Exceptions for Electron Configurations
Consider Cu (29 electrons):
Aufbau principle gives
1s22s22p63s23p63d94s2
Correct configuration is
1s22s22p63s23p63d104s1
Chromium: Another Exception to Aufbau
The Aufbau principle gives the incorrect electron configuration for
chromium (24 electrons).
Aufbau principle gives
1s22s22p63s23p63d44s2
Correct configuration is
1s22s22p63s23p63d54s1