1. Orbitals and Their Energies
Orbital energy increases in this order: s< p < d < f
Degenerate orbitals: all orbitals in the same subshell. All have same energy.
Example: p-orbitals are degenerate

2. Electron Configurations
Tells which orbitals are assigned for each electron in an atom.
There are three rules to guide configurations.
Pauli Exclusion Principle
Aufbau Diagram
Hund’s Rule

3. Pauli’s Exclusion Principle
At most, there can be two electrons in a given orbital and they must have opposite spin.
One spins clockwise, the other counter-clockwise.
In block diagrams, we show this as:  = clockwise;  = counterclockwise

4. Electron Spin
One electron spins clockwise, the other counter-clockwise.

5. Aufbau Diagram
Aufbau Diagram: electrons fill orbitals from lower energy to higher energy.
Electrons do NOT always fill all orbitals in same energy level before starting to fill the next level!

6. Aufbau Diagram
Use the Aufbau Diagram to write the e- configuration for nitrogen.
Use the Aufbau Diagram to write the e- configuration for iron.

7. Write the e- configuration for lead.
Aufbau Diagram
Write the e- configuration for lead.

8. Hund’s Rule
Hund’s Rule: For degenerate orbitals, need to maximize the number of electrons with same spin.
Fill each degenerate orbital with a single electron spinning in one direction before adding each orbital’s second electron spinning in the opposite direction.

9. Reading Electron Configurations from the Periodic Table

10. Condensed Electron Configurations
Ne completes the 2p subshell.
Na marks the beginning of a new row.
So, we write the condensed electron configuration for Na as
Na: [Ne] 3s1
[Ne] represents the electron configuration of neon.

11. Core and Valence Electrons
Core e-’s: electrons in closed shells. Usually in [brackets] in condensed e-configs.
Valence electrons: Electrons in the highest principal quantum level and unfilled lower quantum levels of an atom.
[Ar]3d84s [Core] Valence
[Ar]3d104s24p [Core] Valence

12. Unusual Electron Configurations
Lanthanides & Actinides = rare earth elements
4f and 5f block
First three lanthanides and actinides have unusual electron configurations.
La: [Xe]6s25d1 Ce: [Xe]6s25d14f1 Pr: [Xe]6s24f3

13. Anomalous Electron Configurations
Anomalous = unusual
Know Cr, Mo, Cu, and Ag!
Element
Actual
Expected Cr
[Ar]3d54s1
[Ar]3d44s2 Cu
[Ar]3d104s1
[Ar]3d94s2 Ag
[Kr]4d105s1
[Kr]4d95s2