1. Electron Arrangement in Atoms
OBJECTIVES
Describe how to write the electron configuration for an atom.
Explain why the actual electron configurations for some elements differ from those predicted by the aufbau principle

2. ELECTRON CONFIGURATIONS
Three principles are used to distribute electrons in orbitals
Aufbau or Building up Principle. Orbitals are filled in order of increasing energy: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4 p, 5s, 4d, 5p, 6s, 4f, 5d, 6p.
Pauli Exclusion. No more than 2 electrons can occupy an orbital and then only if they have opposite spins.
Hund’s Rule. One electron is placed in each equal-energy orbital so that the electrons have parallel spins, before pairing occurs. (Substances with unpaired e- are paramagnetic-they are attracted to a magnetic field.)

3. Aufbau Principle

4. Pauli Exclusion Principle

5. Hund’s Rule

6. Hund’s Rule Examples

7. Hund’s Rule Examples

8. Exceptions to the Rules!
The Aufbau orbital filling principle can be used for elements up to Vanadium (atomic # 23).
Some actual electron configurations differ because of their tendency to want to be stable.
These exceptions are due to subtle e- to e- interactions in orbitals with very similar energies.
Examples: Chromium and Copper

9. Exceptions to the Rules!
Cr 1s2 2s2 2p6 3s2 3p6 3d4 4s2 (aufbau)
Cr 1s2 2s2 2p6 3s2 3p6 3d5 4s1 (correct)
Cu 1s2 2s2 2p6 3s2 3p6 3d9 4s2 (aufbau)
Cu 1s2 2s2 2p6 3s2 3p6 3d10 4s1 (correct)

10. Practice!!!!
In addition: include the orbital box diagrams for each and the planetary model for each.