1. Electron Configuration
The distribution of electrons among the orbitals of an atom
Describes where the electrons are found and what energies they possess.
Determined by distributing the atom’s electrons among levels(n=1,2,3etc.), sublevels(1s,2s,2p,etc.), and orbitals(s,p,d,f), based on a standard set of principles

2. Principles for Configurations
Aufbau Principle
Pauli Exclusion Principle
Hund’s Rule

3. The Aufbau Principle
Electrons are added one at a time to the lowest energy orbitals available until all the electrons of the atom have been accounted for.
Ex. 1s fills before 2s before 2p before 3s, etc.

4. Exceptions to the Aufbau Principle
1st Chromium(Cr)
Expected 1s2 2s2 2p6 3s2 3p6 4s2 3d4
Actual 1s2 2s2 2p6 3s2 3p6 4s1 3d5
2nd Copper
Expected 1s2 2s2 2p6 3s2 3p6 4s2 3d9
Actual 1s2 2s2 2p6 3s2 3p6 4s1 3d10

5. Pauli Exclusion Principle
An orbital can hold a maximum of 2 electrons and to occupy this orbital simultaneously they must have opposite spins.

6. Pauli Electron pairs
Pairs of electrons with opposite spins are said to be PAIRED
Single electrons in an orbital are UNPAIRED
Hydrogen Z=1 Helium Z=2

7. Hund’s Rule
Electrons occupy equal-energy orbitals so that a maximum number of unpaired electrons results.

8. Hund’s equal-energy orbitals Orbital Diagrams
Carbon Z=6
1s 2s 2p

9. Writing Configurations
There are several ways to do configurations.
One method is the arrow diagram,christmas tree, or beehive.
In this method you write out the sublevels for each energy levels and draw arrows

10. Writing Configurations
Another method is using the periodic table as a guide.
The table is broken into 4 blocks, s,p,d,f
7 rows or energy levels
Finally the elements position tells the number of electrons.