1. Orbital Diagrams
And Electron Configurations

2. Electrons behave as though they are spinning on their axis
Electrons behave as though they are spinning on their axis. A half arrow is used to indicate if the direction of the spin is up ( ) or down ( ).
H: 1 electron
, 1s1
Electron configuration

3. Pauli Exclusion Principle:
Each orbital can contain 2 electrons ONLY if the electron spins are in opposite directions.
He: 2 electrons
, 1s2

4. Auf bau principle: the electrons fill the orbitals starting with the lowest energy level and working their way up the energy ‘ladder’
Li: 3 electrons
, 1s2 2s1

5. Be: 4 electrons
, 1s2 2s2

6. B: 5 electrons
, 1s2 2s2 2p1

7. Hund’s Rule: all sublevel orbitals of the same energy must be filled with a single electron before the electrons can begin to pair up.
C: 6 electrons
, 1s2 2s2 2p2

8. Total electrons 12 20 10 21 18 4 2
Sc: 21 electrons
, 1s2 2s2 2p6 3s2 3p6 4s2 3d1
 complete configuration
Shorthand notation:
[Ar]
4s2 3d1
 noble gas core configuration

9. Electron Dot Structures
Chemical reactivity depends upon the number of electrons in the highest energy level (the largest n).
Electron dot structures show only the outermost electrons so that reactivity can be easily predicted.
The outermost electrons are also called the valence electrons.
Rules to Drawing Electron Dot Structures
Electrons 1-4 are added singly.
Exception: If there only two electrons, they’re shown as a pair.
Electrons 5-8 are paired with the single electrons that are already there.

10. Examples:
1. What is the electron configuration of strontium (Sr)?
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2
The highest energy level is n=5 and there are 2 electrons in it. Therefore, Sr has 2 valence electrons. Sr
2. What is the electron configuration of nitrogen (N)?
1s2 2s2 2p3
The highest energy level is n=2 and there are 5 electrons in it. Therefore, N has 5 valence electrons. N
3. What is the electron configuration of xenon (Xe)?
1s2 2s2 2p6 3s2 3p6 4s2 3d10 3p6 5s2 4d10 5p6
The highest energy level is n=5 and there are 8 electrons in it. Therefore, Xe has 8 valence electrons. Xe