1. Electron Configurations
Chapter 5
Section 3

2. Vocabulary electron configuration aufbau principle
Pauli exclusion principle
Hund’s rule
Valence electron
Electron-dot structure

3. Review
Each principal energy level can have the same number of sublevels as the level number
Each sublevel orbital has a different shape
Each orbital can have only 2 electrons
What does this mean?

4. Electron Arraignment follows Rules
Low energy level systems are more stable than high-energy systems
Atoms will assume the electron arrangement that gives the atom the lowest energy
Most stable is the “ground state” (lowest energy)
Three rules/principles for arranging electrons
Aufbau
Pauli Exclusion principle
Hund’s Rule

5. Aufbau Principle
Each electron occupies the lowest energy level available
Learn the sequence of atomic orbitals from lowest to highest:

6. Aufbau Diagram
#4 – also do electron DOT diagram, #7 & 8 – SHOW YOUR WORK

7. Aufbau Expanded

8. Using Aufbau
All orbitals related to the same energy level are of equal energy
All 2p orbitals have the same energy
In a multi-electron atom, the energy sublevels within a principal energy level have different energies:
2p orbitals are higher energy than 2s orbitals
The sequence of sublevels within a principle level in increasing energy is: s, p, d, and f
Orbitals related to energy sublevels within one principle energy level can overlap orbitals related to another principal level
Notice: 4s is lower than 3d

9. Pauli Exclusion Principle
Each electron has an associated spin, like a top
Can spin on bottom or top
Each orbital can hold AT MOST 2 electrons, but only if they have opposite spin
Designated as:

10. Hund’s Rule
Because negatively charged electrons repel each other, they try to get as far away from each other as possible, therefore:
Single electrons with the same spin will occupy each equal energy orbital before additional electrons with opposite spins occupy the same orbitals.
WHAT?

11. Hund’s Rule 2 1 4 3

12. Representing Atom’s Electrons
Orbital Diagrams
Electron Configuration Notations

13. Orbital Diagrams Draw one box for each orbital Example:
Empty box means no electrons
Box with single up arrow means orbital with one electron
Box with two arrows (up/down) means orbital with two electrons
Each box is labeled with the principle quantum number and the sublevel
Example:

14. Example Orbital Diagram - N

15. Example Orbital Diagram – C & O

16. Electron Configuration Notation
Specify principal energy level and energy sublevel
Use a superscript to represent the number of electrons
Use the Aufbau diagram to help you remember the level/sublevel filling order
Examples:

17. Electron Configuration Notation

18. Electron Configuration Notation
Element
ECN
Hydrogen
1s1
Carbon
1s2 2s2 2p2
Helium
1s2
Nitrogen
1s2 2s2 2p3
Lithium
1s2 2s1
Oxygen
1s2 2s2 2p4
Beryllium
1s2 2s2
Fluorine
1s2 2s2 2p5
Boron
1s2 2s2 2p1
neon
1s2 2s2 2p6

19. A Variation – Noble Gas Notation
Similar to Electron Configuration Notation, but:
Start with the previous noble gas, and put that symbol in brackets
Add electrons to the noble gas
Example:
Sodium (Na) = [Ne] 3s1
Ne = 1s2 2s2 2p6
Na = 1s2 2s2 2p6 3s1

20. Exceptions to Aufbau Cr (Chromium) = [Ar] 4s1 3d5
Cu (Copper) = [Ar] 4s1 3d10

21. Valence Electrons
Only Valence Electrons contribute to chemical bonding
Valence Electrons = electrons in outer most principal energy level
Generally highest energy
Examples:
Carbon: [He] 2s2 2p2 has 4 valence electrons
Sulfur: [Ne] 3s2 3p4 has 6 valence electrons

22. Electron Dot Diagrams
Also called Lewis Electron Dot Diagrams or Lewis Electron Dot Structure or Electron Dot Structure
Shows atomic symbol and ONLY valence electrons

23. Electron Dot Diagrams

24. Periodic Table