1. Light and Quantized Energy
Section 5.3

2. Ground State Electron Configurations
Review from Section #2:
According to the quantum mechanical model of the atom…
Principle Energy Level (n)
Energy Sublevel
Orbitals
s, p, d, f, …
2 e-s per orbital

3. Ground State Electron Configurations
The arrangement of e-s in an atom is called the atom’s electron configuration.
The ground state is the “normal” electron configuration of the atom…but why?
The atom wants to be in a state that is as low in energy as possible.
I guess that means that atom’s are lazy???
Atoms are the most stable when they are in their lowest energy state.

4. Ground State Electron Configurations
The Aufbau Principle
Aufbau (in German) = “building up” or “construction
The aufbau principle states that each e occupies the lowest energy orbital available.

5. Ground State Electron Configurations
The Aufbau Principle – Statements:
All orbitals within a specific sublevel are equal in energy.
The energy sublevels contained within a single principle energy level have different energies.
Sublevels in order of increasing energy: s, p, d, f
For related sublevels, the one with the higher principle quantum # is higher in energy.

6. Ground State Electron Configurations
The Aufbau Principle – Statements:
All orbitals within a specific sublevel are equal in energy.
Example: All 2p orbitals are equal in energy
The energy sublevels contained within a single principle energy level have different energies.
For n=2, the p sublevel is higher in energy than the s sublevel
Sublevels in order of increasing energy: s, p, d, f
For related sublevels, the one with the higher principle quantum # is higher in energy.
The 3p sublevel is higher in energy than the 2p sublevel

7. Ground State Electron Configurations
The Pauli Exclusion Principle:
Electrons have a property known as spin…an electron can have an up spin or a down spin.
The Pauli exclusion principle states that only 2 e-s can occupy an orbital at one time and that the e-s must have opposite spin to share the same orbital!

8. Ground State Electron Configurations
Hund’s Rule:
Hund’s “Hungry” Rule
Within an energy sublevel, each orbital must receive one electron before any orbital receives a 2nd electron.
Yum, Yum!!!

9. Orbital Diagrams
There are two major ways to represent an atom’s electron configuration: orbital diagrams and electron configuration notation.
In the orbital diagram method, an empty box represents an orbital.

10. Orbital Diagrams Li Be B

11. Orbital Diagrams C N F

12. Orbital Diagrams Si

13. Electron Configuration Notation
The electron configuration method uses the principle energy levels and the sublevel names in writing the notation. The number of electrons in each sublevel is indicated with a superscript above the sublevel symbol.
Orbital Diagrams e- configuration notation
orbitals sublevels

14. Electron Configuration Notation
How can you decide on the order to fill all of the orbitals? The diagram below can help you! (Hint: Follow the arrows)

15. Electron Configuration Notation
Homework: Practice Problems #18, 19, 20 (p.139)
Section Assessment #24, 26 (p.141)

16. Electron Configuration Notation
Why are the electron sublevels arranged in a non-numerical order? In particular, look at the last two arrows in the diagram below:

17. Electron Configuration Notation
Remember…electrons will stay in the lowest energy orbital that is available to them!!!

18. Electron Configuration Notation

19. Electron Configuration Notation

20. Shorthand Electron Configuration Notation
What is the election configuration notation for…
Argon (Ar)
Potassium (K)

21. Shorthand Electron Configuration Notation
Putting the noble gas in brackets [ ] indicates that the atom of interest contains the same e- configuration up until that point!

22. Electron Configuration Notation
One last note about writing electron configurations… you will not always be able to use the “cheat sheet” that you have.

23. Electron Configuration Notation
But never fear, there is a way for you to get help! You can use…the periodic table!!!

24. Electron Configuration Notation

25. Electron Configuration Notation

26. Valence Electrons
The chemistry of elements only depends on the e-s in the outermost principle energy level.
This level or “shell” of e-s are known as valence electrons.
Using the shorthand e- configuration notation, it is incredibly easy to identify the valence electrons for an atom.

27. Valence Electrons

28. Valence Electrons

29. Valence Electrons
First, let’s get some more practice writing out electron configurations…write out the following elements using both the long and shorthand method:
Copper – Cu
Promethium – Pu
Uranium – U

30. Valence Electrons
An atom can have anywhere between 1 and 8 valence e-s.
Why? Only the outermost s and p orbitals matter when counting valence electrons!!!
How many valence electrons are in the following atoms, according to their electron configurations? Fr Br Sb

31. Electron Dot Structures
As stated earlier, valence e-s are involved in the formation of chemical bonds. Therefore, it is useful to have a visual, shorthand way of representing the valence e-s.
An electron-dot structure is made up of the element’s chemical symbol surrounded by dots that represent the valence electrons.
See page 140 for lots of examples!!!

32. Electron Dot Structures
Rule: You must give each side of the chemical symbol one electron before any side gets two.
Electron Dot Structure Examples:
Carbon Arsenic Iodine
Krypton

33. Electron Configuration Notation

34. Homework (11/4): Practice Problems p. 139: #18d,e (can use shorthand!)
21,22
Practice Problems p. 141: #23a,b,c
Section Assessment p. 141: #24,26,28

35. Homework (11/5): Practice Problems p. 141: #23a,b,c,d,f
Section Assessment p. 141: #25 (e- configuration only), 28