1. Electron Configurations

2. Energy Levels, Sublevels, and Orbitals
Principal energy levels – n, assigned values 1- 7 (Like floors in a hotel)
Energy sublevels- s, p, d, f (Type of suite in a hotel)
s sublevel – 1 orbital
p sublevel – 3 orbitals
d sublevel – 5 orbitals
f sublevel – 7 orbitals
Orbitals – Two electrons per orbital (Two people per room)
(Orbitals are the number of rooms in a suite)

3. Electron Configurations
Electron configuration – the arrangement of electrons in an atom.
Example Sodium (Na) –
Three rules determine electron configurations
the Aufbau Principle,
the Pauli Exclusion Principle
Hund’s rule
1s22s22p63s1

4. The Aufbau Principle
Each electron occupies the lowest energy orbital available
Like filling the hotel from the bottom up
Which should we fill first, the lowest or highest energy orbital?

5. Aufbau Diagram
Which has a lower energy level 4s or 3d orbitals?

6. Filling the Orbitals in the correct order

7. Orbitals in the Periodic Table

8. Pauli Exclusion Principle
A maximum of two electrons may occupy a single orbital
Like only two people sharing one hotel room
How many electrons are found in an orbital?

9. Hund’s Rule
If two or more orbitals of equal energy are available, electrons will occupy them singly with the same spin, before filling them in pairs with opposite spins
A spin is denoted with an up ↑or down ↓ arrow to fill orbitals
This is like trying to find your own room on the same floor before having to share a room with someone else

10. Electron configuration Practice10

11. Question Time What is an electron configuration?
What is the Aufbau principle?
What is the Pauli Exclusion Principle?
What is Hund’s Rule?

12. Writing Electron Configurations
Aufbau diagram for sodium (Na) which has 11 electrons
Na electron configuration1s22s22p63s1

13. Exceptions to Electron Configurations
Copper and chromium are exceptions to the Aufbau principle.
Element
Should be
Actually is
Copper
1s22s22p63s23p63d44s2
1s22s22p63s23p63d54s1
Chromium
1s22s22p63s23p63d94s2
1s22s22p63s23p63d104s1
B. Some configurations violate the Aufbau Principle because half-filled sublevels are not as stable as filled sublevels, but they are more stable than other configurations

14. Valence electrons
Valence electrons are electrons in the outermost orbital
For A group elements the group number corresponds to number of valence electrons.
Electron-dot structures – Element’s symbol surrounded by dots representing the valence electrons 8A

15. Lewis Dot Structure Practice15

16. Question Time
What two elements are exceptions to the Aufbau rule for electron configurations?
What are valence electrons?