1. Section: 3.3 - Electron Configurations and Periodic Trends
Electron Configuration – a shorthand notation that shows:
The number of electrons
The arrangement of electrons in orbitals
An atom’s ground state electron configuration determines most chemical properties
Therefore we usually write electron configurations for atoms in ground state

2. Fig. 3.19 - Atomic Orbitals and Relative Energies

3. Writing Electron Configurations
Using Boron as an example
# of electrons in each sublevel
1s22s22p1
Letters represent orbital shape
Principal Quantum Number (Energy Level)
We do not need spin numbers...they must be opposite if they are in the same sublevel

4. Orbital Diagrams/Energy Level Diagrams
Use boxes or circles to represent each orbital
= empty orbital
= orbital with one electron (+1/2 spin)
= orbital with one electron (-1/2 spin)
= orbital with 2 electrons...with opposite spin

5. Completing Orbital Diagrams and Writing Electron Configurations
For Lithium
1s22s1
1s 2s
For Nitrogen
1s22s2p3
1s 2s p
For Oxygen
1s22s23p4
1s 2s p
* The text uses circles and places orbitals vertically from lowest to highest

6. Condensed Electron Configurations
For atoms with a large number of electrons
The configuration is very large
For Potassium (Atomic # = 19)
1s22s22p63s23p64s1
Condensed Form - Element symbol for previous Noble Gas represents the orbitals up to that point. Additional orbitals are added to it.
For Potassium – previous Noble Gas is argon
Condensed Electron Configuraton
[Ar]4s1

7. Electron Configurations for Period 4
Notice that the 4s orbital has a lower energy level than 3d orbitals
Therefore 4s fills before 3d (Aufbau Principle)
K [Ar]4s1
Ca [Ar]4s2
Sc [Ar]4s23d1
Ti [Ar]4s23d2

8. Fig. 3.19 - Atomic Orbitals and Relative Energies

9. Exceptions to the Aufbau Principle
Some elements are more stable when they do not follow the Aufbau Principle
Eg. Cr is most stable when it have only 1 electron in 4s and 1 in each of the 3d orbitals
ie.
4s d
4s 3d

10. Patterns in Electron Configuration and Periodic Table Location

11. Patterns in Electron Configuration and Periodic Table Location
For Main Group Elements – last number in the group number = # of valance electrons
Eg. O is group # 16 – has 6 valance electrons
The n value of the highest occupied energy level is the period number
Eg. For Li – 1s22s1
Li is in period 2
For K – [Ar]4s1
K is in period 4

12. Patterns in Electron Configuration and Periodic Table Location
n2 = the total number of orbitals in that energy level
For n = 2, there are n2 = 4 orbitals
(one 2s orbital and three 2p orbitals)
2n2 = the maximum number of electrons in an energy level
For n = 2, there are 2n2 = 8 electrons max

13. Examining the Periodic Table
Elements with similar properties in the periodic table are filling the same subshell
Look at the electron configuration for lead
What does it look like?
How does the electron configuration explain why this transition metal can form both Pb2+ and Pb4+ ions?
Magnetism
Look at configuration for iron. What do you notice about the d-orbital electrons?

14. Exceptions to the rules...
Chromium
Predicted
[Ar]4s23d4
Actual
[Ar]4s13d5 (an s-orbital e- is promoted)
Copper
[Ar]4s23d9
[Ar]4s13d (an s-orbital e- is promoted)