1. Electron Configurations

2. 2s 2p 1s 3s Electron energy levels have sublevels of different shapes
orbital
Nucleus 2p
orbitals 1s
orbital 3s
orbital

3. How do electrons fill these orbitals?
Electron Configurations:
location of e–’s in different orbitals
3 principles
describe the
electron configuration
of any atom:
the aufbau principle
the Pauli exclusion principle
and Hund’s rule
The 3 rules work
“in concert”. So you
must be able to use any
one when it is required.

4. e–’s fill orbitals of lowest energy first
Aufbau Principle:
(building up)
e–’s fill orbitals of lowest energy first
-Where should we start placing electrons first? -Opposite spin alleviates repulsion. -All single before double.
nucleus

5. Arrows go as you progress along
the Periodic Table.

6. Pauli Exclusion Principle:
(up/down)
e–’s in same orbital have opposite spin
-Where should we start placing electrons first? -Opposite spin alleviates repulsion. -All single before double.
nucleus

7. e–’s pair after one in each same E orbital
Hund’s Rule:
(up/up/up/
down/down/down)
e–’s pair after one in each same E orbital
-Where should we start placing electrons first? -Opposite spin alleviates repulsion. -All single before double.
nucleus

8. Write the electron configuration for Phosphorus (P):
nucleus

9. Write the electron configuration for Selenium (Se):
1s22s22p63s23p64s23d104p4
nucleus

10. Hund’s Rule
Exception 1:
Copper (Cu)
1s22s22p63s23p64s13d10
nucleus

11. Hund’s Rule
Exception 2:
Chromium (Cr)
1s22s22p63s23p64s13d5
nucleus

12. 1s2 2s2 2p6 3s2 3p3 valence electrons:
Write the electron configuration for phosphorus (P):
1s2
2s2
2p6
3s2
3p3
valence electrons:
e–’s in the outer most (highest) energy level
How many valence e–’s in phosphorus (P)? 5
nucleus

13. Write the electron configuration for Krypton (Kr):
1s2
2s2
2p6
3s2
3p6
4s2
3d10
4p6
(3d fills after 4s)
-Where should we start placing electrons first? -Opposite spin alleviates repulsion. -All single before double.
nucleus

14. Write the electron configuration for Krypton (Kr):
1s2
2s2
2p6
3s2
3p6
4s2
3d10
4p6
valence electrons: e–’s in the outer most (highest) energy level
- At left has a lack of subtlety. For now, ignore d electrons. Highest open s and p only.
-Where should we start placing electrons first? -Opposite spin alleviates repulsion. -All single before double.
How many valence e–’s in Krypton (Kr)?
nucleus 8

15. Write the full electron configuration for Gallium (Ga), atomic number 31.
1s2
2s2
2p6
3s2
3p6
4s2
3d10
4p1
[Ar]
4s2
3d10
4p1 OR
noble gas core
configuration Ar

16. P Br valence e–’s: Lewis dot diagram: e–’s in highest energy level
valence e–’s as dots
Phosphorus (P):
Bromine (Br):
1s2 2s2 2p6 3s2 3p3
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p5
[Ne]3s2 3p3
[Ar]4s2 3d10 4p5
# valence e–’s: ___ 5
# valence e–’s: ___ 7
Group # : ___ 5A
Group # : ___ 7A
Lewis dot diagram:
Lewis dot diagram: P Br

17. Quick Quiz!
1. Which element has following electron configuration: 1s2 2s2 2p6 3s2 Ca Ne Na Mg
How many valence e–’s? 2
Group 2

18. Quick Quiz.
2. Write the electron configuration for an atom of oxygen (O).
1s2 2s6
1s2 2p6
1s2 2s2 2p4
1s2 2s3 2p3
How many valence e–’s? 6
Group 16 (6A)