1. Electron Configurations and Periodicity.
Why is this VERY important?
Chemistry is explained through sharing or transferring electrons.
This determines which elements can bond and what type of bond they form

2. How are the electrons arranged in an atom
How are the electrons arranged in an atom? What orbitals are preferentially filled by electrons?
What is an electron configuration?

3. Electron Configuration
Electrons in Atoms
Electron Configuration
C. Johannesson

4. 3 RULES FOR THE ARRANGEMENT OF ELECTRONS IN ATOMS.
1) The Pauli Exclusion Principle
2) The Aufbau Principle
3) Hund’s Rule
C. Johannesson

5. A. General Rules Pauli Exclusion Principle
Each orbital can only hold TWO electrons with opposite spins.
No 2 electrons in the same atom can have the same set of all 4 quantum numbers
(electrons can not occupy same space nor have the same spin)
Wrong
Right

6. General Rules: Aufbau Principle
Electrons fill the lowest energy orbitals first.
(means building-up in German) in the ground state, the electrons will fill the atomic orbital of lowest energy first; “Lazy Tenant Rule”
C. Johannesson

7. A. General Rules WRONG RIGHT Hund’s Rule
Within a sublevel, place one electron per orbital before pairing them.
“Empty Bus Seat Rule”
WRONG
RIGHT
C. Johannesson

8. 1s2 2s2 2p4 O B. Notation 1s 2s 2p 8e- Orbital Diagram
Electron Configuration
1s2 2s2 2p4
C. Johannesson

9. S 16e- 1s2 2s2 2p6 3s2 3p4 S 16e- [Ne] 3s2 3p4 B. Notation
Longhand Configuration
S 16e-
1s2
2s2
2p6
3s2
3p4
Core Electrons
Valence Electrons
Shorthand Configuration
S 16e- [Ne] 3s2 3p4
C. Johannesson

10. C. Periodic Patterns s p d (n-1) f (n-2) 1 2 3 4 5 6 7 6 7
© 1998 by Harcourt Brace & Company
C. Johannesson

11. C. Periodic Patterns Period # energy level (subtract for d & f)
A/B Group #
total # of valence e-
Column within sublevel block
# of e- in sublevel
C. Johannesson

12. 1s1 C. Periodic Patterns 1st column of s-block 1st Period s-block
Example - Hydrogen
1s1
1st column of s-block
1st Period
s-block
C. Johannesson

13. C. Periodic Patterns p s d (n-1) f (n-2) Shorthand Configuration
Core e-: Go up one row and over to the Noble Gas.
Valence e-: On the next row, fill in the # of e- in each sublevel. s
d (n-1)
f (n-2) p
C. Johannesson

14. [Ar] 4s2 3d10 4p2 C. Periodic Patterns Example - Germanium
C. Johannesson

15. D. Stability Full energy level Full sublevel (s, p, d, f)
Half-full sublevel
C. Johannesson

16. D. Stability Electron Configuration Exceptions Copper
EXPECT: [Ar] 4s2 3d9
ACTUALLY: [Ar] 4s1 3d10
Copper gains stability with a full d-sublevel.
C. Johannesson

17. D. Stability Electron Configuration Exceptions Chromium
EXPECT: [Ar] 4s2 3d4
ACTUALLY: [Ar] 4s1 3d5
Chromium gains stability with a half-full d-sublevel.
C. Johannesson

18. D. Stability 1+ 2+ 3+ NA 3- 2- 1- Ion Formation
Atoms gain or lose electrons to become more stable.
Isoelectronic with the Noble Gases. 1+ 2+ 3+ NA 3- 2- 1-
C. Johannesson

19. O2- 10e- [He] 2s2 2p6 D. Stability Ion Electron Configuration
Write the e- config for the closest Noble Gas
EX: Oxygen ion  O2-  Ne
O e [He] 2s2 2p6
C. Johannesson