1. Electron Configurations
Glenn V. Lo
Department of Physical Sciences
Nicholls State University

2. Electron Configuration
How electrons in an atom or ion are assigned to subshells
Example: 1s2 2s2 2p1 means….
Unwritten superscript is implied to be 1.
Example: 1s2 2s2 2p is same as 1s2 2s2 2p1

3. Example
Which of the following can have an electron configuration of 1s2 2s2 2p1 ?
A. Li2-, B. B, C. C+, D. all of the above

4. Ground and Excited States
Ground state configuration: most stable, lowest energy configuration
Excited state: higher energy configuration
Example: Which of the following is the ground state configuration of H? Which is an excited state? A. 1s1, B. 1s2, C. 2p
Unless otherwise specified, “electron configuration” refers to ground state configuration

5. Example
Write the an excited state configuration for He.

6. Ground Configuration
Use Aufbau principle: assign electrons to lowest energy orbitals. Typically, in atoms with more than one electron,
Lower n + l , lower energy
Same n + l , lower n has lower energy
Use Pauli principle: no more than two electrons per orbital.

7. Example
In a typical atom, in which of these orbitals would an electron have a higher energy? A. 3p, B. 3d, C. 4s

8. Memory Aid 1s 2p 2s 3s 3p 4s 3d 4p 5s 4d 5p 6s * 5d 6p 7s * 6d 7p 4f

9. Example: Sc 1s 2p 2s 3s 3p 4s
Electron configuration of Sc is….

10. Abbreviating noble gas core
Ne electron configuration is….
Na electron configuration is….1s2 2s2 2p6 3s1 or [Ne] 3s1

11. Patterns Valence shell (largest n): row number
Valence configuration: same for same column

12. The noble gases
Valence s and p subshells are completely filled.
He: 1s2
Ne: [He] 2s2 2p6
Ar: [Ne] 3s2 3p6
Kr: [Ar] 4s2 3d10 4p6
Xe: 1s2 2s2 … ???

13. The “s block” elements H: 1s1 Li: [He]2s1 Na: [Ne]3s1 K: Rb: Cs:
Be: [He]2s2
Mg: [Ne]3s2
Ca:
Sr:
Ba:

14. The “d block” elements
Sc: [Ar]4s23d1
Ti: [Ar]4s23d2
Mn: Y:

15. The “p block” elements B: [He] 2s2 2p1 Al: [Ne] 3s2 3p1
Ga: [Ar] 4s2 3d10 4p1 C:
Si:
Ge: N: O: F:
Ne:

16. Example
Which atom has an electron configuration of 1s2 2s2 ….etc… 4p5?

17. Test Yourself
Which of the following is not a valid electron configuration for a lithium atom, which has 3 electrons?
A. 1s3 B. 1s2 2s1 C. 1s2 2p1 D. 1s1 2p1 3s1

18. Test Yourself
In a typical atom, which of these orbitals is filled last? A. 3p B. 3d C. 4s D. 4p

19. Test Yourself
Which of the following is the ground state electron configuration of Sulfur? A. 1s2 2s2 2p6 2d6 B. 1s2 2s2 2p6 3s2 3p4 C. 1s2 2s2 2p7 3s1 3p3 D. 1s2 2s2 2p6 3s2 3p6

20. Test Yourself
Which of the following is the ground state electron configuration of Al? A. [Ne] 3s2 3p1 B. [He] 2s2 2p6 3s2 3p1 C. [Mg] 3p1 D. all of the above

21. Test Yourself
What is the ground state electron configuration for zirconium?
A. [Kr]5s2 5p2 B. [Kr]5s2 5d2 C. [Kr]5s2 4d2

22. Orbital Diagram
Shows assignment of electrons to orbitals for a given configuration.
Arrows are used to represent electrons. Direction of arrows represent spin (up or down).
Must follow Pauli’s principle
There is more than one way of assigning if a subshell is partially filled.
Example: 1s2 2s2 2p4. The p sublevel is partially filled; 15 ways!

23. Hund’s Rule of Maximum Multiplicity
Lowest energy assignment of electrons for a given configuration, if a sublevel is partially filled
fill orbitals in the sublevel singly, with parallel spins, before pairing up.
Example: O (1s2 2s2 2p4)

24. Example
Draw arrows in the diagram to represent electrons in an excited state of Oxygen (8 electrons)

25. Test Yourself
How many electrons are unpaired in the ground state of Oxygen?
A. 0 B. 1 C. 2 D. 3

26. Paramagnetism Paramagnetic: affected by magnets
Diamagnetic: opposite of paramagnetic
Atoms with partially filled subshells are paramagnetic.
Examples:
Explain why N is paramagnetic.
Why are Be atoms not paramagnetic?

27. Test Yourself Which of the following atoms is not paramagnetic?
A. Na B. Mg C. C D. Ti