1. Electron Configuration
Chemistry

2. Learning Objectives
TLW express the arrangement of electrons in atoms through electron configurations (TEKS 6.E)

3. Agenda Electron Configuration Overview Aufbau Principle and Diagram
Pauli Exclusion Principle
Hund’s Rule
Writing Electron Configurations
Full, Noble Gas, For Ions
Orbital (Box) Diagrams
Exceptions
Group and Individual Practice

4. Electron Configurations
The way electrons are arranged in atoms:
1. Every element has a specific electron configuration – NO two are alike
2. Very important = scientists know how an atom will react and how to manipulate that element

5. Take your Periodic Table to add info
Break into Lab Teams – each with an Atom Building Game as we Inquire about e- config
Take your Periodic Table to add info

6. Three Rules for Electron Configuration
Aufbau Principle
Pauli Exclusion Principle
Hund’s Rule

7. a. Electrons enter the lowest energy level first
1. Aufbau Principle
a. Electrons enter the lowest energy level first
b. Orbitals do overlap – so Aufbau’s principle plays a very important part in writing electron configuration

8. c. so, 1s would fill first; followed by 2s, then 2p etc
Aufbau’s principle

9. 2. Pauli Exclusion Principle
a. at most there is only 2 electrons per orbital
b. Different spin for each
c. there are four orbitals =s, p, d, f 2p 1s 2s

10. 3. Hund’s Rule
a. electrons do not pair up until they have to
b. the empty bus seat
Wrong
Right

11. C. Writing electron configuration
1. Remember the 3 rules: Aufbau’s, Pauli’s and Hund
2. Orbitals DO NOT fill in a nice neat order; orbitals overlap
3. Must fill lowest energy FIRST (Aufbau’s)
4. 1st energy level is s; 2nd is s & p; 3rd is s, p & d; 4th is s, p, d & f

12. 5. This is the order electrons fill up in going from lowest to highest energy level per Aufbau Principle
1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p 6f 7d 7f

13. 1s2 2s2 2p4 Oxygen 6. Orbital Diagram (Box Diagrams)
Electron Configuration
1s2 2s2 2p4

14. D. Practice
Let’s determine the electron configuration for Phosphorus
A total of 15 electrons

15. Aufbau diagram Increasing energy 7p 6d 5f 7s 6p 5d 6s 4f 5p 4d 5s 4p

16. The first two electrons go into the 1s orbital
Increasing energy 1s 2s 3s 4s 5s 6s 7s 2p 3p 4p 5p 6p 3d 4d 5d 7p 6d 4f 5f
The first two electrons go into the 1s orbital
Notice the opposite spins
only 13 more to go...

17. The next 2 electrons go into the 2s orbital
Increasing energy 1s 2s 3s 4s 5s 6s 7s 2p 3p 4p 5p 6p 3d 4d 5d 7p 6d 4f 5f
The next 2 electrons go into the 2s orbital
only 11 more...

18. The next 6 electrons go into the 2p orbital
Increasing energy 1s 2s 3s 4s 5s 6s 7s 2p 3p 4p 5p 6p 3d 4d 5d 7p 6d 4f 5f
The next 6 electrons go into the 2p orbital
only 5 more...

19. The next 2 electrons go into the 3s orbital
Increasing energy 1s 2s 3s 4s 5s 6s 7s 2p 3p 4p 5p 6p 3d 4d 5d 7p 6d 4f 5f
The next 2 electrons go into the 3s orbital
only 3 more...

20. The last three electrons go into the 3p orbitals.
Increasing energy 1s 2s 3s 4s 5s 6s 7s 2p 3p 4p 5p 6p 3d 4d 5d 7p 6d 4f 5f
The last three electrons go into the 3p orbitals.
They each go into separate shapes
3 unpaired electrons
= 1s22s22p63s23p3

21. The easy way to remember
2s 2p
3s 3p 3d
4s 4p 4d 4f
5s 5p 5d 5f
6s 6p 6d 6f
7s 7p 7d 7f
1s2
2 electrons

22. Fill from the bottom up following the arrows
2s 2p
3s 3p 3d
4s 4p 4d 4f
5s 5p 5d 5f
6s 6p 6d 6f
7s 7p 7d 7f
1s2 2s2
4 electrons

23. Fill from the bottom up following the arrows
2s 2p
3s 3p 3d
4s 4p 4d 4f
5s 5p 5d 5f
6s 6p 6d 6f
7s 7p 7d 7f
1s2 2s2 2p6 3s2
12 electrons

24. Fill from the bottom up following the arrows
2s 2p
3s 3p 3d
4s 4p 4d 4f
5s 5p 5d 5f
6s 6p 6d 6f
7s 7p 7d 7f
1s2 2s2 2p6 3s2 3p6 4s2
20 electrons

25. Fill from the bottom up following the arrows
2s 2p
3s 3p 3d
4s 4p 4d 4f
5s 5p 5d 5f
6s 6p 6d 6f
7s 7p 7d 7f
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2
38 electrons

26. Fill from the bottom up following the arrows
2s 2p
3s 3p 3d
4s 4p 4d 4f
5s 5p 5d 5f
6s 6p 6d 6f
7s 7p 7d 7f
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2
56 electrons

27. Fill from the bottom up following the arrows
2s 2p
3s 3p 3d
4s 4p 4d 4f
5s 5p 5d 5f
6s 6p 6d 6f
7s 7p 7d 7f
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2
88 electrons

28. Aufbau Principle Fill from the bottom up (lowest energy levels) first following the arrows
2s 2p
3s 3p 3d
4s 4p 4d 4f
5s 5p 5d 5f
6s 6p 6d 6f
7s 7p 7d 7f
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6 7s2 5f14 6d10 7p6
108 electrons

29. S 16e- 2s2 3s2 3p4 1s2 2p6 Practice Again Core Electrons
Valence Electrons 6

30. Practice one more time
Cl 17 e-
1s2
2s2 2p6
3s2 at this point we need 5 more
3p5
So Cl is 1s22s22p63s23p5
with 7 valence electrons

31. Shorthand Writing e- Configs
Noble Gas Notation
Noble Gas Notation: 
C = [He]2s22p2
Br = ?
[Ar]4s23d104p5

32. Group Practice
Using the Build-an-Atom Game determine the electron configurations for the 36 elements in Lab C-1, Section 4 Table

33. E. Exceptions to the rules
1. Orbitals fill in order
2. Lowest energy to highest energy
3. Adding electrons can change the energy of an orbital
4. Half filled orbitals have a lower energy

34. 5. Half filled orbitals make an atom more stable
6. this stability can change the order in which electrons fill up an orbital
7. so if you have 4s2 and 3d9
it is better to have 4s1 and 3d10
8. Because a full or half is more stable than a 7, 8, 9
9. BUT only in certain elements

35. 10. Example
Titanium - 22 electrons =
1s22s22p63s23p64s23d2
Vanadium - 23 electrons =
1s22s22p63s23p64s23d3
Chromium - 24 electrons =
1s22s22p63s23p64s23d4
But this is wrong for chromium!!

36. We expect chromium to be
1s22s22p63s23p64s23d4
BUT it is really:
1s22s22p63s23p64s13d5
Why?
This gives two half orbitals
Which means more stable atom
And slightly lower energy

37. 11. Another Exception Copper
Copper has 29 electrons so we expect: 1s22s22p63s23p64s23d9
But the actual configuration is:
1s22s22p63s23p64s13d10
This gives one filled orbital and one half filled orbital.
Remember these exceptions:
d4, d9

38. 12. Writing e- Configs for Ions
For anions – add arrows to boxes and add to orbitals to written format
Example – O2-
1s2 2s2 2p6
For cations – remove arrows from boxes and subtract from orbitals in written format
Example – Li1+
1s2 2s1 1s 2s 2p

39. 1. What is the electron configuration for Strontium?
Group Practice
1. What is the electron configuration for Strontium?
Sr has 38 electrons
Electron configuration is
1s22s22p63s23p64s23d104p65s2
7s 7p 7d 7f
6s 6p 6d 6f
5s 5p 5d 5f
4s 4p 4d 4f
3s 3p 3d
2s 2p 1s
1s2
2s2
2p6
3s2
3p6
4s2
3d10
4p6
5s2

40. 2. What is the electron configuration of Cadmium? Cd has 48 electrons
2p6
3s2
3p6
4s2
3d10
4p6
5s2
At this point we have 38
Need 10 more 10 2 6 10 2
4d10 6 2 6 2
1s22s22p63s23p64s23d104p65s24d10 2

41. 3. What element has an electron configuration of:
1s22s22p63s23p64s23d104p65s24d105p3
Answer:
The element with 51 electrons
Antimony

42. 4. Determine electron configuration of Oxygen?
8 electrons
1s2
2s2
2p4
Answer: 1s22s22p4

43. 5. How many valence electrons does oxygen have?
if the electron configuration is
1s22s22p4
then is has 6 valence electrons

44. Electron Configuration Shorthand
You can write an element’s electron configuration beginning with the closest Nobel Gas
For example – Rather than write Strontium as 1s22s22p63s23p64s23d104p65s2 you could just write [Kr]5s2

45. Summary 1. Fill lower energy levels 1st (Aufbau Principle)
2. Remember 2 electrons per orbit (Pauli Exclusion Principle)
3. Electrons don’t pair up until they must - Fill “up” arrows 1st (Hund’s Rule)
4. Energy Levels = Periods = Rows
1st energy level has an s orbital 2nd has s and p = 8
3rd has s, p & d = th has s, p, d & f = 32
5. The order of electrons
1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d
6. Exceptions – chromium and copper (shift an electron between 4s and 3d orbitals for stability)
7. Can use shorthand by starting with closest Nobel Gas and building from there – ex. Strontium [Kr]5s2
8. Count superscripts to determine the element
Valence electrons determined by counting electrons in highest energy level
Can also show ions – add or subtract appropriate number of electrons

46. Individual Practice
Atom Building Game – Lab C-1 “How is the Periodic Table Organized?”
Guided Practice from Chemistry book
Worksheets on Aufbau Principle, Pauli Exclusion Principle, Hund’s Rule, Orbital Diagrams, Writing Electron Configurations
Practice Set #1
Practice Set #2
Practice Set #3