1. Electron Configuration
History of Modern Atomic Theory
Energy Levels and Sublevels
The Energy Levels

2. History of Modern Atomic Theory
Niels Bohr noticed that electrons could only gain certain amounts of energy
This implied that there were energy levels in the electron cloud
Electrons could jump from one energy level to another, but could not exist between levels. The levels were still pictured as circular orbits.
This model of the atom was called the Bohr model and agree with experimental data for hydrogen atoms. It failed to explain other atoms though.

3. History of Modern Atomic Theory
In the modern description of the electrons in atoms, the quantum mechanical model, the electron does not travel in a circular orbit, instead it exists somewhere within a region of space called an orbital
We don’t know it exact position only the probability of finding it
We should no longer picture the electron as tiny particle whizzing around the nucleus
The propeller blade has the same probability of being anywhere in the blurry region, but you cannot tell its location at any instant. The electron cloud of an atom can be compared to a spinning airplane propeller.

4. Energy Levels and Sublevels
Remember that e- are what hold atoms together to form molecules. Understanding more about how they are arranged in the atom will help us better understand chemical reactions
The electron cloud is divided into energy levels
Theoretically there are an infinite number of levels, but even the biggest atoms only use seven
The energy levels closest to the nucleus are lower in energy
Electrons always try to achieve the lowest energy state possible; this is called the Ground State.
This tendency is known as the Aufbau Principle (which means to “Build up”)
When we start to place electrons into energy levels we begin with the lowest levels and build our way up as needed.
Each energy level contains one or more Sublevels
The first energy level has only one sublevel (called the s sublevel)
The second energy level has two sublevels (called s and p sublevels)
The third energy level has three sublevels (called s, p, and d sublevels)
The fourth energy level has four sublevels (called s, p, d, and f sublevels)

5. Energy Levels and Sublevels
Each sublevel is divided into Orbitals which are 3 dimensional regions of space where a pair of e- is most likely to reside
The s sublevel (the first sublevel in every energy level) has one orbital
The p sublevel (the second sublevel in energy levels 2-7) has three orbitals
The d sublevel (the third sublevel in energy levels 3-7) has five orbitals
The f sublevel (the forth sublevel in energy levels 4-7) has seven orbitals
Each orbital can house two electrons
The Apartment Building Analogy: You can think of the electron cloud as an apartment building
The energy levels are the same as the floors
The sublevels are the same as the apartments
The orbitals are the same as a bunk bed for two people
Each person represents an orbital

6. 4 3 2 1 f s p s p s p s 4 sublevels 1+3+5+7=16 orbitalsd 4 s p
4 sublevels
=16 orbitals
= 32e- d 3
3 sublevels
1+3+5=9 orbitals
= 18e- s p
2 sublevels
1+3=4 orbitals
2 + 6 = 8e- 2
1 sublevel
1 orbital
2e- s 1

7. Comprehension Check
Knowing that each orbital can hold 2 e-, how many electrons can the following sublevels hold?
s sublevel
1 orbital x 2 e- = 2 e-
p sublevel
3 orbitals x 2 e- = 6 e-
d sublevel
5 orbitals x 2 e- = 10 e-
f sublevel
7 orbitals x 2 e- = 14 e-

8. Comprehension Check How many electrons can each energy level hold?
1st Energy Level
2nd Energy Level
3rd Energy Level
4th Energy Level

9. Answers to Comprehension Check
How many electrons can each energy level hold?
1st Energy Level
2 (2)
2nd Energy Level
8 (2+6)
3rd Energy Level
18 (2+6+10)
4th Energy Level
32 ( )

10. Energy Levels and sublevels
Fill out the following table using what you know about electrons and energy levels
Energy Level
Sublevels
Number of Orbitals
Total Number of Electrons 1 2 3 4 s 1 2
s p
1 + 3 = 4
2 + 6 = 8
s p d
= 9
= 18
s p d f
= 16
= 32

11. The First Energy Level
The first energy level is the lowest energy level in all atoms
This level only has an s sublevel and can only hold two electrons because it has only 1 orbital

12. The Second Energy Level
Second lowest energy level
Has as s sublevel and a p sublevel
The 2s sublevel has one orbital and can therefore hold 2 electrons
The 2p sublevel has 3 orbitals and can therefore hold up to 6 electrons

13. The Third Energy Level
The third energy level has three sublevels s, p, and d
The 3s and 3p sublevels have the same number of orbitals as 2s and 2p
The 3d orbital has 5 orbitals and
Can therefore hold up to 10 electrons

14. The Fourth & Higher Energy Levels
The 4th energy level has 4 sublevels: s, p, d, and f
The 4s, 4p, and 4d are all the same shape, except bigger than lower energy levels
The 4f sublevel has 5 orbitals

15. Electron Arrangement in Atoms
There are three rules to help us understand where electrons are in the electron cloud
Aufbau principle – electrons will occupy the lowest energy level possible
Pauli Exclusion Principle – only two electrons may occupy an orbital and they must have opposite spin (clockwise and counterclockwise)
Hund’s Rule – when filling a sublevel such as 3p, e- will go to an empty orbital prior to pairing up (spread out before pairing up)

16. Writing Electron Configurations
Electron Configurations tell you the ground state location of each electron in an atom
Example 1: What is the Electron Configuration of nitrogen
First of all, how many electron does a nitrogen atom have? 7
Example 2: What is the Electron Configuration of silicon
First of all, how many electron does a silicon atom have? 14
# of e-
Energy Level
1s2
2s2
2p3
Sublevel
1s2
2s2
2p6
3s2
3p2

17. The Energy of Orbitals
Higher sublevels do not necessarily have a higher energy
The order from lowest to highest energy is as follows
Lowest Energy
Highest Energy 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s

18. A Trick 1s 2s 2p 3s 3p 3d 4s 4p 4d 4f 5s 5p 5d 5f 6s 6p 6d 6f 7s 7p 7d
Fill the sublevels in the order that the arrows pass through, beginning with the top arrow. 2s 2p 3s 3p 3d 4s 4p 4d 4f 5s 5p 5d 5f 6s 6p 6d 6f 7s 7p 7d 7f

19. Comprehension Check
Write the electron configurations for the following elements
Be, Mg, S, Ar, Sc, Fe, Ge, Kr, Tc

20. Comprehension Check
Write the electron configurations for the following elements Be
1s2 2s2 Mg
1s2 2s2 2p6 3s2 S
1s2 2s2 2p6 3s2 3p4 Ar
1s2 2s2 2p6 3s2 3p6

21. Comprehension Check
Write the electron configuration for the following elements. Sc
1s2 2s2 2p6 3s2 3p6 4s2 3d1 Fe
1s2 2s2 2p6 3s2 3p6 4s2 3d6 Ge
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p2 Kr
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 Tc
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 5d10

22. Congratulations, you can now write long electron configurations!