1. Electron Configuration

2. Objectives
Describe the relationship between orbitals and energy levels for the electrons of an atom
Describe how to write the electron configuration for an atom
Explain quantum numbers and how they affect electron configuration of atoms
Explain why the actual electron configurations for some elements differ from those predicted by the Aufbau principle

3. Important Vocabulary Atomic orbital Quantum numbers
Principal quantum number
angular momentum quantum number
Magnetic quantum number
Electron configuration
Pauli exclusion principle
Hund’s rule
Aufbau principle

4. Atomic Orbitals
An atomic orbital is best though of as a region of space in which there is a high probability of finding an electron
Electrons are found in orbitals within energy levels
Within each level, electrons occupy orbitals that have the lowest energy
4 different kinds of orbitals
s, p, d and f

5. s Orbitals
Are the simplest
Are spherical in shape
Have the lowest energy
Hold only 2 electrons

6. p Orbitals
Are dumbbell shaped
They can be oriented three different ways in space
Has more energy than a s orbital
Each p orbital can hold 2 electrons for a total of 6 electrons

7. d & f Orbitals Are much more complex
There are 5 possible d orbitals which are clover leaf shapes
There are 7 possible f orbitals
f orbitals have the greatest energy
Each orbital holds a maximum of 2 electrons

8. Orbitals

9. Relationship between Levels & Sublevels
1st energy level has 1 sublevel = s
So it contains 1 orbital and holds 2 electrons
2nd energy level has 1 s orbital and 3 p orbitals
So it can hold 8 electrons
3rd energy level has s, p, & d orbitals
So it can hold 18 electrons
4th energy level has s, p, d, & f orbitals
So it can hold 32 electrons

10. Periodic Table Orbitals

11. Quantum Numbers
Scientists have defined the region in which electrons can be found by using 4 quantum numbers
A quantum number is a number that specifies the properties of electrons
The principal quantum number, n, indicates the main energy level occupied by an electron
Values of n are positive integers
As n increases, the electron’s distance from the nucleus and its energy increases

12. orbital

13. Quantum Numbers Continued
Main energy levels can be subdivided
The sublevels are represented by the angular momentum quantum number, l
This number indicates the shape or type of orbital of a particular sublevel
Chemists use a letter code for this quantum number
l = 0 = an s orbital
l = 1 = an p orbital
l = 2 = an d orbital
l = 3 = an f orbital

14. Magnetic Quantum Numbers
Symbolized by m
Is a subset of the angular quantum number
It also indicates the numbers and orientations of orbitals around the nucleus
The value of m is in whole-number values but depends on the value of l
1 s orbital, 3 p orbitals, 5 d orbitals and 7 f orbitals

15. Spin Quantum Number Symbolized by + ½ and – ½ or ↑ and ↓
It indicates the orientation of an electron’s magnetic field relative to an outside magnetic field
A single orbital can hold a maximum of 2 electrons, which must have opposite spins

16. Principal Quantum Number
Angular Momentum Quantum Number
Magnetic Quantum Numbers
Spin Quantum Numbers

18. Electron Configurations
Electrons in atoms tend to assume arrangements with the lowest possible energies
An electron configuration is the written arrangement of electrons in an atom
It shows the lowest-energy arrangement of the electrons for an element
It is a shorthand notation
3 rules determine it

19. Pauli Exclusion Principle
Was established by the German chemist Wolfgang Pauli in 1925
It states that each orbital can hold a maximum of 2 electrons
In other words, no two electrons in the same atom can have exactly the same four quantum numbers
In addition, if one electron has a spin quantum number of + ½ than the other must be – ½

20. The Aufbau Principle
Also helps to write the electron configuration for an atom
It states that electrons fill orbitals that have the lowest energy first
Order for filling orbitals is as follows:
1s ‹ 2s ‹ 2p ‹ 3s ‹ 3p ‹ 4s ‹ 3d

21. Hund’s Rule
States that orbitals of the same n and l quantum numbers are each occupied by 1 electron before any pairing occurs

22. Electron Configuration Example
Let’s do magnesium (Mg)
It has 12 electrons

23. Electron Configuration
1s2 2s2 2p6 3s2 or
[Ne]3s2

24. Electron Configuration Practice
Let’s try a few together Na Ar Ca

25. Electron Configuration

26. Electron Configuration Practice
Now you try! F S

27. Exceptional Electron Configurations
The elements after Vanadium (atomic #23) do not follow the Aufbau principle
Filled energy sublevels are more stable than partially filled sublevels.
Exceptions to the aufbau principle are due to subtle electron-electron interactions in orbitals with very similar energies.
For Example: Cu and Cr

28. Exceptions to Aufbau’ Principle
INCORRECT
CORRECT