1. Quantum Mechanical Model: Electron Configurations
Chemistry 11

2. Electron Configurations
It is the nature of things to seek the lowest possible energy.
High energy systems are unstable and tend to lose energy in order to become more stable.
In terms of the atom, electrons and the nucleus interact to form the most stable arrangement possible.

3. Electron Configuration:
The ways in which electrons are arranged around the nuclei of atoms.

4. Rules to Remember Three rules govern the filling of atomic orbitals by
electrons within the principle energy levels.
Aufbau Principle
Pauli Exclusion Principle
Hund’s Rule

5. Aufbau Principle Electrons enter orbitals of lowest energy first
The various orbitals within a sublevel of principle energy level are always equal.
Within a principle energy level, the “s” orbital is always the lowest energy level.
The range of energy levels within a principle energy level can overlap the energy levels of an adjacent principle energy level.

6. Using the Aufbau Principle
Note:
4s orbital is lower in energy than 3d.
4f orbital is lower in energy than 5d.
As you can see from the diagram
the filling order is as follows:
1s 2s 2p 3s 3p 4s 3d 4p 5s
4d 5p 6s

7. Energy Level Template

8. Pauli Exclusion Principle
An atomic orbital contains a maximum of 2 electrons
In order to occupy the same orbital, these electrons must have opposite spins (clockwise and counter clockwise)
Vertical arrows are used to show directions of spins
Paired electrons

9. An Easy Way to Remember 7s 7p 7d 7f 6s 6p 6d 6f 5s 5p 5d 5f
1s2 holds 2 electrons

10. Fill the Orbitals From the Bottom-up
2s 2p
3s 3p 3d
4s 4p 4d 4f
5s 5p 5d 5f
6s 6p 6d 6f
7s 7p 7d 7f
1s2 2s2 holds 4 electrons

11. Fill the Orbitals From the Bottom-up
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 holds 12 electrons

12. Fill the Orbitals From the Bottom-up
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 holds 20 electrons

13. Fill the Orbitals From the Bottom-up
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
holds 38 electrons

14. Fill the Orbitals From the Bottom-up
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
holds 56 electrons…and so on!

15. Hund’s Rule
When electrons occupy orbitals of equal energy, one electron enters each orbital until all orbitals contain one electron with parallel spins.
Second electrons then add themselves to the orbital to “pair” their spins Px Py Pz

16. Using Hund’s Rule
When electrons occupy orbitals of equal energy they don’t pair up until they have to.
Let’s determine the electron configuration for Phosphorus…energy level diagrams
Need to account for 15 electrons

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

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

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

20. Increasing energy 7p 6d 5f 7s 6p 5d 6s 4f 5p 4d 5s 4p 3d 4s 3p 3s
The next electrons go into the 3s orbital
Only 3 more

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

22. Remember the following:
Lowest energy to higher energy.
Adding electrons can change the energy of the orbital.
Half filled orbitals have a lower energy.
Makes them more stable.
Changes the filling order