1. Electron Configurations and the Aufbau Principle
Mr. Tsigaridis

2. The Periodic Table
When creating electron configurations we must first take a look at how the periodic table of the elements is separated based on orbitals of atoms
There are 4 major orbitals that we will be discussing, with a focus on the first three

3. The Periodic Table
The following is a periodic table that is based on the orbitals

4. Filling the Orbitals
When filling the orbitals with electrons, we must fill the orbitals with the lowest energy first
Not only do the orbitals with the lowest energy have to be filled first, but they also must become half filled with electrons with one type of spin first in order to follow the Pauli exclusion principle

5. Filling the Orbitals
There are two ways to show electron configurations
The first way is pictorially using boxes and arrows and the second way is the condensed electron configurations
Before continuing with the electron configurations, we must discuss the order in increasing energy of the orbitals in question

6. Electron Orbital Energies

7. “3d” and “4s” Orbitals
You may have noticed that the 3d orbitals are at a higher energy than the 4s obritals
To understand why this happens we have to go back to some of the fundamentals of quantum chemistry
Electrons are attracted to the nucleus and so to pull an electron farther away from the nucleus, you have to work against this attraction
That gives rise to the idea that electrons that are further out have higher energies

8. “3d” and “4s” Orbitals
For atoms that are heavier than copper this effect dominates and the 4s orbitals have a higher energy than the 3d electrons
Raising the energy however has another effect and that is the fact that you get more nuclear nodes
Those are areas where you have a 0 probability of finding electrons close to the nucleus

9. “3d” and “4s” Orbitals
This gives higher energy orbitals (like d orbitals) more energy than the lower energy orbitals like the s orbitals
As the orbitals fill, with more unpaired spins, it means a lower energy overall
This causes the 4s orbitals and the 3d orbitals to equalize in terms of the energy of the electrons in each orbital

10. Aufbau Principle
When building the electron configurations, they must follow the Aufbau Principle
It states that the electrons fill the available orbitals from lowest energy to highest energy. In the ground state all the electrons are in the lowest possible energy level.

11. Aufbau Principle
A simple diagram can help you identify the order of the orbitals as they are filled
You will notice that the d orbitals will fill with in an order of n-1 and the f orbitals will fill in an order of n-2

12. Hund’s Rule
Hund’s Rule states that the electrons must be placed into the orbitals in such a way that no pairs are put together unless absolutely necessary.
That is, single electrons must be placed into boxes first and then paired up if necessary.

13. Hund’s Rule