1. Electron Configuration

2. Electron Configuration
The arrangement of electrons in an atom is called its electron configuration.
Electrons in an atom tend to assume the arrangement that gives the atom the lowest possible energy.
This is the most stable arrangement.
Known as the ground-state electron configuration

3. Electron Configuration
Rules for how electrons are arranged in orbitals
Aufbau principle – each electron occupies the lowest energy orbital available
Pauli exclusion principle – maximum of 2 electrons in a single atomic orbital – must have opposite spin
Hund’s Rule – Single electrons with the same spin occupy each equal enegy orbital before pairing.

4. aufbau diagram

5. Increasing energy of sub-levels

6. aufbau principle

7. Pauli Exclusion Principle
Maximum of 2 electrons in each orbital
Must have opposite spin
The maximun number of electrons related to each principal energy level equals 2n2
Image taken from

8. Hund’s Rule
Recall that electrons have a negative charge and therefore repel each other. Electrons are distributed so that the electrons can maximize the distance from each other.
Hund’s rule – single electrons of like spin occupy different equal-energy orbitals before opposite spins occupy the same orbital.

9. Hund’s Rule

10. Orbital Diagrams
Electrons are represented by arrows in boxes. Each box is labeled
with the principal quantum number and sublevel.

12. Electron Configuration
Is there an easier way to write the electron configuration? Nobel Gas Configuration – write the symbol for the Noble gas that is closest to and prior to the element in brackets then continue with the rest of the electron configuration
YES

13. Warm-Up Write both configurations for the following element: Calcium
What element has an electron configuration of [Kr] 5s2 4d9
Which of the rules is being broken (aufbau, pauli or hund)? Why?
⇅ ⇅ ⇈ ⇅

14. Exit Pass Write both configurations for the following elements:
Aluminum 1s2 2s2 2p6 3s2 3p [Ne]3s2 3p1
Calcium 1s2 2s2 2p6 3s2 3p6 4s [Ar]4s2
Sulfur1s2 2s2 2p6 3s2 3p [Ne]3s2 3p4

15. Nobel Gas Configuration

16. Valence Electrons
Only certain electrons are responsible for the chemical properties of an element. These are the valence electrons.
Usually in the atom’s highest principal energy level
Cl has 17 e- but only 7 valence e [Ne]3s23p5
Francium has 1 valence e- but 87 e [Rn]7s1