1. Electron Arrangement in Atoms
Section 5.2

2. Electron Configuration
The arrangement of electrons into specific energy levels and orbitals around the nucleus.

3. What role does energy and stability play in the way electrons are arranged in an atom?
In an atom, the electrons and the nucleus interact to make the most stable arrangement possible.
How? Electrons occupy the lowest possible energy orbitals first.
Change always proceeds toward the lowest possible energy state.
High energy systems are unstable (i.e. unfilled or incomplete energy levels or orbitals)

4. Rules for Electron Configuration

5. Aufbau Principle
Rule #1 - Electrons enter orbitals of lowest energy first.
Lowest energy state is called the ground state. Highest energy state is called the excited state.
Energy levels can overlap adjacent energy levels.
Aufbau means building up.

6. Electron Configuration
Aufbau Principle
Orbital Diagram 1s
H =
1s1
Electron Configuration

7. Aufbau Principle
energy
Electrons fill low energy orbitals (closer to the nucleus) before they fill higher energy ones

8. Writing Electron Configurations
Aufbau Principal: Fill subshells in order by counting across periods, from hydrogen up to the element of interest

9. Pauli Exclusion Principle
Rule #2 - An atomic orbital may hold a maximum of two electrons.
A vertical arrow is used to indicate an electron.
In order for the two electrons (with negative charges) to occupy the same orbital they must have opposite spins, clockwise (up arrow) and counterclockwise (down arrow).

10. Pauli Exclusion Principle
For example: 1s 2s
Be = 1s 2s 2 2
The first electron in an orbital is represented with an “up” arrow or clockwise arrow.

11. Hund’s Rule
Rule #3 - When electrons fill orbitals of equal energy, one electron will enter each orbital until all the orbitals contain one electron (all “up” or clockwise spins).
Then, electrons with opposite spin fill each orbital (all “down” or counterclockwise).
In other words… for p, d and f orbitals: First fill all orbitals “up”, then fill “down”

12. Hund’s Rule
C = 1s 2s 2p 2 2 2 1s 2s 2p
Unpaired electrons

13. Hund’s Rule
F = 1s 2s 2p 2 2 5 1s 2s
Unpaired electron 2p

14. C = 1s2 2s2 2p2 F = 1s2 2s2 2p5 Valence electrons 4 7
An atom’s valence electrons are the electrons in the highest energy level of an atom. 4
valence electrons
C = 1s2 2s2 2p2
F = 1s2 2s2 2p5 7
valence electrons

15. Ti = 1s2 2s2 2p6 3s2 3p6 4s2 3d2 Valence electrons 2
Why aren’t the 3d 2 electrons considered valence electrons?
An atom’s valence electrons are the electrons in the highest energy level only of an atom.

16. That’s all for today!