1. Electron Configurations
The quantum mechanical model of the atom

2. Quantum Mechanical Model
Developed by Erwin Schrodinger
aka “Electron Cloud” model
Doesn’t define an exact path of electron; estimates probability of finding electron in a certain location
Uses atomic orbitals = a 3-D region around nucleus that describes the electron’s probable location.
Each orbital can hold a maximum of 2 electrons

3. The Quantum Mechanical Model of the Atom
The wave function predicts a three-dimensional region around the nucleus called the atomic orbital.

4. Atomic Orbitals
Electrons cannot exist between energy levels (just like the rungs of a ladder).
Principal quantum number (n) indicates the relative size and energy of atomic orbitals.
n specifies the atom’s major energy levels, called the principal energy levels.

5. Electron Energy Level Energy levels are broken up into sublevels:
There are at least 4 possible types of sublevels—given labels: s, p, d, or f

6. Sublevels and Orbitals
Maximum Number of Electrons
In Each Sublevel
Sublevel Number of Orbitals Max. # e- s p d f

7. Electron Configurations
The electron configuration of an atom is the arrangement of the electrons around the nucleus of an atom.
RULES:
Aufbau Principle: Electrons are added one at a time to the lowest energy orbitals available until all the electrons of the atom have been accounted for.

8. Rules Continued
Pauli Exclusion Principle: An orbital can hold a maximum of two electrons. To occupy the same orbital, two electrons must spin in opposite directions.
Hund’s Rule: Electrons occupy equal-energy orbitals so that a maximum number of unpaired electrons results.

9. Filling Order of Orbitals

10. Example of Electron Configurations
Hydrogen
Lithium
3. Carbon

11. More Examples
Iron:
Sulfur

12. Valence Electrons
The electrons occupying the outermost energy levels of an atom
Located in the highest occupied s and p sublevel
Maximum Number = 8
Determined by the location of the element on the periodic table.
Determine the physical and chemical properties of the element

13. Finding # of Valence Electrons
Group #1 = 1 valence electrons
Group #2 = 2 valence electrons
For Groups #13-18
Subtract 10 from the group # = # valence electrons
Exception Helium only has 2

14. Noble Gas Stability Noble gases are usually unreactive
This is because they have max. # valence electrons
For two atoms to join together atoms must gain, lose or share electrons
Elements with max. # of valence electrons do not easily gain or lose electrons

15. Practice Problems Sodium Chlorine Neon Magnesium Aluminum
Determine the number of valence electrons for the following elements:
Sodium
Chlorine
Neon
Magnesium
Aluminum

16. Electron (Lewis) Dot Diagrams
Model used to display the valence electrons of an element.
Includes the symbol of the element and the valence electrons represented as dots.
Example: Oxygen

17. Practice Problems
Draw the electron dot diagram for the following elements.
Calcium
Arsenic