Electron Configurations
THE QUANTUM MECHANICAL MODEL OF THE
ATOM
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
The Quantum Mechanical Model of the Atom
 The wave function predicts a three-dimensional
region around the nucleus called the atomic
orbital.
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.
Other Quantum Numbers (QN)
 Angular Momentum QN (l = 0 to n  1) - relates to
shape of the orbital.
 Magnetic QN (ml = l to l) - relates to orientation
of the orbital in space relative to other orbitals.
 Electron Spin QN (ms = +1/2, 1/2) - relates to the
spin states of the electrons.
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
Sublevels and Orbitals
Maximum Number of Electrons
In Each Sublevel
Sublevel
Number of Orbitals
Max. # e-
s
1
2
p
3
6
d
5
10
f
7
14
Shapes of Orbitals
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.
Rules Continued
 Pauli Exclusion Principle: In a given
atom, no two electrons can have the same set of
four quantum numbers (n, l, ml, ms).
Therefore, an orbital can hold only two
electrons, and they must have opposite spins.
 Hund’s Rule: Electrons occupy equalenergy orbitals so that a maximum number
of unpaired electrons results.
Filling Order of Orbitals
Example of Electron Configurations using Aufbau
Box Diagrams
1.
Hydrogen
2. Lithium
3. Carbon
More Examples
 Iron:
 Sulfur
“Short Version” Electron Configurations
 Write the Aufbau box diagram e- configuration for
phosphorus:
The “short version” would be:
Filling Order
 Start with 1s
 Follow arrows to next
sublevel.
 Can only go to next
sublevel when current
one is full.
 Go until you run out of
electrons.
Examples
1.
Sulfur
2. Strontium
3. Antimony
Noble Gas Abbreviated e- Configurations
 The symbol for a Noble Gas (Group 18 elements) can
be used to shorten the configuration.

Choose the noble gas ending the period (row)
above the element in question.
 Example:
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
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
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 valence electrons
 Elements with max. # of valence electrons
do not easily gain or lose electrons
Practice Problems
 Determine the number of valence electrons for the
following elements:
 Sodium
 Chlorine
 Neon
 Magnesium
 Aluminum
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: Phosphorus
Practice Problems
 Draw the electron dot diagram for the following
elements.
 Calcium
 Arsenic