NOTES: 5.2 –
Electron Configurations
Electron Configuration!

VIDEO
Electron Configurations
● The quantum mechanical model of the atom
predicts energy levels for electrons; it is
concerned with probability, or likelihood, of
finding electrons in a certain position.
Electron Configurations
● Electron configurations are the ways in which
electrons are arranged in various orbitals
around the nuclei of atoms
Electron Configurations
● Regions where electrons are likely to be
found are called orbitals. EACH ORBITAL
CAN HOLD UP TO 2 ELECTRONS!
Principal Quantum Number (n)
● Describes the energy level that the electron
occupies
n=1
● n = 1, 2, 3, 4…
n=2
n=3
n=4
● The larger the value of n, the farther away
from the nucleus and the higher the energy of
the electron.
Sublevels (l)
● The number of sublevels in each energy level
is equal to the quantum number, n, for that
energy level.
● Sublevels are labeled with a number that is
the principal quantum #, and a letter:
s, p, d, f
(ex: 2p is the p sublevel in the 2nd energy level)
RECALL:
Sublevel
# Orbitals
Max # elec.
s
1
2
p
3
6
d
5
10
f
7
14
Summary of Atomic Orbitals
Principal
Quantum
Number
# of
Energy
Sublevels
Type of
Sublevel
# of
Orbitals
Total # of
Electrons
1
1
s
1
2
2
2
s, p
4
8
3
3
s, p, d
9
18
4
4
s, p, d, f
16
32
Electron Configurations:
● The way in which electrons are arranged in
atom follow three rules:
Rule #1: Aufbau Principle
● electrons enter orbitals of lowest energy
first
● the orbitals within the same sublevel (e.g. the
3 orbitals in the 3p sublevel) are always of
equal energy
● the s sublevel is always the lowest-energy
sublevel
● in some cases, the energy levels within a
principal energy level may overlap with energy
levels in a different principal energy level
Electron Configurations
Electron Configurations
Start Filling
Here!
Rule #2: Pauli Exclusion
Principle
● an atomic orbital can describe (contain) at
most two electrons
● to occupy the same orbital, 2 electrons must
have opposite “spins” (1 is clockwise, 1 is
counterclockwise)
● opposite spins are shown as ↑↓ or with
oppositely charged “spin quantum numbers”
Spin quantum number (m):
● Labels the orientation of the electron
● Electrons in an orbital spin in opposite
directions; these directions are designated
as +½ and -½
Rule #3: Hund’s Rule
● Orbitals of equal energy are each occupied
by one electron before any pairing occurs.
 Repulsion between electrons in a single
orbital is minimized
● All electrons in singly occupied orbitals must
have the same spin.
● When 2 electrons occupy an orbital they have
opposite spins.
Orbital Diagrams:
● Each orbital is represented by a box.
● Each electron is represented by an arrow.
Ne
Orbital Diagrams
● hydrogen
1s
● helium
1s
● carbon
1s
2s
2p
Write the electron configuration
for the following:
● Fluorine
● Phosphorus:
● Potassium:
Electron Configurations
Write the electron configuration
for the following:
● Fluorine: 1s22s22p5
● Phosphorus:
● Potassium:
Write the electron configuration
for the following:
● Fluorine: 1s22s22p5
● Phosphorus: 1s22s22p63s23p3
● Potassium:
Write the electron configuration
for the following:
● Fluorine: 1s22s22p5
● Phosphorus: 1s22s22p63s23p3
● Potassium: 1s22s22p63s23p64s1
Electron Configurations
Complete electron
configurations:
● helium
● boron
● neon
● aluminum
● uranium
Electron Configurations
Complete electron
configurations:
● helium
1s2
● boron
1s22s22p1
● neon
1s22s22p6
● aluminum 1s22s22p63s23p1
● uranium
1s22s22p63s23p64s23d104p65s24d10
5p66s24f145d106p67s25f4
Abbreviated electron diagrams:
● helium
● boron
● aluminum
● cobalt
● uranium
Electron Configurations
Abbreviated electron diagrams:
● helium
1s2
● boron
[He]2s22p1
● aluminum [Ne]3s23p1
● cobalt
[Ar]4s23d7
● uranium
[Rn]7s25f4
Exceptional Configurations:
● Consider: Cr and Cu
● By following the Aufbau diagram for filling
energy levels, you would assign the following
electron configurations:
Cr: 1s22s22p63s23p63d44s2
Cu: 1s22s22p63s23p63d94s2
However, the correct configurations are…
Exceptional Configurations:
● Consider: Cr and Cu
● Correct configurations:
Cr: 1s22s22p63s23p63d54s1
Cu: 1s22s22p63s23p63d104s1
**filled energy sublevels (3d10) are more stable
than partially filled sublevels (4s1)
**half-filled levels can be more stable than
other configurations (as in Cr)