Ch. 4 - Electrons in Atoms
IV. Electron
Configuration
(p. 105 - 116,
128 - 139)
C. Johannesson
Pauli Exclusion Principle
No two electrons in an atom can have
the same 4 quantum numbers.
Each e- has a unique “address.”
1. Principal #

2. Azim(Ang. Mom.) # 
3. Magnetic # 
4. Spin #

energy level
sublevel (s,p,d,f)
orbital
electron
A. General Rules
Pauli Exclusion Principle
Each orbital can hold TWO electrons
with opposite spins.
Spin is the only difference between
these two electrons’ quantum states.
B. Notation
Orbital Diagram: shows all the
electrons in an atom as arrows.
Each box is an orbital.
Joined orbitals are sublevels.
O
8e-
1s
2s
2p
 UP arrows have a ms of +1/2
 DOWN arrows have a ms of -1/2
A. General Rules
Aufbau Principle
Electrons fill the
lowest energy
orbitals first.
“Lazy Tenant
Rule”
C. Johannesson
A. General Rules
Hund’s Rule
Within a sublevel, place one e- per
orbital (spin-parallel) before pairing them.
“Empty Bus Seat Rule”
WRONG
RIGHT
C. Electron Configuration
Gives the energy state for ALL the
electrons in an atom.
1
Hydrogen’s
2
configuration is 34
5
simplest and
6
shortest
7
Just one
because H only
contains one
electron in
electron.
the 1s
1
1s
sublevel.
C. Electron Configuration
Gives the energy state for ALL the
electrons in an atom.
12
Lithium
3
contains 3
electrons:
2 in the 1s
(#1 & #2)
1 in the 2s
(#3)
2
1
1s 2s
C. Electron Configuration
Gives the energy state for all the
electrons in an atom.
12
Sulfur’s
5 6 7 8 9 10
34
11 12
13 14 15 16
configuration
needs to
include ALL 16
of its electrons.
Sulfur’s last
2
2
6
2
4 4 electrons
Powers should add up to atomic # are in 3p.
All e-configs
start at 1s.
(2+2+6+2+4=16)
1s 2s 2p 3s 3p
B. Notation
Orbital Diagram
O
8e-
1s
2s
Electron Configuration
2
2
4
1s 2s 2p
C. Johannesson
2p
B. Notation
Longhand Configuration
S 16e- 1s2 2s2 2p6 3s2 3p4
Core Electrons
Valence Electrons
(up to previous Noble)
Shorthand (Noble Gas) Configuration
S
16e
2
4
[Ne-10] 3s 3p
C. Johannesson
C. Periodic Patterns
1
2
Aufbau Principle: electrons always fill in
order from lowest energy to highest
energy.
Atomic numbers on the periodic table
count electrons in the order they fill, so the
order is always the same.
Don’t forget where the f-block goes!
3
4
5
6
7
C. Johannesson
C. Periodic Patterns
Aufbau Series: the order in which
electrons fill energy levels & sublevels.
2
6
10
6
2
2
2
2
6
5s
1s 2s 2p 3s 3p 4s 3d 4p
4d10...
…5p6 6s2 4f14 5d10 6p6 7s2 5f14 6d10 7p6
C. Periodic Patterns
 Orbital diagrams
show the same
information as
electron
configuration in a
different way.
The Aufbau Series
still applies!
C. Electron Configuration
Example – Nickel (Ni #28)
12
34
11 12
5 6 7 8 910
13 14 15 16 17 18
19 20 21 22 23 24 25 26 27 28
Ni
2
2
6
2
6
1s 2s 2p 3s 3p
2
8
4s 3d
2+2+6+2+6+2+8 = 28
C. Periodic Patterns
Shorthand Configuration
Core e-: Start with the noble gas from
the previous period.
Valence e-: On the next row, fill in the
# of e- in each sublevel.
1
2
3
4
5
6
7
C. Johannesson
C. Periodic Patterns
Example - Germanium
1
2
3
4
5
6
7
2
[Ar-18] 4s
10
3d
C. Johannesson
2
4p
C. Periodic Patterns
Period #
energy level (remember d & f don’t line
up with s & p!)
A/B Group #
total # of valence eColumn within sublevel block
# of e- in sublevel
C. Johannesson
C. Periodic Patterns
s
p
1
2
3
4
5
6
7
f (n-2)
d (n-1)
6
7
© 1998 by Harcourt Brace & Company
C. Johannesson
C. Periodic Patterns
Elements in the same column have
similar valence configurations.
1
2
3
4
5
6
7
Hydrogen:
1st Period
1st
column of
s-block
1
1s
s-block
C. Johannesson
C. Periodic Patterns
Rubidium (Rb #37)
1
2
3
4
5
6
7
1
[Kr-36]5s
5th Period
1st
column of
s-block
s-block
D. Stability
Full energy level
Full sublevel (s, p, d, f)
Half-full sublevel
1
2
3
4
5
6
7
C. Johannesson
D. Stability
Electron Configuration Exceptions
Copper
EXPECT:
[Ar] 4s2 3d9
ACTUALLY:
[Ar] 4s1 3d10
Copper gains stability with a full
d-sublevel.
C. Johannesson
D. Stability
Electron Configuration Exceptions
Chromium
EXPECT:
[Ar] 4s2 3d4
ACTUALLY:
[Ar] 4s1 3d5
Chromium gains stability with a half-full
d-sublevel.
C. Johannesson
D. Stability
Ion Formation
Atoms gain or lose electrons to become
more stable.
Isoelectronic with the Noble Gases.
1
2
3
4
5
6
7
C. Johannesson
D. Stability
Ion Electron Configuration
Write the e- config for the closest Noble
Gas
EX: Oxygen ion  O2-  Ne
2O
10e
[He]
C. Johannesson
2
2s
6
2p