Electron
Configurations
The Bohr Model
• Electrons are in fixed orbits.
• Orbits are associated with very specific energies, and are
called energy levels.
The Quantum Mechanical Model
• Like Bohr Model, QMM restricts energies of electrons to
certain values.
• Unlike Bohr Model, QMM does not define an exact path
electron takes around the nucleus.
• According the QMM, the exact location of an electron cannot
be exactly determined at a given time.
• Estimates probability of finding an electron in a certain
position.
• Regions in which electrons are going to be found are called
Orbitals
Electron Configurations—The Basics
• Chemists describe the placement of electrons based on
principal energy levels n=1,2,3,4, etc.
• Within these energy levels, there are sublevels labeled s, p, d,
and f.
• Within these sublevels are orbitals
• Every orbital can hold 2 electrons.
Principal energy levels (shells)
• Principal energy levels are assigned values in order of
increasing energy
n=1, 2, 3, 4, …
• What do the numbers tells us?
• Distance from the nucleus increases with increasing n.
• Energy increases with an increasing n.
Sublevels (subshells)
• Within each principal energy level, electrons occupy energy
sublevels
• Sublevels denoted by the letters s, p, d, f
• In terms of energy, s < p < d < f
• What do the letters tell us?
• Shape of orbitals
• s = sphere, p= dumbbell, d = clover leaf
Orbitals
• Each sublevel contains a certain number of orbitals
• Orbital = region in which electrons are likely to be found
• What else do the letters tell us?
•
•
•
•
s = 1 orbital
p = 3 orbitals
d = 5 orbitals
f = 7 orbitals
Electrons
• Each orbital can only contain 2 electrons
• In order for electrons to occupy the same orbital, they must
have opposite spin
Orbitals
• S sublevel
• 1 orbital
Orbitals
• S sublevel
• 1 orbital
• 2 e-
Orbitals
• P sublevel
• 3 orbitals
Orbitals
• P sublevel
• 3 orbitals
• 6 total e• 2 in each orbital
Orbitals
• d sublevel
• 5 orbitals
Orbitals
• d sublevel
• 5 orbitals
• 10 total e• 2 in each orbital
Summary
Energy Levels
Sublevel
# of Orbitals
# of Electrons
1
1s
1
2
2
2s
2p
1
3
2
6
3
3s
3p
3d
1
3
5
2
6
10
4
4s
4p
4d
4f
1
3
5
7
2
6
10
14
Summary
Energy Levels
Sublevel
# of Orbitals
1
1s
1
2
2s
2p
1
3
# of Electrons
2
2
2
6
8
3
3s
3p
3d
1
3
5
2
6
10
18
4
4s
4p
4d
4f
1
3
5
7
2
6
10
14
32
Summary
• What is the relationship between energy level and number of
sublevels?
Summary
• What is the relationship between energy level and number of
sublevels?
• Energy Level # = # of sublevels
Let’s put it together!
Let’s put it together!
• It was mentioned earlier that increasing n = increasing energy
• Not always the case.
• Notice 3d is higher energy than 4s.
• This is not the only time when that happens!
Writing electron configurations in
orbital notation
• RULES
• Aufbau Principle: Electrons fill the lowest possible energy levels
first.
• Pauli Exclusion Principle: Two electrons in the same orbital have
opposite spins.
• Hund’s Rule: Electrons fill in separate orbitals of the same
subshell (sublevel) with parallel spins before pairing.
Writing electron configurations
in orbital notation
• Examples:
• Carbon
• Argon:
• Vanadium:
Writing electron configurations
in superscript notation:
• See Periodic Table
• Use Aufbau Principle: Fill lowest energy levels first
Writing electron configurations
in superscript notation:
• Ex:
• Carbon
• Argon
• Vanadium
Writing electron configurations
in superscript notation:
• Ex:
• Carbon: 6e• 1s22s22p2
• Argon: 18e• 1s22s22p6 3s23p6
• Vanadium: 23e• 1s22s22p6 3s23p64s23d3
Kernel configurations:
• To shorten, place previous noble gas (8A) in brackets and
continue writing configuration.
• Ex:
• Vanadium: 23 e• [Ar] 4s23d3
• Bromine:
• Al:
Kernel configurations:
• To shorten, place previous noble gas (8A) in brackets and
continue writing configuration.
• Ex:
• Vanadium: 23 e• [Ar] 4s23d3
• Bromine: 35 e• [Ar] 4s23d104p5
• Al: 13 e• [Ne] 3s23p1
Valence shell/valence electrons
• The outermost principal energy level of an atom that includes
at least one electron is called the valence shell.
• The electrons in the valence shell are called valence electrons.
• Valence electrons determine properties and reactivity or
elements. Very important!
Valence shell/valence electrons
• How many valence electrons in examples above?
• Carbon:
• Argon:
• Vanadium:
Valence shell/valence electrons
• How many valence electrons in examples above?
• Carbon: 1s22s22p2 = 4 v.e.
• Argon: 1s22s22p63s23p6 = 8 v.e.
• Vanadium: 1s22s22p63s23p64s23d3 = 2 v.e.
Electron Configurations:
Special Cases
• Writing electron configurations for ions
• For cations (+ charge) subtract electrons
• For anions (- charge) add electrons
• Ex:
• Na+ 11 – 1 = 10 e- 1s22s22p6
• N3- 7 + 3 = 10 e-
1s22s22p6
*Same as Neon. These are isoelectronic = Same e- configuration*
Exceptional configurations:
• For d orbitals only
• The atom will be at lowest energy (most stable) when the d
sublevel is half full or full.
• Chromium and Copper Cr, Cu
Exceptional configurations:
• Chromium and Copper Cr, Cu
• Chromium expected configuration:
• 1s22s22p63s23p64s23d4
• Actual Cr Configuration:
• 1s22s22p63s23p64s13d5 ← HALF FULL
• Copper expected configuration:
• 1s22s22p63s23p64s23d9
• Actual Cu Configuration:
• 1s22s22p63s23p64s13d10 ← FULL