Electron Configurations
…and now, the rest of the story
Recall
An electron configuration (EC) shows the
location of all electrons in an atom or ion.
In an atom, number of electrons = number
of protons = atomic number
Electrons are found around the nucleus of
an atom in specific energy levels.
1st energy level
--has one sublevel, the 1s (for spherical) sublevel.
Z
--Any s sublevel has one orbital,
--Any orbital can hold two electrons
Y
• So the electron configurations of hydrogen and
helium are written
• 1H 1s1
• 2He 1s2
X
• 1H 1s1
• 2He 1s2
• The 1 refers to the energy level
• 1H 1s1
• 2He 1s2
• The s refers to the sublevel
• 1H 1s1
• 2He 1s2
• The superscripts are the number of
electrons in this sublevel.
2nd energy level
--has two sublevels, the 2s and 2p sublevels.
The 2s sublevel
-is spherical,
-has one orbital, and
-can hold two electrons (just like the 1s),
but it is larger than the 1s sublevel.
• The 2s sublevel…
-- is in a higher energy level,
--the electrons in it have more energy
than 1s electrons, and
--average a greater distance from the
nucleus.
The 2p sublevel has three p orbitals
--they have two lobes each,
--lie in three perpendicular axes (x, y, and z),
--can hold two electrons each,
…so the 2p sublevel (and any p sublevel)
can hold six electrons
Z
Y
Z
X
Y
Z
X
Y
X
2nd energy level
• -has 2s (bigger, still spherical) and 2p (has
3 bi-lobed orbitals in x, y, and z directions)
• -holds up to (2 in s + 3 x 2 in p)=8
electrons total
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•
•
•
•
22s1
Li
1s
3
22s2
Be
1s
4
22s22p1
B
1s
5
22s22p2
C
1s
6
22s22p3 …
N
1s
7
The 3rd energy level
 has three sublevels, 3s, 3p, and 3d. The s
and p sublevels are similar in structure
(but bigger) than the s and p sublevels
seen before.
The 3d sublevel
• Any d sublevel has five orbitals of varying
shapes.
• These orbitals can hold two electrons each
for a total of ten electrons.
• The 3d sublevel is the highest energy
sublevel of energy level 3,
• Energy level 4 begins to fill (the 4s
sublevel fills) before the 3d sublevel
D sublevels
Z
Z
Y
Y
Z
Y
X
X
Z
X
Y
Z
X
Y
X
The 4th energy level
 has four sublevels (notice the trend?)
They are 4s, 4p, 4d, and 4f.
 The s, p, and d sublevels are structured as
before.
 The f sublevel has seven orbitals, and can
hold up to fourteen electrons.
 The 5s sublevel is filled before the 4d, and the
5p and
6s sublevels
precede the 4f.
 Let’s look at a picture instead
Out of Order!
•
•
•
•
•
•
•
1)
2)
3)
4)
5)
6)
7)
1s
2s
3s
4s
5s
6s
7s
2p
3p
4p
5p
6p
7p
3d
4d
5d
6d
4f
5f
…
(…g?)
Please review
S
1 orbital
2 electrons
P
3 orbitals
6 electrons
D
5 orbitals
10 electrons
F
7 orbitals
14 electrons
The Aufbau diagram
Boxes are orbitals, each can hold two electrons
7p
7s
6p
6s
5p
6d
5d
4d
5s
4p
4s
3s
3p
2p
2s
1s
3d
5f
4f
The Aufbau order of sublevels
• All Electron configurations are some
subset of the order shown below. Only
the last sublevel might be incomplete
• 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10
5p6 6s2 4f14 5d10 6p6 7s2 5f14 6d10 7p6…
Three rules for filling orbitals
– Aufbau (building up) principle—lower energy
sublevels are filled first
– Pauli exclusion principle—electrons sharing an
orbital must have opposite spins
– Hund’s Rule—when a sublevel has several
orbitals, electrons will distribute to separate
orbitals with parallel spins, before sharing
orbitals with opposite spins
Watch out for two things
• 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2
4f14 5d10 6p6 7s2 5f14 6d10 7p6…gets really old
eventually. Look for the last octet and describe it
as a noble gas core electron configuration
• Ex: 87Fr 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10
5p6 6s2 4f14 5d10 6p6 7s1
can be written
• 87Fr [Rn] 7s1 (because radon is a noble gas,
and accounts for the first 86 electrons)
Practice
• Write the full EC for Titanium (element 22)
and write the EC with a noble gas core
Practice
• Write the full EC for Titanium (element 22)
and write the EC with a noble gas core
• A) Ti 1s2 2s2 2p6 3s2 3p6 4s2 3d2
• and Ti [Ar] 4s2 3d2
Watch out for two things
• Chromium, copper and a few others
rearrange electrons to get a more stable
arrangement. The sublevel energies go
down when a sublevel is full or half full.
• Cr 1s2 2s2 2p6 3s2 3p6 4s2 3d4 becomes
• Cr 1s2 2s2 2p6 3s2 3p6 4s1 3d5 and
• Cu 1s2 2s2 2p6 3s2 3p6 4s2 3d9 becomes
• Cu 1s2 2s2 2p6 3s2 3p6 4s1 3d10
Problems:
1. Write the electron configurations for
phosphorus and nickel. Then draw the aufbau
diagrams for these elements.
2. Write the complete electron configurations for
magnesium, sulfur, and potassium. Then write
their electron configurations using the symbols
for the noble gases.
3. What element is represented by [Ne]3s23p6?
4. Determine the electron configuration for the last
SUBLEVEL of the following elements: S, Pt, Sr,
K, and Al.
5. The an unknown element has an electron
configuration of 1s22s22p63s23p4.
A. What is the element?
B. What does the superscript 6 refer to?
C. What does the letter s refer to?
D. What does the coefficient 3 refer to?
6. Write the electron configuration for calcium, a
nutrient essential to healthy bone growth and
development.
7. Write the electron configuration for copper,
which is used in pennies.
8. Use the symbols for the noble gases to write
the electron configurations for the following
elements:
A. Zr
B. U
C. Rn
9. Write the electron configuration and draw the
orbital diagrams for the following elements:
A. Carbon
B. Silver C. Aluminum
VOCABULARY
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Electron configuration
Atomic number
Energy level
Valence level
Sublevel
s,p,d,f
Orbital
Spin
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Proton
Electron
Spherical
Two-lobed
Dumbell-shaped
Aufbau principle
Pauli’s exclusion
principle
• Hund’s rule
Be able to:
• Describe the levels, sublevels, and orbitals.
• Recreate the aufbau order from the periodic
chart
• Write a complete EC and EC with a noble gas
core for any element
• Determine the last sublevel, and number of
electrons there from a position on a periodic
chart
• Identify a position on the chart and the element
from an EC
• Fill out an aufbau diagram for any element