MR. SURRETTE
VAN NUYS HIGH SCHOOL
CHAPTER 19: MODERN ELECTRON CONFIGURATIONS
CLASSNOTES
LOCATION OF ELECTRONS
The location of electrons found in the first three rows of the periodic table (hydrogen  argon) are
predicted by the Haufbau Principle and Pauli Exclusion Rule (electrons in rows 4 – 7 do not always
follow these rules!).
HAUFBAU PRINCIPLE
The Haufbau Principle determines that electrons fill up orbitals one available space at a time.
PAULI EXCLUSION RULE
The Pauli Exclusion Rule states that every orbital can hold up to two electrons of opposite spins. Spin
means orbital direction, i.e. “clockwise” or “counter-clockwise” movement around the nucleus.
PAULI EXCLUSION RULE
To differentiate between clockwise and counterclockwise motion, electrons are labeled as up spin or
down spin.
ELECTRON CONFIGURATIONS
Electron configurations use shorthand notation to describe the location of electrons. Electron
configurations list the orbitals in sequence and the number of electrons per orbital (see examples below).
RULES FOR ELECTRON CONFIGURATIONS
1. Every orbital holds up to two electrons.
2. The number of electrons per orbital is represented in super-script.
ELECTRONS AND THE PERIODIC TABLE
Rows 1 – 3: Electrons in atomic elements
1 – 18 fill up orbitals in the following order: 1s2, 2s2, 2p6, 3s2, then 3p6.
Example 1. What is the electron configuration for helium?
1A.
Helium [He] is atomic number two. This means the helium atom contains two protons. Since helium is
electrically neutral, it must have two electrons. The first energy level contains a 1s shell and has room
for two electrons. Electron configuration for helium: 1s2
Example 2. What is the electron configuration for oxygen?
2A.
Oxygen is atomic number eight, and has eight electrons. The 1s orbital contains the first two electrons.
The 2s orbital contains the next two electrons. The remaining electrons fill up four out of the six
available openings in the 2p orbital. Electron configuration for oxygen: 1s22s22p4
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CHEMISTRY
MR. SURRETTE
VAN NUYS HIGH SCHOOL
ELECTRONS AND THE PERIODIC TABLE
Row 4: With two exceptions, valence electrons in elements 19 – 36 fill up: 4s2, 3d10 (transition
metals), then 4p6.
d and f SUBSHELLS: DEGENERATE ORBITALS
The d and f subshells contain degenerate orbitals. This means electrons in the d and f subshells have
lower than expected energy levels.
d and f SUBSHELLS: DEGENERATE ORBITALS
As a result, some of these electrons move easily into neighboring orbitals, creating some unexpected
electron configurations.
FILLING OF THE 3d ORBITALS
ANAMOLOUS CONFIGURATIONS
Chromium and copper only have one electron in the 4s orbital. This gives them each an extra electron in
the 3d orbitals.
ELECTRONS AND THE PERIODIC TABLE
Row 5: Most valence electrons in elements 37 – 54 fill up: 5s2, 4d10, then 5p6.
ELECTRONS AND THE PERIODIC TABLE
Row 6: Most valence electrons in elements 55 – 86 fill up: 6s2, 5d1, 4f14 (lanthanide, or rare earth,
series), 5d9, then 6p6.
ELECTRONS AND THE PERIODIC TABLE
Row 7: Most valence electrons in elements 87 – 118 fill up: 7s2, 6d1, 5f14 (actinide series), 6d9,
then 7p6.
ABBREVIATED ELECTRON CONFIGURATIONS
Electron configurations can get quite long. For example, the electron configuration for silver is
1s22s22p63s23p64s23d9.
ABBREVIATED ELECTRON CONFIGURATIONS
To save time and energy, chemists sometimes use abbreviated electron configurations. These consist
of a noble gas symbol followed by valence electron configurations.
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CHEMISTRY
MR. SURRETTE
VAN NUYS HIGH SCHOOL
CORE ELECTRONS
All non-valence electrons within an atom are called core electrons. Core electrons are found between
the nucleus of an atom and the valence electrons.
CORE ELECTRONS
Core electrons are “hidden” from other atoms by the valence electrons, and do not take place in
chemical reactions.
NOBLE GAS CONFIGURATIONS
Core electrons within an element are represented by the noble gas symbol on the previous row of the
periodic table.
NOBLE GAS CONFIGURATIONS
For example, lithium (atomic number 3) is on row 2 of the periodic table.
NOBLE GAS CONFIGURATIONS
Helium [He] is the noble gas at the end of row 1. Therefore, the core electrons for lithium are
represented as [He].
Example 3. Provide the abbreviated electron configuration for lithium.
3A. [He]2s1
Example 4. Provide the abbreviated electron configuration for vanadium.
4A.
[Ar]4s23d3. Vanadium [V] is a transition metal on row 4 of the periodic table. The noble gas at the end
of row 3 is argon [Ar]. Starting with potassium (element number 19), count the number of elements
from left to right until you get to vanadium (element number 23). Since vanadium is the fifth element
you count on row 4, it has five valence electrons. The first two valence electrons go in the 4s orbital.
The next three go in the 3d.
Example 5. Provide the abbreviated electron configuration for neodymium.
5A.
[Xe]6s25d14f3. Neodymium [Nd] is a lanthanide metal. It belongs in row 6 of the periodic table.
Starting with cesium (element number 55), count the number of elements from left to right until you get
to neodymium (element number 60). Neodymium is the sixth element as you count on row 6, it has six
valence electrons. The first two valence electrons go into 6s, the next into 5d, then the last three into 4f.
ORBITAL DIAGRAMS
Orbital diagrams are similar to electron configurations, but provide details about up spin and down
spin electrons.
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CHEMISTRY
MR. SURRETTE
VAN NUYS HIGH SCHOOL
RULES FOR ORBITAL DIAGRAMS
1. Determine the number of electrons per atom.
2. Determine the subshells needed to hold these electrons.
3. Fill in open orbital spaces left to right across each orbital. Use up spin electrons (represented as “up
arrows”) first.
4. When a subshell runs out of open space, go back to the left side and fill in down spin electrons (
“down arrows”) next to the up arrows.
DRAWING ORBITAL DIAGRAMS
Before drawing an atom’s orbital diagram, it may be easier to first determine its electron configuration.
DRAWING ORBITAL DIAGRAMS
That way, you can transfer the orbitals found in the electron configuration into your orbital diagram.
Then you add your arrows on top of the orbital lines.
Example 6. Draw the orbital diagram for helium.
6A.
As seen in Example 1, the electron configuration for helium is 1s2. The 1s subshell consists of a single
1s orbital. To begin the orbital diagram, copy the 1s orbital:
At this point, add the two electrons, up-spin arrow first.
Example 7. Draw the orbital diagram for oxygen.
7A.
As seen in Example 2, the electron configuration for oxygen is 1s22s22p4. The 1s and 2s subshells
consist of the 1s and 2s orbitals. The 2p subshell, however, consists of the 2px, 2py, and 2pz orbitals.
Copy all the orbitals.
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CHEMISTRY
MR. SURRETTE
VAN NUYS HIGH SCHOOL
7A. (continued…)
Now fill in the electrons from left to right.
Example 8. Draw the orbital diagram for iron.
8A.
Iron is found on the fourth row of the periodic table. Therefore, it automatically contains the [Ar] core
electrons: 1s22s22p63s23p6. Counting left-to-right across row 4, iron is the eighth atom. These
additional electrons are: 4s23d6. Electron configuration for iron: 1s22s22p63s23p64s23d6. When we
transfer the orbitals from iron’s electron configuration we get:
Now, fill in the electrons from left-to-right.
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