MR. SURRETTE
VAN NUYS HIGH SCHOOL
CHAPTER 15: MODERN ELECTRON CONFIGURATIONS
CLASS NOTES
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. 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 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. The means electrons in the d and f subshells have
lower than expected energy levels. As a result, some of these electrons move easily into neighboring
orbitals, creating some unexpected electron configurations.
FILLING OF THE 3d SUBSHELL
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. To save time and energy, chemists sometimes use abbreviated electron
configurations. These consist of a noble gas symbol followed by valence electron configurations.
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 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. For example, lithium is on row 2. of the periodic table. 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
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CHEMISTRY
MR. SURRETTE
VAN NUYS HIGH SCHOOL
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 (19), count the number of elements from left to right
until you get to vanadium (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 (55), count the number of elements from left to right until you get to neodymium
(60). Neodymium is the sixth element as you count on row 6, it has six valence electrons. The first two
valence electrons go into6s, 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.
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 subshell. 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.
Example 6. Draw the orbital diagram for helium.
6A. Helium has two electrons in the 1s orbital:
Example 7. Draw the orbital diagram for oxygen.
7A. Oxygen has eight electrons found in the 1s, 2s, and 2p orbitals:
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CHEMISTRY
MR. SURRETTE
VAN NUYS HIGH SCHOOL
Example 8. Draw the orbital diagram for iron.
8A. Iron has 26 electrons. It is a transition metal found on row 4 of the periodic table:
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CHEMISTRY