Energy Shells (Levels) and Subshells (Sublevels)
(continued)
Electron Shielding vs. Electron Penetration
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© 2023 M. Ilies
► What does determine the splitting of main energy levels and
the distance between them?
A “push-pull” game between attraction & repulsion:
- each proton in the nucleus attracts one electron;
- electrons repel each other.
When there is a perfect balance between attraction and repulsion,
the atom is stable. More stable = lower energy = more symmetry.
This balance is achieved via 2 effects:
1. Shielding (screening) effect: describes interaction between electrons
on different energy levels and sublevels
2. Penetration effect: describes interaction between the nucleus and
electrons on energy levels higher than n=1
© 2023 M. Ilies
1. Shielding (screening): less nuclear attraction for outer shell electrons
Electrons can shield electrons in higher energy sublevels from the
total nuclear charge given by all protons in the nucleus (Z).
Li (Z=3): 1s22s1
Inner shell (core) electrons that
shield the outer shell electron
+3 = total nuclear charge = Z
+1 = effective nuclear charge
= Zeff
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© 2023 M. Ilies
Effective Nuclear Charge (Zeff)
= a fraction of the total nuclear charge = the actual, decreased nuclear
charge experienced by outer-shell e- due to shielding by core e-.
will experience an effective
nuclear charge smaller than the
actual nuclear charge, Z:
Zeff < Z
http://wikis.lawrence.edu/display/CHEM/5.1+Effective+Nuclear+Charge+%28Gil+Shank%29
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© 2023 M. Ilies
P (Z = 15): 1s2 2s2 2p6 3s2 3p3
(splitting not shown)
(because it contains 8e- total)
experienced
by the 5 e- in
the last shell
https://ch301.cm.utexas.edu
As electron shells are added, the inner e- will repel and shield outer e-.
Hence, the attraction of the nucleus for the outer e- is diminished.
Consequence: the orbitals in higher energy levels increase in volume.
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E.g., a 3p-orbital has a larger volume than a 2p-orbital
© 2023 M. Ilies
In which orbital does an electron in a phosphorus atom
experience the smallest effective nuclear charge, Zeff?
1. 2s
2. 2p
3. 3s
4. 3p
smallest effective nuclear charge =
most shielding
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2. Penetration (Orbital Overlap): due to electron-proton attraction
SHIELDING
PENETRATION
(radiation is emitted
when an electron
moves to a lower
energy level, thus
losing kinetic energy;
similar to electron
relaxation)
https://chem.libretexts.org/LibreTexts/Valley_City_State_University
© 2023 M. Ilies
What is similar and what is different?
Li (Z=3): 1s22s1
When penetration of the 2s1 electron
happens, this e- experiences a full
nuclear charge attraction, so Zeff =Z.
more stable state
(lower kinetic energy
of the electron;
more attraction to
the nucleus)
compare to shielding
Zeff = +1
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© 2023 M. Ilies
What determines the energy of a 3p-electron in an iodine atom?
(choose all that are correct)
1.
2.
3.
4.
5.
shielding by 3d-electrons
penetration of the 4th main energy level into the 3rd energy level
shielding by 4s-electrons
shielding by 2p-electrons
shielding by 1s-electrons
4p
4s
Energy
3s
2s
3d
3p
2p
2, 4 & 5
1s
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The Periodic Table and Electron Configurations
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© 2023 M. Ilies
The Periodic Table
periodic = happening at regular intervals
Similar properties appear after a certain number of units of increasing Z:
17 (Cl) - 9 (F) = 8 units of Z
11 (Na) - 3 (Li) = 19 (K) - 11 (Na) = 8 units of Z
group
period
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© 2023 M. Ilies
A group is a column.
Noble gases
Alkali metals
Halogens
Alkaline
earth
metals
elements with
similar properties
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© 2023 M. Ilies
“A” = “main-group”
”B” = “transition”
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The number of a main group = how many e- in the last energy shell.
A period is a horizontal row
Period
number
how many units of increasing Z
until some properties repeat
The number of a period = how many main energy shells filled with
14
electrons in the atom
© 2023 M. Ilies
= the highest “n” for that element.
Electron Configurations:
Core vs. Valence Electrons
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© 2023 M. Ilies
● Write the electron configuration for Ge:
1. Find the element in the periodic table:
Z = 32; group 4A; period 4
2. How many electrons totally to arrange around the nucleus?
32
3. How many main energy levels (shells) around the nucleus?
4 (period 4)
- the number of each energy shell also gives you how many
types of subshells in that shell:
s < p < d < f (in the increasing order of energy)
4. How many electrons expected in the last shell?
4 (group 4A)
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© 2023 M. Ilies
Core vs. Valence Electrons
Core e- = e- in completely filled inner energy shells & sub-shells.
in the ground-state
Valence e- :
For main-group elements (elements in taller groups):
= e- in the outermost (highest energy) occupied main shell
For transition elements:
= e- in the outermost occupied main shell +
e- in incomplete d or f sublevels
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© 2023 M. Ilies
4p
4s
Energy
3s
2s
3d
Recall: 4s orbitals fill before 3d!
3p
2p
Main-group Elements
(Z = 14; group 4A)
1s
(Z = 32; group 4A)
For main-group elements,
the # of the group = the # of valence electrons.
d-sublevel
completely filled
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© 2023 M. Ilies
4p
4s
Energy
3s
2s
3d
3p
2p
incomplete
d-sublevel
1s
Transition Elements
core electrons
Z
valence electrons
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27
28
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© 2023 M. Ilies
What did you find most difficult to understand?
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the screening (shielding) effect
the penetration effect
core vs. valence electrons
how to write electron configurations for atoms
Nothing – understood everything.
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