Quantum Theory Explanations
Periodic Table
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Bohr’s model could not adequately explain why only two electrons fit in the first
shell and eight electrons in the shell after that.
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Quantum theory shows that the maximum number of electrons in the s.p,d, and f
orbitals corresponds exactly to the number of columns of elements in the s.p,d.f
blocks in the periodic table
Table 1 – Electron Subshells and the Periodic Table
Electron Distribution
Groups:
1-2
13-18
Period
# of Elements
Orbitals:
s
p
Period 1
2
2
Period 2
8
2
6
Period 3
18
2
6
Period 4-5
18
2
6
Period 6-7
32
2
6
Table 2 – Electron Subshells and Elements
Sublevel
# of Elements
# of Orbitals
s and p
2+6=8
1 +3 = 4
# of Electrons
2 + 6 =8
d
10
5
10
f
14
7
14
3-12
d
f
10
10
10
14
Series of Elements
Representative
Elements
Groups 1-2, 13-18
Transition Elements
Groups 3-12
Lanthanides and
Actinides
Ion Charges
- Previously could not explain transition-metal ions and multiple ions formed by
heavy representative metals
- Quantum theory can now explain what could not be explained previously
Example: Explain why lead can form two different ions given that the electron
configuration for a lead atom is Pb: [Xe] 6s24f145d106p2
Magnetism
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Some elements that are ferromagnetic (strongly magnetic) include iron, cobalt,
and nickel. These elements have several unpaired electrons. However,
ruthenium, rhodium, and palladium, immediately below iron, cobalt, and nickel in
the periodic table are only paramagnetic (weakly magnetic). The reason is that
ferromagnetism is based on the properties of a collection of atoms (each acting
like a little magnet), rather that just one atom.
Anomalous Electron Configuration
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Some elements are exceptions to the rules for writing electron configuration.
Two examples are chromium and copper.
Element
Predicted Electron Configuration
chromium
[Ar] 4s23d4
Actual Electron
Configuration
[Ar] 4s13d5
copper
[Ar] 4s23d9
[Ar] 4s13d10
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This suggests that half-filled and filled subshells are more stable (lower energy)
than unfilled subshells. In the case of chromium, an s electron is promoted to the
d subshell to create two half-filled subshells. In the case of copper, an s electron
is promoted to the d subshell to create a half-filled s subshell and a filled d
subshell.