ALEKS Alex H. Yuan - 09/08/2014 2:47:10 PM EDT - Copyright © 2014 UC Regents and ALEKS Corporation
9/8/14 2:47 PM
Alex H. Yuan - 09/08/2014 2:47:10 PM EDT
General Chemistry (First Semester) / Chem 110 FA 2014 – CH110 FA14 S25-36 (Dr. Barber)
1.
Writing the electron configuration of a first transition series atom
Write the ground-state electron configuration for a neutral atom of each element:
chromium
manganese
Chromium and manganese are transition metals:
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ALEKS Alex H. Yuan - 09/08/2014 2:47:10 PM EDT - Copyright © 2014 UC Regents and ALEKS Corporation
1A
9/8/14 2:47 PM
8A
1 H 2A
3A 4A 5A 6A 7A He
2 Li Be
B
C
N
O
F Ne
3 Na Mg 3B
4B 5B 6B 7B --- 8B --- 1B 2B Al Si P
4 K Ca Sc
Ti
5 Rb Sr Y
Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te
6 Cs Ba La
Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn
7 Fr Ra Ac
Rf Db Sg Bh Hs Mt Ds Rg Cn
S Cl Ar
V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr
I Xe
The electron configurations of many transition metals are not exactly what you'd expect:
element
electron configuration
expected
actual
(Madelung rules)
In fact, for many transition elements in Period 5 and up, and particularly for the lanthanides and actinides, the exact
electron configuration can only be determined by experiment or detailed calculation.
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When using the aufbau method to construct the electron configurations of transition metal atoms, simple rules
(like the
or Madelung rules) are often not enough to determine the subshell with the lowest energy unfilled
orbital.
The reason is that the energy of the outermost
and subshells in transition metal atoms are relatively close,
and the small energy shifts that always occur with the addition of each electron can easily change the energy
ordering of the subshells.
It is still possible to calculate the lowest energy unfilled orbital for each atom, and then use the aufbau method to
construct the correct electron configuration. However, much more detailed and precise quantum mechanics
calculations of the energies of the electron orbitals are required.
Fortunately, for elements in the first transition series, like chromium and manganese, there is only one deviation
from the predictions of the simple aufbau method, and it is relatively easy to explain:
In the first transition series, only the electron configurations of chromium (
) and copper (
predictions of the aufbau method using the
or Madelung rules, or the Periodic Table.
For
and
) differ from the
atoms, the actual configuration differs from the simple aufbau prediction only by the
movement of one
electron to the
subshell.
The explanation for this deviation is that it makes the
subshell either exactly half filled (
an exactly half-filled or filled subshell turns out to have unusual stability.
) or filled (
), and
You're already familiar with the idea that filled electron subshells are unusually stable. We've used that fact to
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ALEKS Alex H. Yuan - 09/08/2014 2:47:10 PM EDT - Copyright © 2014 UC Regents and ALEKS Corporation
9/8/14 2:47 PM
explain why halogen atoms like or
form
anions, why Group 1A metals like
or
form
cations,
why the octet rule determines the composition of small molecules, and in general why atoms participate in chemical
reactions that leave them with the electron configuration of a noble gas -- that is, with filled subshells. What's new
here is that exactly half filled subshells turn out to have unusual stability also. (The extra stability is not nearly as
large, however.)
In this case, you have one of the first-series exceptions. After you use the aufbau method, be sure you move the
second
electron to the
subshell for the exception.
chromium
manganese
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