Date: Fri Feb 25 15:36:03 2000
Posted By: Dan Berger, Faculty Chemistry/Science, Bluffton College
Area of science: Chemistry
[Xe] 4f14 5d4 6s2 unlike the other 2 elements in its period (Chromium and
Molybdenum). I was wondering why does Tungsten NOT "kick" up an electron
from the s sublevel.
There is no way to explain this within a simple, qualitative model.
"Electron configurations" in atoms are, to the best of my knowledge,
obtained by fitting spectroscopic information from that atom to a hydrogen-
like model of the atom. I suppose that high-enough-level calculations might
predict the failure for tungsten to follow the family trend, but the
best we can do on a qualitative level is this hand-waving explanation:
- The reason we get such fluid electron shuffling between 3d/4s, 4d/5s and
5d/6s is that the two types of subshell are very close in energy. Any small
variation in energy is enough to affect the configuration.
- In chromium and molybdenum, the sum of the energy required to force two
electrons together in the s-orbital (the "correlation energy") and that gained
by getting a half-full subshell is less than the energy required to promote
an electron from 4s to 3d or from 5s to 4d.
- As you go down the periodic table, the s-orbitals get lower in energy
relative to other atomic orbitals of the same main shell.
This is called a "relativistic contraction" and is discussed in
- Because of this effect, the 6s orbital is lower in energy relative to the
5d orbital than the 5s is relative to 4d or the 4s to 3d. Thus, the energy
gained by not shoving two electrons into the 6s orbital, plus that from a
half-filled 5d subshell, is not enough to compensate for promoting an
electron from 6s to 5d.
Of course, this explanation is very qualitative!
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