MadSci Network: Chemistry

Re: Why does Tungsten not 'Kick' up an electron from the s sublevel ?

Date: Fri Feb 25 15:36:03 2000
Posted By: Dan Berger, Faculty Chemistry/Science, Bluffton College
Area of science: Chemistry
ID: 951444950.Ch

Tungsten is [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:

  1. 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.

  2. 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.

  3. 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 this answer.

  4. 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!

Dan Berger
Bluffton College

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