Solving a subatomic shell game: Physicists decode hidden properties of the rare Earths

March 23, 2009

Physicists at Michigan Technological University have filled in some longtime blank spaces on the periodic table, calculating electron affinities of the lanthanides, a series of 15 elements known as rare earths.

"Electron affinity" is the amount of energy required to detach an electron from an anion, or negative ion (an atom with an extra electron orbiting around its nucleus). Elements with low (like iron) give up that extra electron easily. Elements with high electron affinities (like chlorine) do not.

"I remember learning about electron affinities in 10th grade chemistry," said Research Associate Steven O'Malley. "When I began working as a grad student in atomic physics, I was surprised to learn that many of them were still unknown."

Among them were the lanthanides, which are used in the production of lasers and sunglasses. In terms of their atomic structure, lanthanides are among the most complex elements on the , which is why no one had been able to calculate their electron affinities before.

Here's what makes them so tricky. orbit in shells around an atom's nucleus, something like the layers of an onion, but in stranger shapes. Within each shell are a number of subshells. A subshell is like an egg carton: it can hold from one to a certain number of electrons, but no more.

Typically, as you work your way down and across the periodic table to larger and larger atoms, the inner shells fill up with electrons, and then new shells and subshells are formed and fill up pretty neatly.

That's not what happens with the lanthanides. Before their so-called 4f subshell fills up, the additional electrons begin making new shells. Then, gradually, as you move across the periodic table to heavier atoms in the lanthanide series, that 4f subshell finally fills up with its maximum number of 14 electrons.

Why would this matter for electron affinity? A number of forces hold electrons in their orbits around the atom's nucleus. Two simple ones are electrons' attraction to protons in the nucleus and repulsion away from their fellow orbiting electrons, what physics professor Don Beck calls "the B.O. effect."

The forces exerted by a full shell on the electrons orbiting farther from the nucleus are pretty constant, which had made it relatively easy to calculate the electron affinities of most elements. But if there are vacancies in the shell—as there are in the lanthanides—the electrons in that shell can move around, playing musical chairs, as it were.

The forces an electron exerts from each spot in the shell are different. And, in addition to simple electrical factors, there are many complex variables to contend with at the subatomic level, including relativistic and many-body effects.

"It's a nightmare," says Beck. With several electrons bouncing around in those 14 slots, over 200 different arrangements of electrons of the 4f subshell are possible in some of the lanthanides.

In 1994, the Beck research group, supported by the National Science Foundation, began work on one of the simpler lanthanide atoms, cerium. Then they started to approach the "nightmare" middle of the lanthanide row from both ends, one anion at a time. The most difficult was neodymium (Nd-) which took about six months.

In 2007, O'Malley and Beck began a final push to complete the remaining lanthanides (promethium through erbium) by

1. narrowing down which variables to include in the calculations; and
2. writing scripts and computer codes to automate much of the calculation.

Ultimately, they cut the overall work time by about 85 percent. In just 18 months, they found electron affinities for all the lanthanides, including electron affinities for high-energy, excited states of the anions. All in all, they discovered 118 lanthanide anion states, 63 of which were new.

What's next? The team's theoretical results have already been partially verified by experimentalists, but they are still working to better understand the lanthanides theoretically, to help identify just what is being measured experimentally.

In the meantime, they are turning their attention to the next row in the periodic table.

"We expect to have electron affinities for a portion of the actinides—actinum through plutonium—available sometime this summer," Beck said.

More information: For more in-depth information, visit www.phy.mtu.edu/~donald/ri.html#lea .

Source: Michigan Technological University

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5 comments

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jonnyboy
2.3 / 5 (3) Mar 23, 2009
BRAVO (I think)

And the practical use of this, extremely difficult to obtain, information would be what? (other than just scratching that itch)
el_gramador
2 / 5 (1) Mar 24, 2009
Well that's just it. If we know more we can base ideas and thoughts about the reactions of the f-group to concrete ideas about what they can do. It's not so much a matter of how useful it is right now, but more about knowing enough about that group of atoms to make an idea of what it might be useful for.



In other words, we sure as hell don't know now, but at some point we will, and that is why we pursue it.

While I don't understand what the B.O. effect is between electrons I believe it has something to do with the electron repulsion that they naturally have.
jasonpatterson
4.5 / 5 (2) Mar 24, 2009
The B.O. effect (what a term...) he is talking about has to do with the effective cancellation of part of the nucleus's charge by electrons. It's the same basic idea as the mechanism by which atomic radii change as you cross the table from left to right. For an outer electron as the number of protons increases, the attraction to the nucleus grows stronger. The number of inner electrons stays the same for simpler elements, so their repulsion is constant. The two effects in combination result in atoms getting smaller in size as you move across the table because they are being pulled more strongly toward the nucleus.

There is a surprising amount of basic chemistry information we just don't know. Heck, look at boron. It's only element number 5, but it's got incredibly complicated chemistry and a good number of its basic properties are just unknown due to the difficulty of purifying the stuff.
Scotch_Magic
5 / 5 (1) Mar 25, 2009
"Two simple ones are electrons' attraction to protons in the nucleus and repulsion away from their fellow orbiting electrons"

Isn't that one force???
KBK
1 / 5 (1) Mar 25, 2009
It's 2d infinite amplitude stress fields thus...vortex analysis-these interactives, which create 3-d vortexes - which our 'reality' is a fragile (localized time on the cummulative vector) 'foam-like' vector of.

Please begin getting with the program.

Thank you.

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