Proposed Rearrangement of Periodic Table

[Mister T]

I'm sorry for being so vague here, but perhaps my post will ring a bell for someone who can fill in the details. A few weeks ago in a waiting room I was reading about a proposed rearrangement of the Periodic Table based on measurements of the ionization potentials (I think I got that right) of one of those high atomic number elements. You know, the ones that live for only a fraction of a second. It caught my interest. Then a few days later I saw an article about the same issue, this time not in a popular magazine but in a specialized one. I think it was probably Physics Today.

Anyway, the point they made that I can't understand is this. They were saying that the inner electrons are moving so fast that their kinetic energy makes makes a significant contribution to the atom's mass. (They phrased it as the relativistic mass of the electrons, but for those of us who abhor that term I phrase it differently.) I get that. But then they said that the increased mass somehow changes the shielding effect that the inner electrons have on the outer electrons. I can't understand this. I can see how an increased charge on the inner electrons would effect the shielding they provide, but an increased mass! I don't get it.

Anyway, the upshot is that the outer electrons have a different ionization potential, or something like that, as a result of this altered shielding. And for that reason it may be necessary for the chemists to change the way these atoms are grouped in the Periodic Table.

Again, I apologize for the vagueness. Part of it is because I'm working from memory, but most of it is because I'm not a chemist.

 

[Greg Bernhardt]

@Borek @Bystander thoughts?

 

[DrDu]

You are probably referring to relativistic effects changing the properties of heavy elements like Gold or Mercury.
I think this point has been well communicated among chemists in the last 25 years or so (at least with me as a student).
Since its invention in the 1860's, probably hundredths of alternative periodic systems have been proposed.
But the point is, that today nobody cares, as in the meantime we have understood the quantum mechanical basis behind the
periodicity of the properties and don't have to rely on empirical tables. The latter ones are at best mnemonic guides for students, today.

Referring to the increased shielding: This is rather simple. The size of an orbital depends on the electrons mass. So increasing the mass leads to smaller orbitals. Hence an outer electron in a "normal" orbital will overlapp less the region inside the core orbitals as these are contracted. It will therefore see less of the nuclear charge on average.

 

[Borek]

No idea.

To be honest I find all these new ideas about periodic table nitpicking and a waste of time, so I drop them from the memory the moment I see them.

 

[DrDu]

No idea.

To be honest I find all these new ideas about periodic table nitpicking ...

But chemists are always nitpicking :-)

 

[Bystander]

But chemists are always nitpicking :-)

"Lousey" attitude. "Shielding?" I'll second Borek. And, for bonus commentary, I'll throw in the observation that "bond description" fads have oscillated between "VSEPR" and "LFT" several times since Pauling "wore out," and throwing a third effect into the mix of the two doesn't really add to understanding.

 

[Mister T]

You are probably referring to relativistic effects changing the properties of heavy elements like Gold or Mercury.

Much heavier. Heavier than Uranium.

I think this point has been well communicated among chemists in the last 25 years or so (at least with me as a student).

These atoms have such a short lifetime that the behaviors of their electrons are only now being studied.

Referring to the increased shielding: This is rather simple. The size of an orbital depends on the electrons mass. So increasing the mass leads to smaller orbitals. Hence an outer electron in a "normal" orbital will overlapp less the region inside the core orbitals as these are contracted. It will therefore see less of the nuclear charge on average.

Now that you mention it I think I recall at least one of the two articles mentioning smaller inner orbitals. I get that.

Smaller inner orbitals means outer orbitals overlap less. That makes sense. Why does that mean that electrons in outer orbitals will see less of the nuclear charge, on average?

 

[Bystander]

Smaller inner orbitals

less of the nuclear charge, on average?

Google "screening," John Light(?).

 

[ogg]

The periodic table is arranged by (nuclear) charge, not mass. You'd have to be more specific about how relativistic effects can change the number of protons - obviously it can't. The periodic table is utilitarian. That is, imho, it exists for its utility in helping predict chemical (usually chemical, but some physics properties as well) properties. I don't see any utility in making it harder to understand in order to "explain" (or predict) properties of elements which almost don't exist (that is, they're so unstable as to not have much chemistry). This would be like redesigning a car in order to accommodate the hood (bonnet) ornament.