Boy, on re-reading that post, there sure are a lot of “shoulds” and “coulds” in there. It may be that I’m so used to regarding the atom as the fundamental unit of an element that I’m thinking in circles and making myself goofy.
The answer is that you can’t have an element composed of electrons. Electrons don’t have chemical properties in and of themselves.
Sodium is an element. It’s defined as “the thing with 11 protons”. (These are HS definitions mind you, that I have to use to get them to stick into 16 year-old brains.) The reason that we say sodium is an element is that it has a certain character. It loses one electron easily and then fights to retain the 10 it still has. It gives off a certain bright line spectrum when burned. This is because of the unique spacings of its electrons, which are being excited and then releasing light as they fall to their lower, original energy levels.
The point being that sodium and other elements have characters that distinguish them from other types of atoms - the other elements. It’s an axiom that electrons, or any other subatomic particles can’t be told apart from any other. All electrons are interchangable. This unique character is what makes us call them elements. Try to substitute magnesium for sodium, even though it only has one more proton, and you’re going to have a very rude surprise. Nothing will work right with that substitution.
Well, technically, hydrogen doesn’t have a proton AND an electron. It’s defined by the fact of having one proton. It’s still hydrogen if its electrons get messed around with and added to or taken away.
My last month in this HS class has been getting across the idea that the electrons almost never actually stay the same on real atoms in the real world. They get traded and shared by almost everything all the time. Pay attention to the protons and you’ll know what you have.
Oh, and when I said that hydrogen-1 was a proton and nothing else, I meant nothing else in the nucleus. God knows what state its electron cloud is in. What time is it and what atom has come by in the last nano-second?
I admit that the diagram is um … interesting, but it’s not particularly informational. I think it would be a lot harder to memorize than our current arrangement.
As far as calling neutronium an element - that’s a bit of a stretch, although he got the numbering scheme right. The atomic number refers to the number of protons in the nucleus, so calling neutronium element zero at least makes sense from that standpoint. Of course, that would make anti-hydrogen have an atomic number of -1.
However, I don’t think it can truly be considered an element, nor should be included in the periodic table. The periodic table is concerned with chemical reactions. It is grouped the way it is based on chemical properties of elements, which as Cardinal pointed out is dependent on the number of protons in the nucleus. Neutronium would be more appropriately placed in the chart of nuclides, which is related to nuclear properties and reactions. And neutrons certainly play a role there.
Ok, I stand corrected.
For 100 nanoseconds.
Ahh, there it goes.
Yeah, that was pretty nitpicky.
Still, positronium is a better choice for “element zero” than neutronium; at least positronium has one particle in common with all the other elements.
Okay. I have here some hydrogen fluoride (in a sufficiently inert container!). To be specific, it’s protium fluoride; I’ve carefully and repeatedly centrifuged it to remove all the deuterium. It’s an ionic compound. What’s it made up of? Well, half by atom-count (much more by weight or dimensions) is fluorine- ions, with one more electron than protons. The other half is hydrogen+ ions, with one fewer electron than protons. But it started, in the unionized state, with one each. So it has become a naked proton. It’s still hydrogen, though.
Over here, I have deuterium fluoride, likewise in an inert container. Same situation; it’s half fluorine- and the other half hydrogen+, but this time it’s a naked nucleus of one proton and one neutron. Still behaves like hydrogen fluoride, though.
The hydrogen doesn’t cease to be hydrogen through being ionized and losing its electron; it behaves as a hydrogen ion, little different from any Class I element ion with a single positive charge, even though it’s not surrounded by an “inner” electron shell beyond the one that would hold its single outer-shell electron
I suspect I could, with a little finagling, produce lithium trifluoride, where one lithium ion is in ionic bonding with three fluoride ions. (Anyone want to tell me whether this is possible, in terms of ionization potential?) And this time I have a naked nucleus of three protons and three or four neutrons.
So this little thought experiment has established that naked subatomic particles are, in fact, elements, singly or in groups.
In that classic high school level definition that Cardinal mentions, Sodium is an element. It’s defined as “the thing with 11 protons”. And that holds regardless of whether it has 11 or 10 electrons (or, conceivably, less or more). Well, OK, hydrogen is hydrogen, regardless of whether it has 1 or 0 electrons. It’s simply “ionized hydrogen-1” when it’s a lone proton, with no faithful electron companion, Tonto.
And there are elements which have various allotropes in the pure-element state, depending on ambient temperature and pressure. I recall that tin changes allotrope when sufficient temperature or pressure is applied. So, of course, does carbon: naturally it occurs as graphite at STP, but when subjected to intense heat and pressure, it becomes diamond, which is metastable at STP – it’ll revert to graphite, but at a millions-of-years time scale. (Diamonds aren’t forever; sorry, DeBeers!)
Now, go back to deuterium. It’s a one-from-column-A-and-one-from-column-B substance: unionized, one each of proton, neutron, and electron. Ionized, just the first two. It behaves as hydrogen due to its single proton (and, unionized, equalizing single electron). But it’s different than protium because of having the neutron present. (Deuterium is the one isotope that has significantly different chemical characteristics than its dominant isotope, owing to the fact that the weight ratio is a full 2:1 rather than 238:235 or 14:13:12 as in uranium and carbon.)
So we know that an element is composed of a fixed number of protons and a variable number of neutrons, the possible alternatives being specified by what combinations are radioactively stable, metastable, or measurably unstable-but-possible. Hydrogen can be defined as the element with one proton and 0, 1, or 2 neutrons in its nucleus.
But it therefore is possible to define an element with zero protons and one neutron in its nucleus: neutronium. The fact that neutronium is only physically stable when in the sort of pressure produced by quantities on the order of 10[sup]57[/sup] particles has little to do with the concept. That allotrope of tin and diamond are not stable at STP either. Since it has no protons, it needs no electrons to stabilize it in terms of ionization. And therefore it becomes exceedingly dense. One might call each neutron star the sole example of an isotope of neutronium: one giant atom with no protons, no electrons, and some enormous, variable number of neutrons. (This describes the neutron star’s core; the star itself is this central mega-atom plus, of course, the degenerate and normal matter that accrue to it by gravity.)
Therefore neutronium does in fact qualify as an element.
Neutron stars are more complicated than that:
What generates the magnetic field of a neutron star ?
If you’re going to go to extreme conditions to form your elements, you’ll also have to admit things like Mu mesons orbiting protons into your table.
The Mendeleev table has “patterns too perfect” because it represents the way that elements interact; elements with similar chemical properties tend to appear in columns owing to their relative electronegativity and so forth. That “perfection” (and believe me, it’s not so perfect–once you study PChem you realize how many seemingly arbitrary rules have to be created in order for things to make sense) is a result of making the table fit reality, and only later coming back to fill in the missing elements.
Neutronium is (by some) considered Element 0, but since it does not interact chemically with any other element that seems a rather pointless designation. Free electrons can interact chemically (or, at least, can react with other elements in an electrochemical fashion), but aren’t considered elements themselves, so that seems a little arse-back’ard to me. “Positronium” is just silly; given that it has no nucleus I’m not clear by what basis (other than particle physicists wanting to publish in the Journal of Physical Chemistry just to piss off the bretheran across the quad) one could consider it an “element”; it certainly can’t form chemical compounds. And as Chronos points out, while free neutrons exist, they don’t clump together except in massive quantities (and there’s some question as to exactly what form the matter in a so-called neutron star takes, anyway–some scientists assert that neutronium isn’t stable, and that the star goes from degenerate matter directly to a quark-gluon plasma with only a brief, energetic phase in which electrons and protons combine and then decay).
And that Stewart periodic table is a mess. It isn’t manifestly different in detail from the Mendeleev table, but it scatters information to and fro without cause. I can’t see any real utility in it.
Stranger
Thanks muchly for spending so much time laying out the framework for this notion, Polycarp. If I understand this argument correctly, then, would all subatomic particles other than the proton be categorized as element zero also, since they contain zero protons and a number of neutrons (which also happens to be zero?)
There have been many redesigns of the periodic table over the years. Some of them ar pretty interesting, and give you insights into the symmetries involved. I’ve seen many of these, but can’t recall where. So this isn’t exactly the first redesign.
My two-or-so-cents:
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First, the visual design of Stewart’s chart makes my skin crawl. I’ve never pitted anything before, but I may have just found something worthy.
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Second, I see two questions here, one implied and the other direct from the OP’s keyboard:
(1) Does the word “element” include neutrons/neutronium. The arguments above have searched for a clean rule to define “element”, but no such rule need exist. I posit that, logic and rationales be damned, the word “element” has a clear English meaning to the lay public and to scientists in general, and that that meaning does not include neutronium. In other words, if you walk up to a random English-speaking chemist, physicist, etc. and use the word “element” in a sentence, that scientist will not be including neutronium in whatever you are saying. Semantics being what they are, I say that that alone provides the definition. (Sure, there are reasons why it might be more elegant to have (or not have) neutronium included in the definition, but that’s not the way the language evolved, so we’re stuck with it. If the need arises for a term that means “all the elements plus neutronium”, that term will surely be coined, but “element” is already taken.)
(2) “Is there any serious argument in favor of adding neutronium to the periodic table?” If the consumers of the periodic table are helped with its addition, then yes; else no. I argue “no” because if you’re dealing with, umm, elements [see (1)], then you don’t care about neutronium (chemists); and if you’re dealing with neutrons, you don’t care about gadolinium (particle physicists). The relations between neutrons and gadolinium are never of particular practical interest, so why clutter the periodic table? Is there a gap in physical intuition that could be filled by added neutrons them to the periodic table? It doesn’t seem so.
Exploring the properties of neutrons is certainly a laudable exercise, but adding neutrons to the period table doesn’t really help that process along any.
One important thing that hasn’t been mentioned above is that bare neutrons, in the absence of other forces, are unstable; they decay into a proton, an electron, and an anti-neutrino with a half-life of about 15 minutes. This isn’t a problem, per se, since there are elements on the periodic table whose most stable elements have half-lives shorter than that; but still, it’s kind of strange to think of a particle like that as an element. Why not throw in all the strange baryons as well?
Actually, particle physicists do sometimes concern themselves with heavier nuclei: The Relativistic Heavy Ion Collider at Brookhaven, for instance, uses gold nuclei. So to a particle physicist, a table of nuclides which includes both the neutron and heavy nuceli might be relevant. But such a table would not be organized according to the patterns of electrons around the nuclei, as the Periodic Table is.
And to reiterate, there is no structure, macroscopic or microscopic, in nature which consists of more than one neutron and no particles other than neutrons. At the subatomic scale, you can’t get them to stick together without other particles (specifically protons) present, and at the neutron-star scale, you can’t keep them from collapsing to a black hole without a significant number of electrons and protons.
I thought about bringing RHIC up specifically, but I did not because RHIC cannot accelerate free neutrons, so even RHIC physicists aren’t ever treating neutrons and gold on the same footing. And, if they ever did, the periodic table would be much less useful than a table of nuclides, so my conclusion that the period table doesn’t need a neutron stays unmodified.