What is an element?

Positronium is significantly different from “real” hydrogen, because the two particles have the same mass. That’s a very different situation from the nucleus being 1800 times the mass of the electron. It’s like the old question of whether the planets orbit around the Sun, or if the planet and Sun both orbit around their common center of mass: Technically, it’s the latter, but the Sun is so much more massive than the planets that it’s almost completely correct to say that the planets orbit the Sun.

Indeed, but note that the only constraint in 74westy’s hypothetical was charge, not mass. Therefore, we have free rein in not assuming this alternate nucleus has similar mass to one made from protons and neutrons.

It’s possible to go the other way as well (i.e., more massive nuclei). For instance, you can take a helium nucleus and add both a muon and electron. The muon orbits so much closer to the nucleus that it may as well be a part of it. The single electron behaves chemically like ordinary hydrogen, but the overall mass is ~4.1 instead of ~1.

That one, I suspect, is not all that different from ordinary hydrogen (just a little further along the spectrum than deuterium or tritium).

I wouldn’t call O2 or O3 elemental oxygen.

Why not? Elemental oxygen shouldn’t be confused with atomic oxygen, which indeed would mean bare O atoms. But elemental oxygen (or anything) should include any of its allotropes.

Because they can be broken down by chemical reactions.

Chemical reactions can change a bunch of O3 molecules into O2, but they can’t make it into something other than oxygen.

Overall a chemist regards an element as something that defines the QED of its electrons, and even then almost entirely just the outer shell.

Given the wildly different QED properties of positronium, it isn’t a form of hydrogen. But it does have a distinct signature. So ignoring its lack of stability, it should count as an element. I’m amused to note that the standard program for an-initio QED chemistry, Gaussian*, has been extended to include positronium. So one might argue that the deal is done.

A nuclear physicist regards an element by its atomic number and ignores electrons. They really care about how many neutrons it has as well. Just as much really.

If I’m trying to avoid a quick death due to an incoming nuclear device I’m probably interested in nuclear cross sections. I’ve often wondered if a water tank filled with a sodium borate solution would be useful.

*aka “that chemistry virus” by those of us that ever managed high performance computer systems.

Wouldn’t a molecule comprising two atoms

Remarkably, Helium-3 exists, and despite having two protons and just one neutron, it is stable. I know of no other stable isotope of any element that has only half as many neutrons as protons, though I’m certainly not an expert.

IIRC the opposite of hydrogen would be of antimatter, an anti-proton with a positron. Naturally, as soon as any antiparticle (antiproton, positron) encounters it opposite number in annihilates so stability isn’t likely.

While elements with a different number of neutrons - isotopes - may react chemically the same, there are subtle differences due to the extra neutron(s) increasing the mass of the atom. This is exploited in various ways to refine and isolate certain isotopes, particularly the ones that are useful in nuclear applications.

Well of course, when you get to the border cases, like hydrogen and helium, you can count one as a huge difference. Deuterium has one more neutron than simple hydrogen, or an infinitely larger proportion of neutrons, depending how you describe it.

But neutrons don’t tend to repel each other.

Sort of. But you can’t add arbitrary neutrons either. Plus or minus a few neutrons you need the same number as protons. So Tritium and Helium-3 are not really outliers. Tritium is of course not stable, but it isn’t wildly unstable either.

Too many neutrons and nuclei get unhappy and start spitting out stuff until they are closer to balanced.

Protons do, but you can still get 249 or more of them to glom together… That’s what neutrons do for them.

Yep. One thing an element is absolutely NOT is a molecule. A molecule is a mixture of elements, like Na (sodium} and Cl (chlorine) make NaCl, sodium chloride.

Here’s the definition I found: “each of more than one hundred substances that cannot be chemically interconverted or broken down into simpler substances and are primary constituents of matter. Each element is distinguished by its atomic number, i.e. the number of protons in the nuclei of its atoms.”

But there are molecules consisting of multiple atoms of one element, such as O2, O3 and N2.

True, but they should be put on a diet. LOL

Yes an element is as much an adjective as it is a noun. Its like wood or stone. Something is made of wood or made of stone, but outside of golf you don’t talk about “a wood”. You have an atom of Oxygen, a molecule of Oxygen or a liter of Oxygen, you don’t have an Oxygen.

So far as I know, the noble gases are the only substances ever found in monatomic form, under any reasonable conditions. I don’t think that justifies calling them the only elements, though.

As for nuclear compositions, there are two forces at work, the strong nuclear force, and the electromagnetic force. The strong nuclear force can be attractive or repulsive, depending on a lot of complicated details, and it still isn’t fully understood, but for now, it’s enough to say that it’s most attractive when there are about the same number of protons and neutrons (and ideally, an even number of both). As the name implies, it’s much stronger than electromagnetism, but it’s also much shorter-range: Pack a bunch of protons and neutrons together, and the strong nuclear force will pretty much only act between nearest neighbors. For lightweight atoms, all of the nucleons (protons and neutrons) are close together, and so the stablest and most common isotopes are the ones with equal numbers of both (and also, the even-numbered elements tend to be more common than the odd-numbered elements).

The electromagnetic force, meanwhile, doesn’t care at all about the neutrons, but always repels the protons from each other. And for the high-mass nuclei, there are enough nucleons that most of them aren’t close enough together for the strong nuclear force to be relevant. And so for the higher elements, you need more and more neutrons to prevent the protons from flying apart from each other.

Eventually, you get so many neutrons relative to protons that the strong force starts repelling, too, or at least not attracting enough, and for sufficiently-massive nuclei, there aren’t any stable isotopes.

If there was a nucleus with six positrons , surrounded by 6 electrons, it would be very likely to unstable (quick to undergo fission or fusion ) due to chemistry or ionisation.

The reason protons form stable nucleus … well its a goldilocks principle, like for life on earth, but for atoms life times…the relative strength of weak and strong nuclear force and all that… tiny changes would affect the nucleus greatly…

You’re right, it would be unstable, but the reason would not be fission, fusion, or due to chemistry or ionization. The positrons would fly apart due to repulsion of like-charged particles. Likely you’d get some, perhaps all, of them anihilating with the electrons.