I’ve seen various defnitions for an element (eg. arsenic)
Are all of the following acceptable definitions?
1.
An element is an atom with a specific number of protons, neutrons and electrons.
2. An element is a molecule consisting of one type of atom
3. An element is a molecule consisting of the same types of atoms
You could have free elements consisting of more than one atom (e.g., an oxygen molecule). The IUPAC handbook
https://goldbook.iupac.org/terms/view/C01022
mentions both definitions (a species of atom with a given number of protons / a pure chemical substance) but says the term “chemical element” is used for both concepts.
IANAChemist but 1 and 3 are flat out wrong.
It’s 1 but with a restriction: an element is defined by an atom having a certain number of protons.
neutrons may vary, but stay the same element ( 235U and 238U are both uranium, even if they behave differently)
electrons may vary too (Fe and Fe++ are both iron, but one in metal form and the other in ionic form).
a molecule (formed by 2 or more atoms) is never an element.
None of them are true, because an element is a substance, not a single particle (atom or molecule).
More precisely, it’s a substance which consists entirely of atoms all of which have the same number of protons.
So a single atom of Californium is not an element?
It is. So is two atoms of californium, or ten atoms, or 10^24 atoms of it.
The usual chemical definition of an element is something that cannot be broken down by any chemical reaction. Elements have a fixed number of protons, and in a neutral state have the same number of electrons. Varying the number of electrons produces ions. Varying the number of neutrons produces different isotopes. But they are still the same element.
Some isotopes are stable, some are not.
The concept started with the ancient Greeks (or I believe is at least first documented in ancient Greece). IIRC it was Aristotle who said something to the effect that an element is something that cannot be broken down further. Other things can decay into elements, but elements cannot themselves be broken down. Of course they started with fire, water, air, and earth as the “elements”, so their understanding of modern chemistry was a bit lacking. But hey, you gotta start somewhere.
When we learned enough to create the periodic table, we thought that we had nailed it. All that remained was to fill in some gaps in the table. And then some other folks came along and screwed everything up by figuring out sub-atomic particles. Bastards. 
Thanks Chronos. Would it be correct to say that the uniqueness of an element rests solely on the number of protons (not on their neutrons or electrons etc) ?
Yes. The number of protons in the atom of the element is its identity.
That number is called the atomic number of the element. Every distinct box in a periodic table of elements is an element, and they increase in atomic number from 1 to (currently) 118.
Since this is factual, let’s move it to FQ (from IMHO).
It’s true that how we classify an atom as an “element” is based solely on the number of protons, there’s a bit of nuance involved. Two atoms with the same number of protons but different number of neutrons are obviously not identical in all respects. When we say, for example, that carbon-12 and carbon-14 are merely different isotopes of carbon, we mean that they act (mostly) the same in chemical reactions. They don’t act the same in situations involving nuclear reactions (for example, 14C is radioactive while 12C is not). But since effects of chemical reactions dominate human’s lives while effects of nuclear reactions are pretty rare, we call all atoms that act the same chemically “the same” element. If we lived inside a star, where chemical reactions don’t occur and all activity is based on nuclear reactions, we might consider 12C and 14C to be completely different substances.
I’ve heard it asserted, I don’t remember where, that if something could exist that had a nucleus with the charge of 6 protons surrounded by 6 electrons then it would be chemically indistinguishable from a carbon atom.
This assertion may be right or wrong. I suspect it’s “not even wrong” because we neither know of nor suspect the existence of anything other than a carbon nucleus that has the charge of 6 protons and could be surrounded by 6 electrons.
So is my assertion nonsensical and unworthy of comment by real scientists or does it elucidate some principle of chemistry?
It would BE carbon. You have described Carbon-6.
Whether this universe allows such a thing to exist given the known subatomic forces I do not know.
Certainly it has not yet been found or created, as the lightest known isotope is Carbon-8, and Carbon-8 has a half life that is mind-bogglingly short… but if it does, that’s just another isotope of carbon.
I wasn’t very clear. My atom has a nucleus that is not made of protons and neutrons. I don’t know what it’s made of but it has the charge of 6 protons and the ability to hold electrons. The assertion I heard was that whatever that was would be chemically identical to a carbon atom.
The question may be much less interesting than it sounds but I recall it being offered seriously.
I’d call that definition a bit iffy… Ozone (O3), for instance, can break down into O2, but both of them are pure oxygen, and both the same element.
I’ve seen this in the context of hypothetical extremely small charged black holes. Such a black hole would have a mass much, much greater than an ordinary atomic nucleus, but that has surprisingly little relevance for chemistry: For most chemistry, it’s already an excellent approximation to treat the mass of the nucleus as infinite, relative to the mass of the electrons.
And you can maybe quibble a bit about the number of electrons in an atom: A negative chlorine ion, for instance, behaves very differently chemically from a neutral chlorine atom. But at the bulk scale, for elements, this doesn’t matter much: Any amount of a pure element that’s large enough to measure will have the vast, vast majority of the atoms having the same number of protons and electrons.
It would not be chemically identical to Carbon-12, because bond energy and reactions dynamics for a carbon atom that has half the mass of normal carbon will be sufficiently different for many reactions, and especially in biological systems.
You would expect that heavy water (water with one or both of the hydrogens replaced by deuterium) would be chemically identical to water for most purposes. However, at a level above ~25% of water mass of a living creature, there are significant biological impacts including sterility, and is generally fatal in small mammals at over 50%.
I would expect very similar issues to arise with a hypothetical Carbon-6, if such a thing were stable enough to be integrated into the tissues of a living creature.
On the other hand, the mass difference for Carbon-14 is much less significant, and this isotope of carbon integrates into living organisms with no biological issues, which provides us with our Carbon-14 clock running back over 50K or more years
An atom is not a single particle, but rather contains several different particles.
74westy didn’t even say that this hypothetical nucleus weighed the same as 6 protons, just that it had the same charge.
I don’t think there’s any clear answer for carbon, but we do have an answer for hydrogen: positronium. Instead of a proton for a nucleus, we have a positron. It’s much lighter than a single proton.
It’s highly unstable. It does have some similarities with hydrogen (and can even bond with a hydrogen to make an H2-like “molecule”), but overall is pretty dramatically different. The mass difference significantly alters the energy levels of the electron.
Exactly. That’s where the word atom comes from. The Greek word literally means “without a cut.”