Just published today in IUPAC's Chemistry International Magazine

[ERIC SCERRI]

[Larry T.]

👏

V."Larry" Tsimmerman

On Sat, Feb 14, 2026 at 1:39 PM ERIC SCERRI <sce...@g.ucla.edu> wrote:

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Regards

Eric

ERIC SCERRI PhD

UCLA

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[johnmarks9]

Dear Eric, 

I humbly suggest the word "essence" (from Latin esse "to be", "to exist") may be what you (and Doug Templeton) are looking for, for Mendeleyev´s "basic substance" and "element" for Lavoisier´s "simple substance".

"Substance" applies to anything, usually solid, from wood to lava and Lavoisier chose "element" as first principle of matter, following Boyle and the Ancients. Lavoisier used "element" in this sense, replacing the five elements of antiquity. The essence of an element is that which is essential wherever that element is found in compounds, allotropes, isotopes or, indeed, anywhere. It seems necessary to retain "element" as distinct from "substance" since all substances are composed of elements but elements are distinctive substances, a natural kind, each composed of identical atomic number.

So "simple substance" is renamed "element"

"Basic substance" is renamed "essence" or ´elemental essence´ or ´essence of the element` under discussion.

In general, "hydrogen" would refer to the element and "H" to its essence.

Regards, 

John

On Saturday, February 14, 2026 at 7:39:57 PM UTC+1 scerri wrote:

[Mario Rodriguez]

Hi Eric,

I’d like to add a thought aligned with how we define an element or simple substance.

Classical chemistry is based on bond formation between atoms, a process in which only electrons participate. In this context, the atomic number matters not because it counts protons per se, but because, in a neutral atom, it equals the number of electrons, the quantity that actually governs chemical behaviour. It just happens that the number of protons remains unchanged when atoms become ionised. Because isotopes of the same element have the same number of electrons in their neutral state (and the same proton number), they exhibit identical chemical reactivity and are regarded as the same element in classical chemistry.

In nuclear chemistry, however, each isotope has its own half-life (stability) and its own probabilities for absorbing or emitting alpha, beta, or gamma radiation, as well as other nucleons. Each isotope also has distinct tendencies toward fission or fusion and follows specific decay pathways. For this reason, nuclear chemistry is governed by both proton and neutron numbers, and under this perspective each isotope can be considered a different simple substance. In this framework, even the free neutron (sometimes referred to as “neutronium”) is treated as a simple substance, with its own half-life and capacity to interact with radiation and nucleons. However, the neutron is not an element in classical chemistry, since it has no electrons to participate in chemical bonding. This is the reason why the Segrè table of nuclides is used in nuclear chemistry, where each isotope (and the neutron) is depicted as a distinct entity.

The broader point is that what we call a “simple substance” is relative to the domain of study. In classical chemistry, a simple substance is defined by atoms that, in their neutral state, have the same number of electrons, hence isotopes are considered the same element. In nuclear chemistry, where protons and neutrons are the relevant particles, each isotope constitutes a different simple substance, and this also applies for the neutron. In other words, isotopes form a single simple substance in classical chemistry, but distinct simple substances in nuclear chemistry.

If you think this perspective is useful, I can send it as a formal reply.

Best,

Mario RP

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[Mario Rodriguez]

PS: Perhaps an example in lay terms helps clarify my point. Protium and tritium are both hydrogen and form chemical bonds in the same way, so in classical chemistry they are the same simple substance. However, protium is stable, whereas tritium is radioactive and can cause certain materials to glow; it is unstable and requires radiation safety precautions when handled. From a nuclear chemistry perspective, therefore, they are different simple substances.

To view this discussion visit https://groups.google.com/d/msgid/PT-L/493260566.1363322.1771237155970%40mail.yahoo.com.

[ERIC SCERRI]

Thanks for your comments Mario. 

This may be a useful point you raise but first let’s discuss it further. 

You appear to be drawing a distinction between how simple substance is regarded in classical as opposed to nuclear chemistry. 

I’m not sure that’s the case. 

In the literature on this topic the doctrine that one element encompasses many simple substances applies equally well to what you are calling classical chemistry. For example consider the allotropes of any given element. 

please explain your main point further. 

Eric Scerri

On Feb 16, 2026, at 2:36 AM, Mario Rodriguez <mavo...@yahoo.es> wrote:



[Eric Scerri]

Thanks for your suggestion and comments, John.

On Feb 15, 2026, at 8:54 AM, johnmarks9 <johnm...@hotmail.com> wrote:

Dear Eric, 

I humbly suggest the word "essence" (from Latin esse "to be", "to exist") may be what you (and Doug Templeton) are looking for, for Mendeleyev´s "basic substance" and "element" for Lavoisier´s "simple substance".

The trouble is that “element” was originally used, and has has traditionally been used, for the abstract sense rather than the concrete sense.

Given a choice of which of the two senses the word element should be associated, I believe most people who have discussed this issue would opt for the abstract sense.

"Substance" applies to anything, usually solid, from wood to lava and Lavoisier chose "element" as first principle of matter, following Boyle and the Ancients. Lavoisier used "element" in this sense, replacing the five elements of antiquity.

If we need to go etymological, please bear in mind that the literal meaning of substance is that which lies underneath.  So in one sense “substance" could be used to designate the abstract element which lies beyond, or under, the more concrete and mundane sense.

Terminology has been a big issue in the literature.

I used the words simple substance and basic substance because they are translations of Einfacherstoff and Grundstoff used by Paneth who revived the issue some 100 years ago.  The English translation has generally been attributed to his son Heinz Paneth (later Heinz Post and my PhD advisor in London).

But as I learned from an article by Conal Boyce, a former member of this forum, these words first came from Paneth’s daughter Eva Paneth who translated an earlier Paneth article on “element".

Conal Boyce’s article is well worth reading and readily available online from the now defunct journal Hyle,

Boyce suggests using the words substance     and      element respectively for

                                                     Einfacherstoff.  and     Grundstoff

[Larry T.]

Unlike grains of sand for example, all atoms of the same isotope are absolutely indistinguishable. In the microworld environment, stating that a certain phosphorus atom in DNA is replaced by another phosphorus atom has no meaning, unless it was a different isotope. The phosphorus in DNA is just that, phosphorus. You can count those atoms, but any of them could be at any location in that DNA strand at any time without us being able to tell any difference. In that sense, abstract and real notions merge in the microworld. The words "atom" and "element" become redundant unless the word "element" means all isotopes of an atom.

Best Regards,

V. "Larry" Tsimmerman

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In fact almost every author has suggested an alternative terminology.  I was one talked into using the terms element-1 and element-2 by the then editor of Philosophy of Science, Noretta Koertge.

The late Joe Earley one said “simple substance” as used by Paneth is neither simple, nor a substance.

Here is what he suggested for terminology.

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          And so it goes on.

I personally quite like essence for the abstract sense of element but then people will object by saying that this implies buying into the philosophy of essentialism, which would open up another can of worms.

To conclude, I don’t think the terminology matters all that much.  What does, is that chemistry has a metaphysical underbelly, a metaphysical aspect which admits such concepts as the abstract nature of elements.  This is something that ‘regular chemists’ find difficult to appreciate since they operate on a highly empiricist basis.  This is why the philosophy of chemistry has taken so long to get off the ground.

Most chemists dont see the value of philosophy of chemistry.

The person who responds to my article in Chemistry International is basically saying that we don't need to include talk of the abstract nature of elements in the Gold Book definition.  

I enclose the preface and introductory chapter from the book I co-dited, entitled, “What is a Chemical Element” which contains 14 chapters, each with its own particular take on this question.

 

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Eric Scerri

The essence of an element is that which is essential wherever that element is found in compounds, allotropes, isotopes or, indeed, anywhere. It seems necessary to retain "element" as distinct from "substance" since all substances are composed of elements but elements are distinctive substances, a natural kind, each composed of identical atomic number.

So "simple substance" is renamed "element"

"Basic substance" is renamed "essence" or ´elemental essence´ or ´essence of the element` under discussion.

In general, "hydrogen" would refer to the element and "H" to its essence.

Regards, 

John

On Saturday, February 14, 2026 at 7:39:57 PM UTC+1 scerri wrote:

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Eric Scerri PhD

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[Eric Scerri]

Is this an application of Pauli’s Principle to entire atoms?

Are you sure this is kosher, Larry?

What is the basis for your claim?  I’m not disputing it, but wanting to know on whose authority you are claiming it.

Incidentally, I was reminded of your periodic table within a glass cube a few days ago by Mark Leach.

What a wonderful piece of work!

Eric

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Eric Scerri PhD

[Eric Scerri]

Just a small correction to my attempt to quote Joe Earley from memory

I think what he said was “a basic substance is neither basic, nor a substance.”

He would surely have accepted that Lavoisier’s elements are substances.


Eric

[Larry T.]

Thank you, Eric.

I base my claim on the Gibbs paradox, or the mixing paradox and the indistinguishability of atoms.

See this PDF: https://bayes.wustl.edu/etj/articles/gibbs.paradox.pdf.

 If two containers A and B separated by a diaphragm are filled with the same gas under the same pressure and temperature, removing the diaphragm does not increase entropy, unlike in the similar setup with different gases where entropy would increase.

Similarly, this would occur in DNA, for example, if atoms of phosphorus switch places there would be no change. The change would occur if those were different phosphorus isotopes. 

Best Regards,

V. "Larry" Tsimmerman

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[Mario Rodriguez]

Hi Eric,

I used the term “classical chemistry” to refer to chemistry based on bond formation and electronic configuration, because I couldn’t find a better adjective. I’m happy to adopt a more appropriate term if you have a suggestion.

Allotropes fall within this “classical chemistry” because their differences arise from how bonds are arranged, while the atoms themselves are identical (same electrons and same nucleons).

In contrast, isotopes differ in an intrinsic property of the atom: the number of nucleons, due to different numbers of neutrons. As mentioned in my previous email, a different neutron count does not change the number of electrons, so isotopes have essentially the same bonding behaviour and very similar chemical reactivity. For classical chemistry, they are treated as the same simple substance.

However, isotopes can have very different properties (due to their nuclei): different stability, different radioactive decay modes, different propensities for fission or fusion, and different reactivity with other nucleons and nuclides. These differences lead to distinct behaviours and applications in nuclear chemistry. A couple of examples:

1. Protium vs. Tritium (both hydrogen, similar chemical reactivity)

In nuclear chemistry, protium (¹H) is stable, while tritium (³H) undergoes spontaneous beta decay:

³H → ³He + e⁻ + ν̄ (half-life: 12.32 years)

Because of this intrinsic atomic/nuclear difference, tritium is radioactive, requires special handling, and decays over time into helium-3. None of these properties apply to protium. This radioactive decay can be exploited to excite a phosphor in a sealed vial to produce self-powered light sources (“Betalights”). Filling the same device with protium would not work, because protium does not undergo the same spontaneous nuclear reaction.

So for a manufacturer of such devices, protium and tritium are clearly not the same simple substance, and the difference lies in an intrinsic nuclear property of the atom.

2. ²³⁸U vs. ²³⁵U (both uranium, similar chemical reactivity)

These two isotopes undergo different nuclear decay reactions:

²³⁸U → ²³⁴Th + ⁴He (α particle), half-life = 4.46 × 10⁹ years

²³⁵U → ²³¹Th + ⁴He (α particle), half-life = 7.04 × 10⁸ years

They also differ strongly in their nuclear reactivity with neutrons:

Plutonium production from ²³⁸U:

²³⁸U + 1n → ²³⁹U + γ

followed by

²³⁹U → ²³⁹Np + e⁻ + ν̄ (half-life 23.5 min)

²³⁹Np → ²³⁹Pu + e⁻ + ν̄ (half-life 2.35 days)

Fission of ²³⁵U (basis of nuclear reactors):

²³⁵U + n → ¹⁴¹Ba + ⁹²Kr + 3 1n

This is why uranium must be enriched in ²³⁵U for use in nuclear reactors, and why the presence of this isotope raises proliferation concerns. From this perspective, if you run a nuclear reactor, ²³⁸U and ²³⁵U wouldn´t be interchangeable substances.

In summary, isotopes have essentially the same chemical behaviour and bonding because they have the same number of electrons (same element in "classical chemistry"), but they can have dramatically different properties and applications due to an intrinsic difference of the atom, a different number of nucleons due to a different number of neutrons (not merely different bonding arrangements, as in allotropes). This is why nuclear chemists work with the Segrè chart of nuclides rather than the periodic table: each isotope (and neutron) is treated as a distinct simple substance with its own stability and reactivity.

Best,

Mario Rodríguez Peña

[Eric Scerri]

Thanks for the clarification, Mario.

I think that what you are saying is rather familiar to chemists, who form the readership of 

Chemistry International, so I’m not sure that a note to them would be of great value, if that’s what you meant.

The central question is whether thinking about elements abstractly has any value in chemistry in general, and whether this should be included in the Gold Book definition, since it is not as things stand at present.

Eric

Eric Scerri PhD

<Screenshot 2023-08-28 at 9.24.32 AM.png>

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[Mario Rodriguez]

Thank you for your quick response.

My conclusion is that grouping all isotopes under a single element makes sense within classical chemistry, because neutral atoms have the same number of electrons and therefore the same bonding behaviour and a similar chemical reactivity.

However, this grouping does not make sense in nuclear chemistry, where each isotope (and the neutron) is treated as a distinct simple substance, with its own stability and nuclear reactivity. From that perspective, isotopes effectively behave as different “elements” with different nuclear reactions.

In other words, what is considered a simple substance or element is relative to the domain of study. There is no single, fully objective definition of “element” that applies in the same way to both "classical" and nuclear chemistry.

Best regards,

Mario RP

https://groups.google.com/d/msgid/PT-L/1472A870-1A25-4524-ABB9-10EB869F16CC%40gmail.com

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[Mario Rodriguez]

PS: Neutron would be an element for nuclear chemistry but not for "classical chemistry".

https://groups.google.com/d/msgid/PT-L/1460541201.1873234.1771282951957%40mail.yahoo.com

.

[ERIC SCERRI]

Nobody is asking for a single definition. Even The Gold book attempts to give 2. 

I think we are talking past each other. Let’s take a break maybe. 

Eric

On Feb 16, 2026, at 3:05 PM, 'Mario Rodriguez' via Periodic table mailing list <PT...@googlegroups.com> wrote:



Eric Scerri PhD

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[johnmarks9]

Dear Eric,

Thanks for the thoughtful reply and the interesting excerpt from "What is an Element?"

Before tackling the question, it is remarkable that a word "element" having two distinct meanings, is an issue at all. English manages without debate "sole" as having three meanings: 1) single, 2) a fish  (Solea solea) and 3) the underside of the foot or of a boot worn on a foot.

So Earley agreed that Paneth´s "basic substance" was neither basic nor a substance and agreed that Lavoisier´s elements are substances - and that they are simple in the sense that they are not compound. I sympathize with the "jobbing chemist" in his disdain for what seem to him scholastic quibbles but I am impressed with  Philip Ball´s foreword to "What is an Element?"

The past is a foreign country and each generation must find its own words or repurpose older ones. While "essence" has whiffs of the "essentialism" of classical philosophy, modern usage in ´the essence of beauty, of courage, of love´, etc., etc., is more metaphorical and eschews any material basis, concentrating on behaviour. Essences are noumenal in the Kantian sense, which was  Mendeleyev´s argument.

In Mendeleyev´s and Paneth´s distinction of "basic substance", they are highlighting the surprising behaviour of atoms, especially in compounds but also as allotropes or in their subtle variations as isotopes. In this sense, the modern usage of essence corresponds better to the idea of basic substance. By contrast, modern usage of "element", exemplified by collections of samples in display cases, corresponds to modern usage of "simple" as uncomplicated, not compound and therefore to Lavoisier´s simple substances.

Ball is certainly right that this discussion is useful, sometimes essential, in an ever-changing world. But it is often the meaning of words that shift and, whatever they meant in past times, essence now means the idea of something unchanging (in this case Moseley´s Z) whether in elements or compounds.

Elements are the final attainment of chemical analysis (Lavoisier). Atomic number remains the essential constant, unchanged in a chemical reaction (Moseley´s update of Mendeleyev).

Dalton presciently noted this.

In your answers to the  "Open Questions Concerning the Two Senses of Element", in your answer to the first, "How Are the Two Senses of Element Related to Each Other?",  you write  "Perhaps element-2 [Grundstoff] can be thought of as subsisting within element-1[einfacher Stoff] in the sense that the essence of an element might be contained within the more tangible and sensible aspect of element, or element-1 . . ."  You are using "essence" in its current sense!

In answer to the second question, you aver that Lavoisier continued to believe in the abstract concept of element as the bearer of properties or as a principle. This is von Hartmann´s transcendental realism espoused by Hendry. It also corresponds to current usage of "essence".

H is the essence of the element hydrogen. Q.E.D.

John

[ERIC SCERRI]

On Feb 17, 2026, at 4:00 AM, johnmarks9 <johnm...@hotmail.com> wrote:

Dear Eric,

Thanks for the thoughtful reply and the interesting excerpt from "What is an Element?"

Before tackling the question, it is remarkable that a word "element" having two distinct meanings, is an issue at all. English manages without debate "sole" as having three meanings: 1) single, 2) a fish  (Solea solea) and 3) the underside of the foot or of a boot worn on a foot.

Hi John,

Thank you for an equally thoughtful response.

So Earley agreed that Paneth´s "basic substance" was neither basic nor a substance and agreed that Lavoisier´s elements are substances - and that they are simple in the sense that they are not compound. I sympathize with the "jobbing chemist" in his disdain for what seem to him scholastic quibbles but I am impressed with  Philip Ball´s foreword to "What is an Element?"

The past is a foreign country and each generation must find its own words or repurpose older ones. While "essence" has whiffs of the "essentialism" of classical philosophy, modern usage in ´the essence of beauty, of courage, of love´, etc., etc., is more metaphorical and eschews any material basis, concentrating on behaviour. Essences are noumenal in the Kantian sense, which was  Mendeleyev´s argument.

Very nicely put.  I am more and more persuaded of “essence” to describe the more abstract sense of element, although I hesitate to introduce yet more terminology into the literature.  

I’m also working on a related project in which I want to connect the element discussion with Kripke’s essentialism.  

There is, not surprisingly, some debate as to whether Mendeleev was aligned to Kant. Some of the contributions to the volume I co-edited with Elana Ghibaudi take up this theme.

According to Paneth, an appeal to the essence of an element, as you prefer to say, is more aligned to Hartmann’s brand of transcendental idealism, than to a striagthforward Kantian philosophy of the phenomenal and the noumenal.  

In Mendeleyev´s and Paneth´s distinction of "basic substance", they are highlighting the surprising behaviour of atoms, especially in compounds but also as allotropes or in their subtle variations as isotopes. In this sense, the modern usage of essence corresponds better to the idea of basic substance. By contrast, modern usage of "element", exemplified by collections of samples in display cases, corresponds to modern usage of "simple" as uncomplicated, not compound and therefore to Lavoisier´s simple substances.

Agreed.

Ball is certainly right that this discussion is useful, sometimes essential, in an ever-changing world. But it is often the meaning of words that shift and, whatever they meant in past times, essence now means the idea of something unchanging (in this case Moseley´s Z) whether in elements or compounds.

Elements are the final attainment of chemical analysis (Lavoisier). Atomic number remains the essential constant, unchanged in a chemical reaction (Moseley´s update of Mendeleyev).

Dalton presciently noted this.

In your answers to the  "Open Questions Concerning the Two Senses of Element", in your answer to the first, "How Are the Two Senses of Element Related to Each Other?",  you write  "Perhaps element-2 [Grundstoff] can be thought of as subsisting within element-1[einfacher Stoff] in the sense that the essence of an element might be contained within the more tangible and sensible aspect of element, or element-1 . . ."  You are using "essence" in its current sense!

Yes indeed.

In answer to the second question, you aver that Lavoisier continued to believe in the abstract concept of element as the bearer of properties or as a principle. This is von Hartmann´s transcendental realism espoused by Hendry.

Yes, but I would be most surprized if Robin Hendry does indeed support this view.  

Can you provide a reference?  I have had many conversations about this issue with him.  

Also some published debate if you are interested in seeing it?

It also corresponds to current usage of "essence".

Yes, I’m all for essence as a way of thinking about the fundamental sense of element but as explained above maybe not in writing.  

Regards

Eric Scerri

P.S.  Please consider writing a response to the IUPAC magazine since regular chemists need convincing of the value of this question and of thinking of elements in an essential sense.  The long response published after my piece boils down to saying, “why bother”.  

H is the essence of the element hydrogen. Q.E.D.

John

On Monday, February 16, 2026 at 4:42:40 PM UTC+1 ericscerri123 wrote:

Thanks for your suggestion and comments, John.

On Feb 15, 2026, at 8:54 AM, johnmarks9 <johnm...@hotmail.com> wrote:

Dear Eric, 

I humbly suggest the word "essence" (from Latin esse "to be", "to exist") may be what you (and Doug Templeton) are looking for, for Mendeleyev´s "basic substance" and "element" for Lavoisier´s "simple substance".

The trouble is that “element” was originally used, and has has traditionally been used, for the abstract sense rather than the concrete sense.

Given a choice of which of the two senses the word element should be associated, I believe most people who have discussed this issue would opt for the abstract sense.

"Substance" applies to anything, usually solid, from wood to lava and Lavoisier chose "element" as first principle of matter, following Boyle and the Ancients. Lavoisier used "element" in this sense, replacing the five elements of antiquity.

If we need to go etymological, please bear in mind that the literal meaning of substance is that which lies underneath.  So in one sense “substance" could be used to designate the abstract element which lies beyond, or under, the more concrete and mundane sense.

Terminology has been a big issue in the literature.

I used the words simple substance and basic substance because they are translations of Einfacherstoff and Grundstoff used by Paneth who revived the issue some 100 years ago.  The English translation has generally been attributed to his son Heinz Paneth (later Heinz Post and my PhD advisor in London).

But as I learned from an article by Conal Boyce, a former member of this forum, these words first came from Paneth’s daughter Eva Paneth who translated an earlier Paneth article on “element".

Conal Boyce’s article is well worth reading and readily available online from the now defunct journal Hyle,

Boyce suggests using the words substance     and      element respectively for

                                                     Einfacherstoff.  and     Grundstoff

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[johnmarks9]

Dear Eric,

Kripke and Putnam argue that properties like atomic number constitute the "real essence" of a substance, not merely its superficial appearances. While essential properties are necessary, they are discovered empirically (e.g., "Water is H₂O

"), bridging the gap between metaphysical necessity and epistemological experience.  I find this convincing. While von Hartmann´s transcendental realism is attractive, I find his wider philosophy incoherent, especially his panpneumatism predicated on an unconsciousness. Paneth observed that Mendeleyev explicitly distinguished Lavoisier´s "simple body" (Kantian phenomenon) from the "element" (Kantian noumenon), that material part which persists through chemical reactions. But it appears that Mendeleyev retained a realist, material view of "essence", even though he called this the "element". Today, this terminology is reversed, with "element" being Lavoisier´s simple substance and "essence" being Mendeleyev´s element. It seems to me this switch of terminology has resulted in the confusion being debated.

Hendry espoused "downward causation" of chemistry on physics. Certainly emergent human consciousness can cause behaviours (such as human flight) otherwise inexplicable in terms of statistical physics. That wouldn´t quite correspond to von Hartmann´s "spirit" of course, since it´s conscious!

As for Templeton´s "why bother?" response, he concedes that chemists subscribe to "scientific realism". And essences of elements, as I am using the term, are real. Further, these essences do have properties, such as valency, weight, electronegativity, etc. Not only that, they exhibit emergent properties in elements (simple substances) and compounds. So Templeton is a transcendental realist! You are pushing at an open door: Templeton substantially agrees with you. In his usage, (1) is the essence of an element and (2) is the element.

John

[Mark Leach]

Hi John & All,

Can I be very, very pedantic and pick you up when you say "Water is H₂O”.

I would have had no issue if you had said “methane is CH4” or "carbon dioxide is CO2” as these are molecular materials with strong covalent bonds between the atoms. What we mean by the CH4 the molecule and methane the substance is clear; there is no ambiguity.

Liquid water is not ‘molecular’ like methane or carbon dioxide. If H2O and D2O are mixed, the system will rapidly and dynamically equilibrate to a mixture of HOH, HOD & DOD due to hydrogen bonding and the Grotthuss mechanism.

I am particularly attuned to this issue because at the philosophy of chemistry conferences I attend, the philosophers often cite “water” as a common, typical, example substance. Yes water is common, but it is so atypical it makes a very poor example substance when discussing philosophical issues.

Imho… 😀

Mark

Mark Leach

meta-synthesis

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[johnmarks9]

Point taken, Mark!

You are, of course, right. Water is indeed very special, probably because of the dual propensity of hydrogen to form H⁺ and H⁻.

This behaviour, to relate to the element/essence debate, is the essence of hydrogen.

John

[johnmarks9]

I should expand my last line to "This behaviour . . . is a property of  the essence of hydrogen" :)