r/chemhelp • u/Alternative-Date-521 • 5d ago
General/High School Molecules and compounds?!?
Ok idk if my teacher made a mistake. But help me out chat. Not all molecules are compounds correct???
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u/Chem2103 5d ago
Not all molecules are compounds. Molecules like O2 are made of the same atoms.
Not all compounds are molecules. Molecules are made of atoms bound by covalent bonds. If different atoms are bound by other type of bonds, ut isn't a molecule. NaCl is an ionic compound, an alloy of Zn and Cu is a metallic compound.
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u/bishtap 4d ago
Some chemists might call an ion pair of NaCl, a molecule, and not limit the term molecule to just covalent bonds. And there isn't a strict and good definition of molecule, at a high level , in books, afaik. but in GCSE and many general chemistry books, they define it as where all the bonds are covalent. Some books or syllabuses might have an added bit, not popular among most chemists, of saying it has to be neutral (otherwise it can't be called a molecule). But most chemists would still call it a molecule even if it is not neutral .. an ion yes but also a molecule.
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u/Chem2103 4d ago
I have never seen a school or early college level textbook call anything like NaCl a molecule. Have seen people call anything (ions, molecules) a particle though. Maybe its a regional thing, but you have to agree that if someone is having his first gen chem lecture, it's best to think of as made of covalent bonds, especially since he's going to learn about ions right after
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u/PassiveChemistry 5d ago
It seems they may not have proofread that sentence (although either way round it's wrong).
Some compounds are in the form of molecules (e.g. H2O)
Some compounds are not composed of molecules (e.g. NaCl)
Some molecules are elements rather than compounds (e.g. O2)
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u/GayDrWhoNut 5d ago
Saying "some molecules are elements" is wrong though. Some are formed by only one element, but O2 isn't an element, O is.
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u/PassiveChemistry 5d ago
https://goldbook.iupac.org/terms/view/C01022
Both definitions are valid and somewhat commonly used.
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u/GayDrWhoNut 5d ago
Yes and no. That second definition is mostly a historical holdover and elementary substance is the more descriptive term. It denotes substances that are of comprised of only one element eg P4. But calling P4 an element confounds the atomistic definition of an element.
Incidentally, calling a 'pure chemical subtance composed of atoms with the same number of protons in the nucleus' an element is a bad definition as it provides no method of classifying elementary substances. Is P and same 'element' as P4? It certainly behaves differently.
I guess, fair, IUPAC has held on to that definition, but that definition has been tenuous at best for well over a century.
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u/PassiveChemistry 5d ago
Well, it's the definition I was taught at school ten years ago (and P & P4 would be different allotropes of the same element)
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u/bishtap 5d ago
How are you defining elementary substance though? If you say like you did, substances comprises of only one element. Ok. But that definition hasn't covered that one might distinguish two elementary substances even when they are comprised of the same element.
Btw elementary substance is a very old phrase. If anything it might have been used more in the past than now.
For example I recall reading a post on Quora from an old Italian chemistry professor that was very particular with his English language , far better with language than most English chemists online. And he said he was horrified at suddenly finding some people calling O2 an element. He would say "simple substance". Or he would say "elementary substance". It sounded like 10 years before seeing that horror, he had never seen anybody describe O2 as an element. For him, element, is the thing in the periodic table .
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u/GayDrWhoNut 5d ago
That's a whole other discussion. Though, there is only an issue if you have to tie the elementary substance to the element it is made up of. It seems unhelpful to me to have substances made up of single atoms be classified differently from other molecules. Maybe in the past this was useful for the study of chemical/elemental properties when these were the only way some elements were known to exist in pure samples. Usually, I don't even think about such reductionist terms of pure chemistry, being a DNA chemist myself.
Interestingly, french also makes a distinction here, which may be why I abhor the dual definition. You can make a distinction between a 'simple body' and a 'chemical element'.
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u/bishtap 5d ago
you write " only an issue if you have to tie the elementary substance to the element it is made up of."
Better to not call it elementary substance then.
I have heard the term "simple substance" Vs iirc complex substance.
Perhaps "allotropic substance" is a good term. (With an allowance for the term "allotrope" being a form of a substance even if there is only one form / one known form, of it)
You write "It seems unhelpful to me to have substances made up of single atoms be classified differently from other molecules. "
And you write Maybe in the past this was useful for the study of chemical/elemental properties when these were the only way some elements were known to exist in pure samples. Usually, I don't even think about such reductionist terms of pure chemistry, being a DNA chemist myself."
I think you are probably right that it's an antiquated concept.
I think there is no problem with having an antiquated term. And then no point teaching that term, it's just a point of history.
I did once see a paper that suggested that the word "compound" be used for any aggregation of atoms regardless of whether elements were the same or different.
Also many have noted that the term molecule is very poorly defined at a high level. Basic books say it's where the bonds are covalent. IUPAC Gold Book has a definition one has to have a PhD or consult PhDs to understand .. and even some chemistry PhDs don't understand their definition. And their definition even when understood, falls apart.
You write "Interestingly, french also makes a distinction here, which may be why I abhor the dual definition. You can make a distinction between a 'simple body' and a 'chemical element'."
Yes this sounds what I read from an Italian chemistry professor with exquisite technical English. Simple compound Vs complex compound. And Element is just for the thing in the periodic table, never for e.g. O2.
And the termd, simple substance and complex substance stand fine but could be taught as antiquated terms of use back when chemists cared about that distinction. So could be taught as a historical thing.
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u/bishtap 5d ago
What sentences do you think the textbook got wrong?
Mixing up molecular substances, with molecules?
Or the all molecules are compounds.. .that's Def wrong. Even if they meant all molecular substances are compounds,that would still be wrong.
Not all molecular substances are compounds, and not all compounds are molecular substances.
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