The Definition Of Chemical Elements

[AdamF]

I don't get it - the definition of a chemical element is:

"Cannot be chemically interconverted or broken down into simpler substances"

-- But isn't every atom of an element made up of further sub-atomic particles, and atoms can be then broken apart, etc...?

Also, how did people tell the difference in the beginning between breaking down a compound structure into constituent "elements" and just forming a new compound?

 

[Drakkith]

-- But isn't every atom of an element made up of further sub-atomic particles, and atoms can be then broken apart, etc...?

Not 'chemically', which means interactions which result in the electromagnetic bonding between atoms, the exchange of electrons between atoms, or the addition/removal of electrons to an atom (as in ionization). To change an element into another element or break it apart involves two other forces which don't affect the things I said above, the weak nuclear force and the strong nuclear force. For chemistry we only care about the electromagnetic force.

Note that the definition you give of a chemical element is only one of several possible definitions. Another equally valid definition (from wikipedia) is: "a species of atom having the same number of protons in their atomic nuclei (that is, the same atomic number, or Z)."

Also, how did people tell the difference in the beginning between breaking down a compound structure into constituent "elements" and just forming a new compound?

I think it was that elements could no longer be broken down into anything else, whereas compounds could.

 

[AdamF]

-- EM is unified with the weak force, so I don't see how they're separated.

The bottom line question of what I'm really though, is, in terms of a definition, I've not seen anything which is of the like:

If I said "Tell me if something is an element or a compound", which specific actions would you take?

 

[Comeback City]

EM is unified with the weak force, so I don't see how they're separated.

The weak force does not affect chemistry, as @Drakkith mentioned... chemical reactions are solely affected by the electrostatic attractions, and the relative strength thereof, between atoms, molecules, ions, etc.

 

[Drakkith]

-- EM is unified with the weak force, so I don't see how they're separated.

They aren't unified at the energy scales you will ever deal with in chemistry and the vast majority of physics.

The bottom line question of what I'm really though, is, in terms of a definition, I've not seen anything which is of the like:

If I said "Tell me if something is an element or a compound", which specific actions would you take?

Do all of the atoms in the material have the same number of protons? If yes, then it's an element. If not, it's a compound.

 

[DrDu]

I don't get it - the definition of a chemical element is:

"Cannot be chemically interconverted or broken down into simpler substances"

-- But isn't every atom of an element made up of further sub-atomic particles, and atoms can be then broken apart, etc...?

Also, how did people tell the difference in the beginning between breaking down a compound structure into constituent "elements" and just forming a new compound?

The german wikipedia entry on Antoine Lavoisier contains a very nice part on the development of the understanding of what constitutes an element. Alas, it is absent in the english and french version.

https://de.wikipedia.org/wiki/Antoine_Laurent_de_Lavoisier#Elemente_und_Verbindungen

 

[TeethWhitener]

The weak force does not affect chemistry, as @Drakkith mentioned... chemical reactions are solely affected by the electrostatic attractions, and the relative strength thereof, between atoms, molecules, ions, etc.

Truth is stranger than fiction. The opposite of beta decay is electron capture, which is strongly dependent on electron density at the nucleus, which is affected by chemical environment. So, for instance, if you take a ⁷Be atom, which normally decays via electron capture to ⁷Li with a half life of 53 days, and put it in an electron-rich environment (like a C₆₀ cage), its half life will decrease to about 52.5 days:
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.93.112501

There are also much more dramatic examples of electronic structure interfering with nuclear structure, but they can only tenuously be called chemical processes. One of the more interesting ones is interconversion of ¹⁶³Ho and ¹⁶³Dy. Normally the decay proceeds from Ho to Dy via electron capture; however, if you completely ionize ¹⁶³Dy, it can undergo bound state beta decay back to ¹⁶³Ho.

 

[Comeback City]

Truth is stranger than fiction. The opposite of beta decay is electron capture, which is strongly dependent on electron density at the nucleus, which is affected by chemical environment. So, for instance, if you take a ⁷Be atom, which normally decays via electron capture to ⁷Li with a half life of 53 days, and put it in an electron-rich environment (like a C₆₀ cage), its half life will decrease to about 52.5 days:
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.93.112501

There are also much more dramatic examples of electronic structure interfering with nuclear structure, but they can only tenuously be called chemical processes. One of the more interesting ones is interconversion of ¹⁶³Ho and ¹⁶³Dy. Normally the decay proceeds from Ho to Dy via electron capture; however, if you completely ionize ¹⁶³Dy, it can undergo bound state beta decay back to ¹⁶³Ho.

I'll admit I did not even consider that, I'll have to learn some more about it...

 

[Comeback City]

but they can only tenuously be called chemical processes.

So the question thus becomes, at what point is a process no longer considered a chemical process? Is it at the point where decay occurs, or can decay be considered a chemical process of some sort as well?

 

[Borek]

at what point is a process no longer considered a chemical process?

The very idea of existence of separate physical and chemical processes is flawed. There is a continuum. As usual, nature laughs at our attempts at putting things into non-existing boxes.

(In other words: don't bother too much with this classification. You will always find examples that won't fit.)

 

[AdamF]

The weak force does not affect chemistry, as @Drakkith mentioned... chemical reactions are solely affected by the electrostatic attractions, and the relative strength thereof, between atoms, molecules, ions, etc.

Okay, so "Chemistry" is kind of inside this philosophical boundary where the forces that hold the actual atoms together are ignored and we're only concerned with the interactions between them, and the "elements" of Chemistry are these kind of discrete collections of sub-atomic particles which are not able to be further decomposed or separated into other things which "look" like them when it comes to feeling the ElectroMagnetic force?

Is that the basic idea?

 

[Comeback City]

Okay, so "Chemistry" is kind of inside this philosophical boundary where the forces that hold the actual atoms together are ignored and we're only concerned with the interactions between them, and the "elements" of Chemistry are these kind of discrete collections of sub-atomic particles which are not able to be further decomposed or separated into other things which "look" like them when it comes to feeling the ElectroMagnetic force?

Is that the basic idea?

I wouldn’t say this is a philosophical thing, just that the terminology we use tends to limit chemistry to electromagnetic attractions. @Borek ‘s post above sums that up well.

 

[AdamF]

I guess linguistic is probably a better word.

 

[James Pelezo]

Not to over simplify, but if one is trying to introduce a student to the world of atoms and elements the basic premise is 'keep it simple'. Right or wrong, a foundation is established and as one progresses, then add the esoteric issues that further define atomic and/or molecular structure.

Knowing that atoms/elements & molecules/compounds fall under a general classification of 'substances' a popular definition found in many chemistry texts is ...

An element is the smallest entity of a substance having characteristic chemical and physical properties that can not be broken down or changed into different substances by ordinary chemical or physical means; e.g., heat, light or electricity. The definition of molecules differs in that they are the smallest entity of a substance that can be broken down into constituent components by application of ordinary chemical or physical means. These 'simplified' definitions lend themselves well to demonstrations showing the difference between elements and molecules; e.g., flame ionization of metal salts showing color generation and spectrum consistency vs electrolysis of water giving molecular hydrogen and oxygen. Much easier to add explanations once completed. Just an opinion. :-)

 

[AdamF]

Not to over simplify, but if one is trying to introduce a student to the world of atoms and elements the basic premise is 'keep it simple'. Right or wrong, a foundation is established and as one progresses, then add the esoteric issues that further define atomic and/or molecular structure.

Knowing that atoms/elements & molecules/compounds fall under a general classification of 'substances' a popular definition found in many chemistry texts is ...

An element is the smallest entity of a substance having characteristic chemical and physical properties that can not be broken down or changed into different substances by ordinary chemical or physical means; e.g., heat, light or electricity. The definition of molecules differs in that they are the smallest entity of a substance that can be broken down into constituent components by application of ordinary chemical or physical means. These 'simplified' definitions lend themselves well to demonstrations showing the difference between elements and molecules; e.g., flame ionization of metal salts showing color generation and spectrum consistency vs electrolysis of water giving molecular hydrogen and oxygen. Much easier to add explanations once completed. Just an opinion. :-)

James,

Thanks for your explanation -- I'm trying to understand what these things actually mean in practice as in how would a person understand this in a way that allows them to construct these concepts from observation and phenomena, as they originally were.

 

[James Pelezo]

For the moment, consider an interest in whether or not a substance (an arbitrarily chosen white powder) is an element or compound. What's a simple way to ID this. In early scientific examinations, flame testing would be high on the experimental list. When applied, many substances gave reproducible color patterns that could be classified and applied and used as standards for ID purposes. Early assumptions/conclusions were limited to the 'naked eye standard' for identifying and differentiating between substances as elements or compounds. Reproducible results were everything in the 18th - 19th century experimental world. For elements, it was generally known that when an element is vaporized in a flame the emission spectrum will appear to be just one color. For example, lithium compounds when excited by flame test appear as a lavender-red color. We know today that this one color results from a combination of all lines of the emission spectrum, in proportion to their intensities. As many elements will still produce distinctive colors under such conditions, simple flame tests can be used to identify these elements. In fact, flame tests were used to identify elements long before the invention of modern techniques, such as emission spectroscopy. At the time of John Dalton (late 18th century - early 19th century), no defined understanding of atomic structure was available in the scientific community. Thus, structure identification was based upon 'fingerprint' properties of matter. At that time, one simply couldn't say this is an element or that is a compound with the detail descriptive accuracy we have today. So, catalogs of physical properties for various substances were compiled from macroscopic observations that would provide reproducible 'fingerprint' reference of a material of interest. Flame tests of a series of salts gave characteristic color emissions that were consistent and reproducible for the purpose of suggesting that a substance contained a specific type element. That is, regardless of the salt, the flame test always gave the same color emission. However, without the sophistication we have today, only a generalization that the same 'stuff' can exist in different combinations giving 'other stuff' that had different properties between them. Confusing? Of course, and it is then that the stage is set for applying the scientific method to make further observations, note trends and add new pieces to the jig-saw puzzle that, over time and study, leads to the more esoteric conclusions we have today of atomic and molecular structure. Great question, keep um coming. :-)

 

[DrDu]

I don't think this does full justice to the historical development. Early scientists, like Dalton, Lavoisier, Davy, Berzelius or Bunsen relied heavily on very precise quantitative metronomic techniques to determine especially the volume and weight of the substances they where handling. Berzelius had a sign over the door of his lab stating that "God ordered everything according to number, volume and weight". This list soon included also coulometry.
E.g. the element germanium was found by Clemens Winkler because he found a discrepancy of only some percent in the analysis of a germanium containing mineral.

 

[AdamF]

For the moment, consider an interest in whether or not a substance (an arbitrarily chosen white powder) is an element or compound. What's a simple way to ID this. In early scientific examinations, flame testing would be high on the experimental list. When applied, many substances gave reproducible color patterns that could be classified and applied and used as standards for ID purposes. Early assumptions/conclusions were limited to the 'naked eye standard' for identifying and differentiating between substances as elements or compounds. Reproducible results were everything in the 18th - 19th century experimental world. For elements, it was generally known that when an element is vaporized in a flame the emission spectrum will appear to be just one color. For example, lithium compounds when excited by flame test appear as a lavender-red color. We know today that this one color results from a combination of all lines of the emission spectrum, in proportion to their intensities. As many elements will still produce distinctive colors under such conditions, simple flame tests can be used to identify these elements. In fact, flame tests were used to identify elements long before the invention of modern techniques, such as emission spectroscopy. At the time of John Dalton (late 18th century - early 19th century), no defined understanding of atomic structure was available in the scientific community. Thus, structure identification was based upon 'fingerprint' properties of matter. At that time, one simply couldn't say this is an element or that is a compound with the detail descriptive accuracy we have today. So, catalogs of physical properties for various substances were compiled from macroscopic observations that would provide reproducible 'fingerprint' reference of a material of interest. Flame tests of a series of salts gave characteristic color emissions that were consistent and reproducible for the purpose of suggesting that a substance contained a specific type element. That is, regardless of the salt, the flame test always gave the same color emission. However, without the sophistication we have today, only a generalization that the same 'stuff' can exist in different combinations giving 'other stuff' that had different properties between them. Confusing? Of course, and it is then that the stage is set for applying the scientific method to make further observations, note trends and add new pieces to the jig-saw puzzle that, over time and study, leads to the more esoteric conclusions we have today of atomic and molecular structure. Great question, keep um coming. :-)

That makes more sense, seems the only scientific way to explain it.

So the definition an "element" is operational (as in what does the "stuff" do it do when we do X or Y to it, for example), and all of the rest of the ways that we write or use this word are just sociological convention/agreed upon terminology to reflect these experiences of what happens when we mess w/ various "stuff" in the ways described in your post?

 

[Comeback City]

So the definition an "element" is operational (as in what does the "stuff" do it do when we do X or Y to it, for example), and all of the rest of the ways that we write or use this word are just sociological convention/agreed upon terminology to reflect these experiences of what happens when we mess w/ various "stuff" in the ways described in your post?

@Borek summed it up rather well...

The very idea of existence of separate physical and chemical processes is flawed. There is a continuum. As usual, nature laughs at our attempts at putting things into non-existing boxes.

(In other words: don't bother too much with this classification. You will always find examples that won't fit.)

There's certainly going to be overlap in any areas of science, and in the case of chemistry, there is certainly an overlap between EM and weak force, as @TeethWhitener explained with examples of electron density.

 

[James Pelezo]

I don't think this does full justice to the historical development. Early scientists, like Dalton, Lavoisier, Davy, Berzelius or Bunsen relied heavily on very precise quantitative metronomic techniques to determine especially the volume and weight of the substances they where handling. Berzelius had a sign over the door of his lab stating that "God ordered everything according to number, volume and weight". This list soon included also coulometry.
E.g. the element germanium was found by Clemens Winkler because he found a discrepancy of only some percent in the analysis of a germanium containing mineral.

No argument about the accuracy applied, but the original question was how does one differentiate between elements and non-elements... All of the 'giants of science' had to start with some simple, fundamental observations that stirred their interest. Then, as I stated in my comment, applied the more esoteric details to better understand the nature of the subject of interest. It was not my intent to minimize the intense efforts applied by the scientist across history to obtain accurate measurements.

 

[symbolipoint]

Once Upon A Time, we studied and read textbooks to learn definitions of things like this. We were happy. Are Chemistry textbooks no longer good for things like this?

 

[AdamF]

Once Upon A Time, we studied and read textbooks to learn definitions of things like this. We were happy. Are Chemistry textbooks no longer good for things like this?

I don't think they are, no.

Raise more questions than they answer.

 

[DrDu]

Lavoisiers "traité élémentare de chimie" from 1789 is available online. I think it is a great resource to understand how the concept "element" evolved:
https://fr.wikisource.org/wiki/Traité_élémentaire_de_chimie

 

[Mark44]

Once Upon A Time, we studied and read textbooks to learn definitions of things like this. We were happy. Are Chemistry textbooks no longer good for things like this?

I don't think they are, no. Raise more questions than they answer.

I think I'm safe in saying that every university chemistry class has a textbook that students are required to get. So your opinion about chem textbooks doesn't seem valid to me -- if the textbooks weren't useful, the instructors wouldn't require their students to get them.

 

[Comeback City]

I think I'm safe in saying that every university chemistry class has a textbook that students are required to get. So your opinion about chem textbooks doesn't seem valid to me -- if the textbooks weren't useful, the instructors wouldn't require their students to get them.

Textbooks are definitely useful... that shouldn't be in question. But... textbooks often leave students with questions that are not directly answered within the text. For instance, the author may make assumptions the reader did not pick up on, since those assumptions were not explicitly written. And that's why many students come to the forums to begin with!