Talk:Molecule

From Wikipedia, the free encyclopedia

This article is within the scope of WikiProject Physics, which collaborates on articles related to physics.

Physics Portal

???

This article has not yet received a rating on the assessment scale. [FAQ]

???

This article has not yet received an importance rating within physics.

Help with this template

Please rate this article, and then leave comments here to explain the ratings and/or to identify the strengths and weaknesses of the article.

This article is within the scope of WikiProject Chemistry, which collaborates on Chemistry and related subjects on Wikipedia. To participate, help improve this article or visit the project page for details on the project.

B	This article has been rated as B-Class on the quality scale.
Top	This article has been rated as Top-importance on the importance scale.

1 Archives
2 collective, collectiion and diamond
3 Intro definition
4 History of the molecule

[edit] Archives

Talk:Molecule/Archive1 ('04)-Jun('06)

[edit] collective, collectiion and diamond

In the introductory paragraph, aggregate (a total considered with reference to its constituent parts) seems quite a bit more suitable than collection (an accumulation; a deposit) or even collective (this last one was masked vandalism).

As for the diamond "molecule": the term molecule refers to a well-defined particle of a chemically pure substance. Atomically pure diamond yields no well-defined molecular particle anyone could point to. Moreover, a common diamond crystal will always carry lattice and surface defects that make its local chemical properties different than the bulk. Even the most perfect diamond crystal will have surface terminal groups (hydrogens, hydroxyls, carbonyls, etc.) and so it will not be a pure chemical substance overall. It will be a covalent structure, though.

--Unconcerned 07:08, 13 September 2006 (UTC)

Let me be devil's advocate. Assume a perfect crystal with no dislocations. That's possible, and it's even possible on quite large scales. The same is true for single polymer molecules, like the DNA making up a chromosome. Thus, you have one big molecule. As for "pure chemical sustance", this one molecule may be the only exemplifier of whatever chemical substance it is. If so, the word "pure" has no meaning. "Pure" applies when we're considering copies, but we might not, and nothing says we must. Any single molecule is by definition pure. If it's one molecule, and it has a formula, it's a bonafide chemical substance. There may not be another molecule in the universe like it, with the exact same formula. So? That might be true of any of your X chromosomes if they happen to have a unique mutation. Again, so? What is or isn't a molecule is a matter of aesthetics, not science. If they get too big, people don't like to call them molecules, is all. We might as well just admit this, and move on. S B H arris 19:16, 15 September 2006 (UTC)

Agree, but if it gets too big then stability and/or "well definedness" become questionable :-) "Purity" is at its best a rather vague concept. From the general definition standpoint ("the smallest particle...") one could always imagine the most extreme case that fits the description; any grain of sand could eventually be considered a unique, pure molecule, right? Kinetic theory of gases and the molecular vs. continuum treatments set the background for the definition of a molecule based on divisibility. (see [1]) I am being bold and so have added some of this discussion in the intro paragraph. As a side note, a chemist with some molecular modeling experience will definitely feel uncomfortable to accept as molecule a structure that has to be decribed using crystallography-specific "unit cells", or "periodic boundary condition" models. Or one that simply won't entirely fit on a decent computer display ;-) --Unconcerned 07:31, 17 September 2006 (UTC)

[edit] Intro definition

I have been reverted by User:Vsmith (reason: per ref Zumdahl). Thus, I am putting the topic here to discuss further. The intro definition as it stands is:

In chemistry, a molecule is an aggregate of two or more atoms in a definite arrangement held together by covalent chemical bonds.

Not all molecules are held together by covalent bonds, an example being DNA which is held together by hydrogen bonds. Moreover, most supermolecules are held together by non-covalent bonds. It would be correct to say that:

In chemistry, a molecule is a aggregate of two or more atoms in a definite arrangement held together by electromagnetic forces, such as covalent bonds.

This would be all inclusive. --Sadi Carnot 17:33, 17 September 2006 (UTC)

Purists would probably insist that any given strand of DNA consists of TWO molecules, not one. The real problem is that the chemical properties and structure of molecules are determined by other than their covalent bonds (the hydrogen bonds of DNA and those that determine the teriary structures of proteins, etc). If we define molecules chemically, we need to include all kinds of bonds. If we define them physically (as in a gas) we don't require any bonds at all. So the LEAD should continue to consist of paragraphs, each of which gives one of the several different definitions of "molecule" in use by chemists and physicists, under various circumstances. They aren't all the same, and they don't have to be. If we can find ONE definition that fits them all, it's not OUR failing. The word has many uses and meanings, that's all. Sorry, but sometimes science is like that. Definitions change. Doctors still can't agree on what constitutes a "fever"! And they keep changing the definition of "diabetes". And organic chemists can't even agree on whether or not organic compounds have to have hydrogen in them. When this happens, you simply have to summarize conflicting uses and points of view, and pass on. And you reverters, beware. The pure and simple truth is rarely pure and never simple (Wilde). The guy trying to say something additional by adding an expansion, is probably right. S B H arris 17:49, 17 September 2006 (UTC)

Expand, yes. But the basic bonding within molecules is covalent. Add info regarding hydrogen and other bonding in and between supermolecules, but the basic starting definition differentiates between covalently bonded molecules and ionic compounds. The electromagnetic forces bit is far too general - start specific and then expand. Vsmith 18:14, 17 September 2006 (UTC)

Also, I provided a definition based on a widely used text reference. A modification of that definition to not be supported by the reference is problematic. Our function here is not to define based on our background in biochemistry or whatever, or our original research, but to report existing info. A basic definition following an introductory level text, followed by expansion/clarification for other usages is preferred. Vsmith 18:24, 17 September 2006 (UTC)

"Existing info" includes how working biochemists use the term. So it needs to be included in the definition. There is no such thing as a "basic" definition when it comes to something like molecules. This is due to the fact that most people working with molecules are somewhat sophisticated, don't you know. The basic bonding in SOME molecules is entirely covalent, and in others it's not. If you have a crystal of human hemoglobin, the hemoglobin "molecules" in it are composed of protein tetramers, which are held together with hydrogen bonds. Worse still, these tetramers in the crystal are held to each other with hydrogen bonds, too-- just not quite so strong ones. Basically, a "molecule" is a chemically-defined repeating unit which you find in a covalent solid or liquid, which "explains" the chemistry of the stuff, without having to go to larger structures. For non-covalent materials like metals or ionic solids, all bets are off and we don't speak of molecules. For network covalent solids like graphite or diamond all bets are off and we don't speak of molecules. For gases, it's easy: "molecules" are the gas particles bouncing around and separated by large empty spaces of vacuum, no matter what their structure. S B H arris 18:31, 17 September 2006 (UTC)

Vsmith, you are very inflexible and you argue in a way that contradicts yourself. Specifically, I was the one who originally provided a definition based on a widely used text reference (see: April 24, 2006 edit):

In chemistry, a molecule is an aggregate of at least two atoms in a definite arrangement held together by special forces. (Chang, Chemistry 6th Ed.)

It was you who modified that definition so to make the situation problematic. You state that your widely used 4th edition text supposedly replaced my 6th edition text. Sounds backward to me? --Sadi Carnot 16:03, 18 September 2006 (UTC)

Moreover, here are more “textbook” definitions:

Molecule – a definite group of atoms that are chemically bonded together, i.e. tightly connected by attractive forces. (Ebbing, General Chemistry, 3rd Ed.)

Molecule – a chemical combination of two or more atoms. (Brown, LeMay, Bursten, Chemistry – the Central Science, 9th Ed.)

Molecule – a group of atoms bonded to one another. (Pauling, General Chemistry, 3rd Ed.)

I hope these definitions clarify my point; I will now change the definition accordingly per these defintions and the above vote. --Sadi Carnot 16:33, 18 September 2006 (UTC)

Where is this vote? This is a supposed discussion, not a vote. Inflexible? Simply said start simple and then expand the definition to cover variables introduced by biomolecules, polymers and supermolecules. Covalent bonds are the most prevalent bonds within molecules - especially simple species such as water, methane, ammonia, etc. The additional text defs provided do indicate a lack of rigor among text writers - the definitions are quite vague and could include almost anything. And that held together by special forces bit makes it sound like a military thing -- leastwise that's the image that special forces pops into my head. Cheers, Vsmith 00:32, 19 September 2006 (UTC)

Okay, I vote 'for' :-) Everyone is right and the views expressed above actually complement each other. I personally tend to favor this current version of the introduction:

molecule - an aggregate of two or more atoms in a definite arrangement held together by chemical bonds.

as a very balanced one. I guess it is as difficult to define a molecule as it is to define a planet, and scientists' subjectivism and conflicting egos will always complicate matters, no doubt. --Unconcerned 03:04, 19 September 2006 (UTC)

Yes, I concur (vote) with Unconcerned and SBharris. This definition is all inclusive. Moreover, both the Oxford Dictionary of Chemistry and the Barnes & Noble Essential Dictionary of Science state that a molecule can be a group of atoms held by either covalent, ionic, or coordinate bonds, thus "chemical bond" is the generic label for all of these types of bonds as well as all others, such as "hydrogen bonding", "non-covalent bonding", etc., I hope this wraps this discussion up. Thanks: --Sadi Carnot 17:58, 19 September 2006 (UTC)

So, is sodium chloride composed of molecules? How about silica or diamond? Or graphite? And how about a protein, DNA? How about acetic acid dimers in gas phase? If you ask me, the answers would be no, no, no, no, yes, two different, two identical. Chemical bonds seem a bit loose for a definition. Does it include ionic bonds? --Rifleman 82 11:54, 6 November 2006 (UTC)

Sodium chloride is a molecule in the gas phase, but not in the solid. I think the key is that a molecule needs to be "discrete" as opposed to "infinite", but the distinction is sometimes blurry. - Itub 17:43, 6 November 2006 (UTC)