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Molecule and compound?

  1. Dec 5, 2006 #1
    Could someone list the similarities and differences between the two?

    Is it correct to say that 'the set of all molecules' is a subset of 'the set of all compounds'. But the vice versa is incorrect. So a 'proper' should be placed in front of the word 'subset' above.

    i.e. any covalently bonded molecule is considerered a compound but an ionic compound like NaCl cannot be a molecule because
    1) It dosen't have a definite arrangement
    2) There is no smallest particle of an element or compound that retains the chemical properties of the element or compound. The whole 'infinite' lattice must be considered which cannot be a molecule.
     
    Last edited: Dec 5, 2006
  2. jcsd
  3. Dec 5, 2006 #2

    Gokul43201

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    Comparing a molecule with a compound is like comparing an atom with an element or a cell wall with fruit.

    A 'compound' is a kind of substance with a fixed chemical composition involving mre than one chemical element - you can have a picogram of a compound or a kiloton of it. Compounds are usually made up of molecules. The term 'molecule' is a label identifying the fundamental chemical unit that certain kinds of substances are made of.

    For more on this, see my last post in this thread: https://www.physicsforums.com/showthread.php?p=1135673#post1135673
     
  4. Dec 5, 2006 #3

    GCT

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    Terms such as molecule and compound aren't definite in their reference, although there are preferences in context when using them in chemical literature.

    Someone might be inclined to mention NaCl as a molecule, as long as there's a good understanding of what the author means in describing the physical situation there should be no qualms on how they are described and the practical terms that are employed.

    For example, one can conceptualize NaCl, existing as a molecule in a water solution; although the dissolution of NaCl results in the interaction of the cation and anion with water, a very few exist at times as NaCl, or for a different "molecule" MgCl2, the descrepancy in the Van't Hoff factor is higher.

    In short, you can use the terms pretty liberally, without backbiting criticism from your peers.
     
  5. Dec 5, 2006 #4
    Gokul43201, You suggest from the earlier post that "In this kind of [molecular] matter, all the chemical properties are manifested by a single unit identified such that the bonding energy between the atoms of the unit is large compared to interaction energies between neighboring units." So this is your definition of molecule (with two or more elements) which distinguishes a molecule from a compound. We should also add the ovbious fact that "A molecule is an aggregate of two or more atoms in a definite arrangement held together by chemical bonds." found in http://en.wikipedia.org/wiki/Molecule
    Although could you explain what they mean by 'definite arrangement'?

    I have made a mistake in the first post. Some molecules are not compounds. i.e. all monotomic molecules are not compounds as only one element is invovled. So molecules and compounds are neither subsets of one another nor mutually exclusive. So a Venn diagram could be set up. But you suggest there is no point in comparing them? If so why?
     
    Last edited: Dec 5, 2006
  6. Dec 5, 2006 #5
    I wonder why they discard these two terms and instead have one term that describes all chemical units of more than one atom. In fact I think I remember such a term used by my lecturers surprisingly a lot. They call them species.

    You suggestion that NaCl in water becomes molecules of NaCl is interesting. ALthough is it in a definite arrangement?
     
    Last edited: Dec 6, 2006
  7. Dec 5, 2006 #6

    GCT

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    Have you ever learned about the Van't Hoff factor yet in relation to the concept of colligative properties? For example with freezing point depression, the formula is dT=Kfm, (where m is the molality, in relation to the concentration, Kf relates to the solvent e.g. water, and dT is the predicted freezing point of the solution which has solutes in it relative to the normal freezing point FPsolvent-FPsolution). If you have an ionic compound which is soluble, and the interaction between the solute and solvent is ideal (you may want to read about what exactly is meant by "ideal" in a standard general chemistry text), then according the colligative property concept, you would need to multiply the formal concentration of the solute with respect to how it dissociates in the solution. For NaCl, the Van't Hoff factor, "i", would be 2, for something more simple, such as sugar "i" is 1.

    However, the empirical results indicate that the real "i" is slightly less then what would be predicted....in understanding that NaCl is completely dissolved as Na+ and Cl- by water molecules. However, there are some components
    of NaCl that exist simply as NaCl "molecules" and this is why the best value for "i" is commonly less then "perfect" for those compounds which have predicted values of i that are greater then 1.
     
  8. Dec 5, 2006 #7

    Gokul43201

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    Because they're completely different beasts.

    Benzene is a compound. Benzene is made up of molecules. These molecules can be refered to as Benzene molecules (which is a shortform for "the molecules that make up Benzene"). Benzene is not a molecule.
     
  9. Dec 6, 2006 #8
    But they are also very same as well in cases where a chemical substance is both a molecule and a compound which is most molecules. Maybe your analogy between atom and elements is good. They are very similar and different at the same time.:confused:
     
  10. Dec 6, 2006 #9

    Gokul43201

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    This is like saying a building can be both a brick and a house.

    A chemical substance can not be a molecule. It can only be made up of molecules. But a chemical substance can be a compound (or an element, or a mixture).
     
  11. Dec 6, 2006 #10
    Why is Benzene not a molecule? Each benzene ring does not and cannot covalently bond with anything unlike C or Si.

    Or are you trying to say that when we talk about Benzene we don't talk about a single benzene molecule but a sample which consists of zillions of benzene molecules. But then you get the problem of vagueness? Would a super small sample consisting of two benzene rings be considered Benzene? If so then why not one benzene molecule in a sample be considered Benzene? The definion of a compound dosen't disallow the latter sample not to be called a compound.
     
    Last edited: Dec 6, 2006
  12. Dec 6, 2006 #11
    So you are saying molecule is a word used to describe a part and compound (or element, or mixture) is used to describe a whole?

    Moreover there is no point in using 'parts' language such as molecule to describe metals or salts because they can only be analysed by considering their whole.

    What about if I want to address a sample consisting of monatomic molecules? Which word can I use since compound is unstable in this case.

    What other "parts" language in chemistry is there? Atom would be another wouldn't it?
     
    Last edited: Dec 6, 2006
  13. Dec 6, 2006 #12

    Gokul43201

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    It's just the way the terminology is used. The terms 'element' and 'compound' are used to specify something about the chemical substance. The terms 'atom' and 'molecule' are used to say something about what these substances are made of.

    As for metals and salts (in crystal form), most of the important properties result from the collective behavior of the whole set of atoms interacting with each other.

    A sample consisting of monoatomic molecules (the noble gases) can not be a compound. A molecular compound must contain atoms of more than one element in each molecule. If the sample were made up of different monoatomic molecules, it would be a mixture (eg: He+Ne).
     
  14. Dec 7, 2006 #13
    What about a sample consisting of only He? How about calling it atomic substance?

    A sample of only H2 we could call molecular substance?
     
  15. Dec 7, 2006 #14

    Gokul43201

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    Correct, and correct.
     
  16. Jul 3, 2011 #15
  17. Jul 4, 2011 #16
    Just to reinforce the idea.
    Elements, compounds etc are terms used to classify different substances into types. These types are composed of very small units which can be atoms or molecules (only molecules in case many compounds). Like Gokul said, you can have different types of houses (bungalows, huts, igloos, skyscrapers, cabins etc) and these houses may or may not be built with the same kind of brick (many houses are not even built of bricks).
     
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