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#1
Old 11-16-2004, 09:49 PM
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Why No Ionic Molecules?

Why would chemistry books claim that NaCl, and all other ionic compunds ARE NOT molecules? I was never taught it like that! Big deal...so, ionic compounds trades electrons and covalent compounds share electrons. So, what's the big fat hairy deal, anyhow? - Jinx
#2
Old 11-16-2004, 09:52 PM
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What books are you reading?

Websters defines it as thus:
1 : the smallest particle of a substance that retains all the properties of the substance and is composed of one or more atoms

Yes, they are compounds. Yes, they are molcules. Those terms have no baggage with bonding arrangments.

unless I am truly a moron....
#3
Old 11-16-2004, 10:04 PM
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Quote:
Originally Posted by ZebraShaSha
What books are you reading?
Chemistry books, probably.
#4
Old 11-16-2004, 10:04 PM
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No, Because...

High school chemistry books are saying this, and I had a passing moment to ask a college prof who confirmed this IS true, but splitting hairs. But, he didn't have time to go into the details.

I always said once I got out of college and studied all this crap of atomic theory and junk, they'd change all the rules! - Jinx
#5
Old 11-16-2004, 10:23 PM
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Alright, then. Skimming my book (yes, it a chemistry book) I can come to a couple of conclusions:

1) Molecules do not generally form ions when placed in a solvent

This is, as you have stated, due to the unequal distribution of charge for ionic compounds. When placed in water, these compounds will dissolve into charged ions. A molecular compound contains a shared electron cloud which does not form ions when placed in water. Usually. Acids, clearly a molecule, do form ions when placed in water.

Which brings us to:

2) Molecules are neutral past their melting points where as ionic compounds turn into ions past theirs. An ionic molecule, then, by this definition, is not possible if molecules melt into neutrality, seeing as to how all ionic compounds melt into ions.

And finally:

3) Ionic compounds form lattice structures which break more evenly than the molecular structures.
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Old 11-16-2004, 10:39 PM
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Imagine for a second what a collection of ionic molecule would be like. Say you had a bunch of NaCl molecules sitting around in a beaker (assuming that each molecule has one Na+ ion and one Cl- ion). Since the only thing holding the molecules together is the electric force of the ions opposite charges, the force between Na+ and Cl- ions in different molecules is the same as the force within the molecule. Intramolecular bond strength = intermolecular bond strength. As a result, there really isn't any point in distinguishing between the two, they both represent the bond strength of the extended structure.
#7
Old 11-17-2004, 12:11 AM
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To illustrate: A two-dimensional section of a salt crystal might look something like this:
Code:
Na Cl Na Cl Na Cl Na
Cl Na Cl Na Cl Na Cl
Na Cl Na Cl Na Cl Na
Cl Na Cl Na Cl Na Cl
Na Cl Na Cl Na Cl Na
Cl Na Cl Na Cl Na Cl
Na Cl Na Cl Na Cl Na
Now look at one particular sodium, there, like, say, the one I colored red. If salt were a molecular compound, then we'd have to say that that sodium is part of one molecule, with one chlorine. But which one? Is that the same molecule as the chlorine above it, or the one below it, or the one to the left, or to the right? Or, for that matter, one on a different layer, not shown here? There's no clear way to say. It's bonded to all six of those chlorines in exactly the same way. So we say that it's not in any particular molecule, it's just in a crystal with a bunch of other ions.
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#8
Old 11-17-2004, 12:33 AM
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Here is the IUPAC Gold Book definition (the most official one):

molecule: An electrically neutral entity consisting of more than one atom (n > 1). Rigorously, a molecule, in which n > 1 must correspond to a depression on the potential energy surface that is deep enough to confine at least one vibrational state.

(I think the 'rigorous' definition refers to the idea that the entity must exist for at least one bond vibration for it to be considered to have existed.)

IUPAC uses 'molecular entity' to refer to a 'molecule' in the Webster's sense: the smallest particle of a substance that retains its properties (this is not the actual definition). Ionic compounds are certainly molecules in this sense.

I'm going to presume that by 'ionic compounds' we mean those in which the bonding is almost wholly ionic, thus excluding substances that are mostly covalently bonded but contain a few ionic bonds (e.g. sodium benzoate or other salts of organic compounds). Here, an ionic bond is one where the electron density in the bonding molecular orbital is almost wholly concentrated on the negative ion and there is almost zero electron density on the positive ion.

An ionic compound, then, is an array of ions stabilized by lattice energy. This is the increased stability that is seen when the ions are arranged in an ordered crystal, relative to the energy they would have if they were free ions in the gaseous state. It arises because each ion is attracted to the surrounding oppositely-charged ions and repelled by the surrounding negatively-charged ions.(I'd suggest consulting a college-level inorganic chemistry textbook for more details on this.)

These lattices certainly pass as 'molecular entities'. But calling them 'molecules' neglects some important things:

1. Ionic solids have indeterminate size. There are 3 atoms in a molecule of water and 9 in a molecule of ethanol, but how many atoms are in a molecule of salt?
2. Ions aren't the only thing that can form crystals. For example, you can have crystals of sugar as well as salt. Are these crystals both 'molecules'? If you break up the crystal into its component parts (e.g. by dissolving it), a sugar crystal yields sugar molecules, but a salt crystal yields sodium and chloride ions. So, ionic solids are made up of ions, not molecules.
3. The component parts of an ionic crystal don't have neutral charge. Again, if you dissolve a salt crystal, you get Na+ and Cl-. The ions in an ionic crystal are not necessarily single atoms, but they are necessarily charged.

Since #3 would appear to violate the IUPAC definition of a molecule, ionic solids can't be called molecules. A better term would be 'ionic crystal' or 'ionic solid' for crystals made up of ions (with non-neutral charges) and 'molecular crystal' or 'molecular solid' for crystals made up of molecules (with neutral charges).
#9
Old 11-17-2004, 12:54 AM
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ZebraShaSha:

Your book's definition of 'molecule' refers to covalently-bonded substances; it's meant to illustrate the differences between 'molecules' (with covalent bonds)and 'ionic compounds' (with ionic bonds). Ionic vs. covalent (much like acid vs. base) is a relative term that really starts to blur in more advanced chemistry, but it's useful at the HS level.

1) Molecules do not generally form ions when placed in a solvent

Many molecules will form ions when placed in solvents, even if only a very small percentage of the molecules in the sample will be ionized. For example, many acids (both organic and inorganic), alcohols, amines, and acid salts will be ionized in solution, even in water. At least some of the molecules in any sample of something that dissolves in a polar solvent will be ionized in solution.

Which brings us to:

2) Molecules are neutral past their melting points where as ionic compounds turn into ions past theirs. An ionic molecule, then, by this definition, is not possible if molecules melt into neutrality, seeing as to how all ionic compounds melt into ions.

Molecules are always neutral; otherwise they'd be ions. 'Ionic molecule' is an oxymoron, though some things (acid salts, for example) are usually called 'molecules' since they have a definite size and consist mostly of covalent bonds, even though they have at least one ionic bond and are thus not technically molecules.

And finally:

3) Ionic compounds form lattice structures which break more evenly than the molecular structures.

Molecular (i.e. neutral, and typically covalent) compounds also can form lattice structures. Some of these may break 'evenly', and some ionic crystals may not break 'evenly', particularly in ionic crystals involving large ions. Remember that the ions in an ionic crystal are not necessarily monoatomic; it's possible for one or more of the ions involved to be quite large, as in a crystal where the negative ion is an organic acid (tartaric acid, for example) and the positive ion is a protonated base (such as many drugs).
#10
Old 11-17-2004, 01:31 AM
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Everyone has done a great job explaining why ionic compounds are not molecules. The only thing I would add is they are technically known as a formula unit.
#11
Old 11-17-2004, 08:32 AM
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And I guess some people would be interested in the idea of a unit cell, which is the smallest unit of a crystal that wholly reflects the arrangement of the ions within the crystal and indicates the geometry of the crystal lattice. The concepts used in describing unit cells also occasionally apply in real life, such as in the packing/stacking of oranges or tennis balls.

Occasionally a formula unit is not adequate to describe an ionic compound. Some minerals are like this -- an example is adamite, which has the formula (Zn,Cu,Co)2(AsO4)(OH). The three metals in parentheses all exist within the mineral in an undetermined ratio. Some compounds also have unknown coefficients, which means that the exact proportion of some of the elements or radicals in its structure may vary. Or they may have non-integer coefficients (to avoid using very large, confusing formulae with integer coefficients). There are examples of this sort of formula in this page about superconductors.
#12
Old 11-17-2004, 05:39 PM
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What about in the gas phase?

If we pump a lot of energy into a pile of NaCl, enough so that it becomes a vapor, will we see evidence of distinc Na-Cl pairs?
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