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Does a grain of salt have exactly equal amount of sodium and chlorine? I know that the formula unit of salt is 1:1, but for example if there is 10000000 atoms of sodium is it possible that that there is 9999999 atoms chlorine? Thanks.

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    $\begingroup$ In the real world nothing is ever completely pure. You'd never have a perfect grain of $\mathrm{NaCl}$. There would be other compounds in it too which would throw off the balance. $\endgroup$
    – Brandon Enright
    Commented Dec 18, 2013 at 16:45
  • $\begingroup$ Short answer: Possibly. $\endgroup$ Commented Dec 18, 2013 at 17:10
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    $\begingroup$ Real world vs. technical definition: salt is NaCl thus anything with Na(x)Cl(x+epsilon) isn't salt. Take this comment with a grain of salt :-) $\endgroup$ Commented Dec 18, 2013 at 18:02

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Crystal solids, however small, have defects in them. These defects are basically irregularities in the arrangement of constituent particles. They are of two types:

  • Point defects
  • Line defects.

Point defects themselves are divided into other kinds of defects. Here, I'll explain only the relevant one.

One type of point defect is a vacancy defect in which some of the lattice sites in the crystal are vacant. This means that some atoms of Na or Cl or both are missing from their lattice sites.

So the answer to your question is, yes. That scenario is very much possible.

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    $\begingroup$ When you say "a grain of salt can never have the same amount of Na and Cl", are you suggesting that perfect crystals can't be grown? $\endgroup$ Commented Dec 20, 2013 at 0:05
  • $\begingroup$ Indeed, I was under that impression. I was looking at this definition. I'll edit my post. $\endgroup$ Commented Dec 20, 2013 at 6:07
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It depends what you mean by "exactly".

A single gram of salt contains about 1022 atoms of sodium and chlorine. The important question is how much of an imbalance would be physically noticeable. If you can't detect the difference it isn't significant. A single atom discrepancy would be impossible to detect with the best equipment available to science. So it doesn't really matter.

More importantly most real world salt will contain a great deal of other stuff. Standard table salt might be 99.9% pure (meaning it could contain 1019 atoms of say, K or Br). But, for most purposes this is very pure and nobody will notice the extraneous elements. Sea salt is sometimes only 98% NaCl and we barely notice that difference.

For some context consider one of the purest objects ever made: the silicon sphere being developed as a new, improved, standard for weight. We have extremely good methods for making pure silicon (because of its importance in the semiconductor industry). As this nature article says:

The researchers spent six months eliminating contaminating elements by repeatedly melting the silicon in an apparatus that does not touch the material. The resulting crystal is thought to contain one foreign atom to every 10 million atoms of silicon.

This state of the art level of purity would allow a gramme of salt to have about 1015 atoms of impurities. Doing better than that is going to be tough to measure: a larger mismatch in atom count is going to be totally insignificant in any practical situation.

We might notice a significant effect if there were a significant imbalance of positive and negative ions, but it wouldn't last long as any crystal would attract opposite charged ions from the environment to make it neutral.

So overall salt contains the same number of Cl and Na ions within any practical limits of detection or effect.

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Yes. The amounts of Sodium and Chlorine are the same. The reason being you need to have the charge balanced.

The defects mentioned in the above answer do exist, but always in pairs. That is, for every missing Sodium atom, you have a Chlorine atom missing.

What can be said is that in 58.44 g (Molar Mass) of NaCl, you won't necessarily have Avagadro number of Sodium or Chlorine atoms. But, whatever their number, there will be equal amounts of Sodium and Chlorine.

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  • $\begingroup$ The impurities could be charged as well. Your answer is only half correct. Potassium could replace sodium and iodine could replace chlorine. Look en.wikipedia.org/wiki/Iodised_salt $\endgroup$ Commented Dec 19, 2013 at 8:28
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    $\begingroup$ Also, although electrical neutrality is held to a high degree of precision, it is not held perfectly. You can very well have a crystals with some excess anions and no cations to compensate their charge, or vice-versa. The charge imbalance probably can't be higher than one part per billion though, very roughly. $\endgroup$ Commented Dec 19, 2013 at 10:19
  • $\begingroup$ But, if I recall correctly, Schottky and Frenkel defects always occur in pairs right? Also, lab-grade NaCl would be almost entirely made up of only Sodium and Chlorine. $\endgroup$
    – Ash
    Commented Dec 19, 2013 at 10:29
  • $\begingroup$ I think even the purest substances used in macroscopic quantities in chemistry today have at least a part per trillion impurities, which would mean a mole of material may have as much as 100 billion other atoms/molecules! Also, although most defects would probably still be electrically neutral or have an oppositely charged defect closeby, there is no rule that says all defects are of those kinds. $\endgroup$ Commented Dec 19, 2013 at 17:48
  • $\begingroup$ I did a bit of reading up. It looks like electrical neutrality isn't perfect and you can't have a 100% pure compound, anyway. Thanks Nicholau, for shedding light on that! $\endgroup$
    – Ash
    Commented Dec 20, 2013 at 5:49
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If it doesn't it isn't "salt" or at least it isn't NaCl, which requires one of each per molecule.

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  • $\begingroup$ Can you elaborate on the answer a bit? $\endgroup$
    – jonsca
    Commented Dec 20, 2013 at 0:03

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