Re: How many atoms of NaCl are in one grain of salt?

First let’s overcome some misperceptions in the question.

NaCl is an ionic solid that strictly speaking has neither atoms nor 
molecules. It is a 3-dimensional "checkerboard" of Na+ ions and Cl- ions. 
That is, the sodium ions have one fewer electron than sodium atom have, and 
the chlorine ions have one more electron than chlorine atoms have.

So I will answer the more accurate question:
How many sodium and chlorine ions are in one grain of salt?

We need to establish how much a grain of salt weighs. For the sake of 
argument, I’ll assume that a grain of salt is 0.10 mg or 0.00010 g. (Your 
grain of salt may vary!)

We also need to look up the following information, which can be found in 
any chemistry textbook (or a general-purpose encyclopedia):
	atomic mass of sodium is 22.9898 (I’ll round to 23.0)
	atomic mass of chlorine is 35.453 (I’ll round to 35.5)
	Avogadro’s number is 6.02 x 10^23

The formula mass of NaCl is 58.5 g/mol.

0.00010 g of NaCl contains 0.00010/58.5 mol =0.00000171 mol
(or 1.71 x 10^-6 mol).

Multiplying by Avogadro’s number:
1.71 x 10^-6 mol * 6.02 x 10^23 formula-units/mol = 1.03 x 10^18 formula-
units

or, rounding to the approximate number implied in "one grain of salt":

1 x 10^18 formula-units of NaCl
1 x 10^18 Na+ ions
1 x 10^18 Cl- ions