How many argon atoms are contained in 7.66x105 mmol of argon?

There is one atom in argon. Argon is a "noble gas", which means that it occurs independently. This is true for all of the atoms on the final column of the periodic table....

18

We need: The atomic weight of Argon (Ar), which is 39.948. We will round to 39.9. Avocado's Number, we'll use 6.02 * 1023 (40 g Ag) / (39.9 g/mol) * ( 6.02e23 atom/mol ) = 6.04 * 1023 atoms Ag...

Same amount as the Proton count - 18

Yes. Because argon is an element the smallest "piece" of it is referred to as an atom.