How many atoms in a mustard seed?

ABOUT 100 QUINTILLION (100,000,000,000,000,000,000)

Source: "Well if you assume that the average Mustard Seed is about .002 g and that the average atomic weight is about that of carbon, then I think it is Avagadro’s number / 12 * .002 which will be about 10^20. A lot more than billions."

FROM a comment made at the StraightDope Q&A site by user Folly.