Answer :

Answer:

4.083 * 10^20 atoms.

Explanation:

One Mole of phosphorus  contains 6.022 * 10^23 atoms (Avogadros number)'

Since 1 mole of Phosphorus  has a mass of  30.974 grams, 21 milligrams has

6.022 * 10^23  * 0.021 / 30.974

= 0.004083 * 10^23

= 4.083 * 10^20

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