Here's how to determine the number of sodium atoms in the molecule 7Na3PO4:

* The subscript "3" after Na indicates that there are 3 sodium atoms in each Na3PO4 unit.

* The coefficient "7" in front of the molecule indicates that there are 7 Na3PO4 units.

Therefore, the total number of sodium atoms is:

3 sodium atoms/unit * 7 units = 21 sodium atoms