Here's how to determine the number of atoms in H₂PO₃:

* H₂: The subscript '2' indicates there are two hydrogen (H) atoms.

* P: There is one phosphorus (P) atom (no subscript means 1).

* O₃: The subscript '3' indicates there are three oxygen (O) atoms.

Therefore, there are a total of 6 atoms in H₂PO₃.