Here's how to determine the number of atoms in a molecule of borax (Na₂B₄O₇):

* Na₂: The subscript '2' indicates there are 2 sodium (Na) atoms.

* B₄: The subscript '4' indicates there are 4 boron (B) atoms.

* O₇: The subscript '7' indicates there are 7 oxygen (O) atoms.

Total: 2 + 4 + 7 = 13 atoms in a molecule of borax.