H₂O is a molecule, not an atom, so it doesn't have an atomic number. Atomic numbers are specific to elements.

Here's how to think about it:

* H₂O is made up of two hydrogen atoms (H) and one oxygen atom (O).

* Hydrogen (H) has an atomic number of 1.

* Oxygen (O) has an atomic number of 8.

Therefore, H₂O is a molecule composed of elements with specific atomic numbers, but it doesn't have its own atomic number.