The oxygen atoms in the oxygen gas produced by photosynthesis come from water molecules (H₂O).

Here's why:

* Photosynthesis equation: The overall equation for photosynthesis is:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

* Water splitting: During the light-dependent reactions of photosynthesis, light energy is used to split water molecules (photolysis). This process releases electrons, hydrogen ions (H+), and oxygen gas (O₂).

* Oxygen release: The oxygen gas released as a byproduct of photosynthesis comes from the oxygen atoms that were originally part of the water molecules.

So, while carbon dioxide (CO₂) provides the carbon atoms for the glucose (C₆H₁₂O₆) produced, it's the water molecules that contribute the oxygen atoms for the oxygen gas released.