The gas released into the atmosphere because of photosynthesis is oxygen (O2).

Here's how it works:

* Photosynthesis is the process plants use to convert light energy into chemical energy (in the form of sugars).

* This process requires carbon dioxide (CO2) and water (H2O).

* As a byproduct, plants release oxygen (O2) into the atmosphere.

So, the equation for photosynthesis is:

CO2 + H2O + light energy → C6H12O6 + O2

(Carbon dioxide + Water + Light energy → Glucose + Oxygen)