The molecule split during the light reactions of photosynthesis to release oxygen is water (H₂O).

Here's the breakdown:

* Photosystem II: Light energy excites electrons in chlorophyll within Photosystem II.

* Water splitting: The excited electrons are then used to replace those lost from the chlorophyll. To get these replacement electrons, water molecules are split.

* Oxygen release: This splitting of water produces oxygen (O₂), which is released as a byproduct.

So, the equation for this process is:

2H₂O → 4H⁺ + 4e⁻ + O₂

This oxygen is what we breathe and is a crucial product of photosynthesis.