Final answer:
Photosystem II splits water (H₂O) molecules during the light reactions of photosynthesis to produce oxygen (O₂), electrons, and hydrogen ions (protons).
Step-by-step explanation:
The molecule that photosystem II (PSII) splits in the light reactions of photosynthesis to produce oxygen is water (H₂O). Within the thylakoid membranes of the chloroplasts, the reaction center of PSII, known as P680, utilizes the energy absorbed from sunlight to extract electrons from water molecules. This process is essential for the formation of diatomic oxygen (O₂) gas that is released as a byproduct of photosynthesis. Two water molecules must be split to form one molecule of O₂, which releases four electrons, four hydrogen ions (protons), and two oxygen atoms. The electrons are then passed down the photosynthetic electron transport chain to photosystem I (PSI), leading to the generation of the energy carriers ATP and NADPH.