Small molecules like O₂, N₂, H₂, CO₂, N₂O, and CH₄ are crucial in biogeochemical cycles and climate change, influencing processes such as respiration and photosynthesis. They are also significant in the chemical industry for energy storage and conversion. As these molecules are central to creating sustainable energy systems and high-value chemical products, mastering their chemistry is a major challenge for the 21st century.