Hydrogen is indeed the most abundant element in the universe, comprising about 75% of its elemental mass. However, on Earth, hydrogen is not as abundant in its elemental form (H2) due to a few key reasons:
Escape from Earth's Atmosphere: Hydrogen is a light gas, and its individual atoms can achieve escape velocity from Earth's gravitational pull more easily compared to heavier elements. Over geological timescales, a significant amount of hydrogen has escaped into space from Earth's atmosphere.
Bonding with Other Elements: Hydrogen readily bonds with other elements, forming compounds such as water (H2O) and various hydrocarbons. Water, for instance, covers about 71% of Earth's surface, and the majority of hydrogen on Earth is present as part of water molecules.
Sequestration in Other Materials: Hydrogen can be trapped or incorporated within other materials, such as minerals and organic compounds. For example, hydrogen is stored in the organic matter of living organisms and is an essential component of organic compounds.
Geological Processes: Geological processes, such as volcanic activity and the Earth's mantle composition, play a role in the distribution and availability of hydrogen. Hydrogen can be released through volcanic emissions, but the rates of release are relatively low compared to the overall abundance of the element.
While hydrogen is not as abundant on Earth in its free form, it is still a crucial component of numerous compounds and plays a fundamental role in various Earth processes, including supporting life as a key component of water and participating in chemical reactions.