Hydrogen is the first and lightest element in the periodic table. It is represented by the symbol H and normally exists as H₂ (diatomic hydrogen gas). Hydrogen is a colorless, odorless, and highly flammable gas.

Hydrogen is the most abundant element in the universe. It is the main fuel of stars, including the Sun, where hydrogen atoms fuse to form helium and release enormous energy. On Earth, hydrogen is mostly found combined with oxygen in water (H₂O).

Hydrogen is used in making ammonia (NH₃), petroleum refining, hydrogen fuel cells, and rocket fuel. It is considered a clean fuel because it produces only water when burned.