A hydrogen atom is the simplest and most abundant type of atom in the universe. It consists of one proton in its nucleus and one electron that orbits around the nucleus. This basic structure makes hydrogen a fundamental building block for more complex atoms and molecules. Hydrogen plays a crucial role in various chemical reactions, particularly in the formation of water (H2O) and organic compounds. It is also essential in processes like combustion and is a key element in stars, where it undergoes nuclear fusion to produce energy.