The most abundant element in the universe, hydrogen is also a promising source of “clean” fuel on Earth. Named after the Greek words hydro for “water” and genes for “forming,” hydrogen makes up more than 90 percent of all of the atoms, which equals three quarters of the mass of the universe. Hydrogen is essential for life, and it is present in nearly all the molecules in living things. The element also occurs in the stars and powers the universe through the proton-proton reaction and carbon-nitrogen cycle.

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