Hydrogen

There are roughly 170 million billion tons of hydrogen tied up in the earth's supply of water. Hydrogen is therefore the most abundant of all elements on Earth. (Remember, there are twice as many hydrogen atoms in water as there are oxygen atoms.) Because the stars are mostly made of hydrogen, it is also the most abundant element in the universe, making up about 93% of all the atoms, and about three-quarters of the mass of the entire universe. Closer to home, 61% of all the atoms in the human body are hydrogen atoms.

Every one of the 13 million known organic compounds contains hydrogen. Hydrocarbons—compounds that contain nothing but hydrogen and carbon atoms—are the foundation upon which the vast world of organic chemicals is built. The proteins, carbohydrates, fats and oils, acids and bases that make up all plants and animals are organic, hydrogen-containing compounds. Petroleum and coal, which are made from ancient plants and animals, are vast deposits of hydrocarbons.

Hydrogen is the source of most of the energy of the sun and stars. At the 10-million-degree temperatures of the interiors of stars, not only are hydrogen molecules separated into atoms, but each atom is ionized—separated into an electron and a nucleus. The nuclei, which are simply protons, fuse together, forming nuclei of helium atoms and giving off a great deal of energy in the process. By a series of such reactions, all of the heavier elements have been built up from hydrogen in the stars.