To understand hydrogen is to understand all of physics. —Attributed to Victor Weisskopf

When scientists analyze materials distributed within the Earth's crust and throughout the atmosphere, they identify ninety-two individual elements that appear in pure form or in combination with other elements. In addition to the naturally occurring elements, some elements are created in the laboratory, but they are short-lived and not observed in nature. All these chemical elements, both natural and laboratory-made, make up the Periodic Table that hangs in the world's science classrooms. Element fourteen, silicon, is prominent in the sandy beaches that attract summer vacationers, whereas elements one and eight, hydrogen and oxygen, make up the waves that surfers ride until those waves break upon the sandy beach. Hydrogen, carbon, nitrogen, and oxygen, elements one, six, seven, and eight, are the primary building blocks of flowers, squirrels, baseball players, and all other forms of living matter. The Earth is blanketed by a thin layer of gases made up mostly of nitrogen, oxygen, and traces of a few other elements.

The element hydrogen, which has consistently turned new ground in the quest to understand the intricacies of the natural world, is the dominant form of matter in stars and the interstellar regions of the cosmos, but the hydrogen atom is largely absent in pure form here on Earth. There is no hydrogen in the Earth's atmosphere. The gravitational pull that successfully holds nitrogen and oxygen to the Earth's surface cannot maintain its grip on the simplest atom. Atomic hydrogen is too ephemeral. Any hydrogen released to the atmosphere slowly works its way through the nitrogen and oxygen surrounding the Earth until it reaches the upper limits of the Earth's atmosphere, then effectively bids adieu to planet Earth as it disappears into the solar system and beyond. There is a touch of irony that the hydrogen atom, so abundant in the universe at large, yet essentially absent in pure form in the world around us, has brought such insights in return for the attention that it has received.

The story of hydrogen in the preceding account has been drawn from physics. Chemists could add to this tale. One topic that begs for inclusion is hydrogen bonding. This bond is not really a chemical bond such as occurs between the oxygen and the two hydrogen atoms in a single water molecule. The hydrogen bond, however, does occur in water between the oxygen of one molecule and the hydrogen of a neighboring molecule. It is this attraction between oxygen and hydrogen of separate molecules that gives water some of its unusual properties. Ice floats because of the hydrogen bond. And because ice floats, it can be argued that life on Earth exists. If water was a typical liquid, its solid form, ice, would be denser than the liquid form. Ice would then sink to the bottom of bodies of water and slowly build up until the Great Lakes and other bodies of water became solid ice. Since living organisms most likely took form first in water, it is questionable whether solid bodies of ice would have been conducive to the fostering of life. There are other stories that chemists would include in their story of hydrogen.

I believe that the hydrogen atom, the essential element, shows the conduct of science at its best. It was my privilege to write a biography of I. I. Rabi.1 I spent many days with Rabi and learned firsthand about his work on the hydrogens. This background surely influenced my decision to write a book about the hydrogen atom. This book was born, however, while I was listening to a talk at a meeting of the American Association for the Advancement of Science. The speaker was expounding boldly on current physics and how a single theory of everything was likely to become a reality. "Come on," I said to myself, "the hydrogen atom, the simplest atom, still beckons. We are still learning from the hydrogen atom." After all, H stands not only for hydrogen, but also for humility. The hydrogen atom still beckons—its story far from over. On occasion we hear that all basic knowledge in science has been acquired. Whenever someone makes such claims, it would be advisable to remember that the simplest atom, one proton and one electron, is still providing insights into natural phenomena. As long as scientists are learning from the essential element, hydrogen, science itself is in no danger of ending.

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