Hydrogen
Hydrogen is the first element on the periodic table of elements. It is a colorless, odorless gas, and is denoted by the atomic symbol H. It has an atomic number of 1 and an atomic weight of 1.0079.
Hydrogen is the source of most of the energy radiated by the sun and other stars. The enormous amount of heat and light that we get from the sun comes from hydrogen-fueled reactions. When we burn oil or other fossil fuels, we are actually using the sun's energy, which was stored millions of years ago by ancient plants.
Hydrogen is remarkable in many other ways as well. Its atom is the lightest, smallest, simplest atomic structure known, consisting of just a single proton and single electron (these are subatomic particles with positive and negative electrical charges, respectively). Yet these minuscule hydrogen atoms make up 90 percent of all matter in the universe. On earth most hydrogen is found combined with other elements in compounds such as water (H2O). Because hydrogen can combine with all elements except the inert gases, it creates more compounds than any other element. By definition, hydrocarbons contain hydrogen, as do almost all other organic compounds.
Before scientists had learned the chemical structure of hydrogen and its compounds, people thought that air and water were two of the four basic elements of nature (along with fire and earth). Then during the 1600s and 1700s, chemists began to realize that air contains individual gases, each with different qualities. The first scientist to collect a gas was Robert Boyle, in the 1650s. Boyle filled a flask with sulfuric acid and dropped iron nails into it, then inverted the flask and submerged it in more acid. As the iron reacted with the sulfur, bubbles of gas rose upward to collect at the top of the inverted flask. Although Boyle called this gas "factitious air, " he had unknowingly isolated hydrogen for the first time.
More than 100 years passed before anyone identified hydrogen as a chemical element. In 1766 English chemist Henry Cavendish discovered a gas that he called " inflammable air," which was hydrogen. Cavendish also produced the gas by dropping pieces of metal into acid. He found that hydrogen is released when zinc, iron, or tin are dropped into hydrochloric acid or dilute sulfuric acid; other metal-acid combinations did not produce the gas.
Cavendish was not only the first chemist to distinguish hydrogen from ordinary air, but also the first to investigate its properties. He found that hydrogen is extremely light, compared to other gases. In fact, it has the lowest density of any known substance. During the 1780s, scientists were just beginning to experiment with hot-air balloons. When French physicist Jacques-Alexandre-César Charles heard about Cavendish's discovery, he realized immediately that hydrogen would work better than hot air. Charles went on to invent a hydrogen-filled balloon that traveled higher and farther than earlier hot-air balloons.
In one of Cavendish's most important experiments with hydrogen, he discovered that when the gas is combined with air and sparked, the mixture explodes and creates water. Cavendish even figured out approximately how much hydrogen and air combine to produce water, which anticipated the development of water's modern chemical formula. Cavendish's proof that water is made of two distinct gases permanently corrected the mistaken idea that water is a chemical element.
As far as we know, however, Cavendish failed to realize that it is only the oxygen portion of the air that combines with hydrogen to form water. Almost 20 years after Cavendish described his inflammable air, French chemist Antoine-Laurent Lavoisier gave it its modern name of hydrogen, which means "water-producer." Lavoisier had developed a new theory of combustion that explained accurately, for the first time, how oxygen combines with burning substances and increases their weight. Although Lavoisier deserves credit for determining which individual gases are contained in water, he did not advertise the fact that it was Henry Cavendish who had performed the original experiments.
By the 1800s chemists had begun trying to turn gases into liquids. Normally, pure hydrogen exists as a tasteless, colorless, odorless gas. Many common gases were liquefied simply by applying pressure, while others had to be both cooled and compressed. But even when more sophisticated liquefaction methods were developed, hydrogen resisted all such efforts until 1898, when it was successfully liquefied by Scottish chemist James Dewar. In addition to cooling the hydrogen via expansion (the Joule-Thomson effect), Dewar adapted a process invented by German chemist Karl von Linde in which compressed gas is cooled repeatedly until very low temperatures are reached. Finally, hydrogen became liquid at -423° F (-253° C). A year later, Dewar solidified hydrogen at a temperature of-435° F (-259° C), not far above absolute zero.
Until then, scientists were aware of only one type of hydrogen atom. Then, in 1932 American chemist Harold Clayton Urey discovered "heavy" hydrogen (deuterium). This second isotope of hydrogen contains not only a proton and electron, but also a neutron (an electrically neutral particle that weighs as much as a proton). Two years later, scientists created hydrogen's third isotope, tritium, which has two neutrons and is radioactive. The thermonuclear fusion of these second and third isotopes is what gives the hydrogen bomb its extraordinary destructive power.
Until recently, scientists doubted that hydrogen could be pressurized into a solid metallic state. If practical, solid metallic hydrogen could be formed into pellets of fuel for fusion power generators. Some astronomers believe that metallic hydrogen is a major constituent of the planets Jupiter and Saturn. In 1989 American physicists H. K. Mao and R. J. Hemley, using diamond tools, compressed an extremely thin layer of frozen hydrogen into what appeared to be a metal. The transformation took place at a pressure some 3 million times greater than normal atmospheric pressure.
Aside from futuristic uses, hydrogen already has many practical applications in today's industry. By far the largest is for manufacturing ammonia (NH2), an important fertilizer and industrial chemical. Margarine, vegetable shortening, and soap are also made with hydrogen, which solidifies oils and fats. Hydrogen is also a critical ingredient in certain oil-refining processes, such as the catalytic cracking of petroleum molecules into the lighter hydrocarbons found in gasoline. Hydrogen is also used to recover pure metals from their compounds, and to create extremely high temperatures for arc welding. Currently, NASA uses substantial amounts of liquid hydrogen as rocket fuel, and many scientists believe that hydrogen will someday be much more widely used as a fuel in automobiles and other vehicles.
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