We continue our review of some of the most important chemical elements in heat treatment and metallurgy.
Hydrogen (Chemical symbol: H)
Hydrogen is the most common element in the universe. In fact, over 90% of the atoms in the known universe are hydrogen. It is the simplest element of all, because the hydrogen atom is made of only one proton, one electron. Hydrogen is the only element that exists without neutrons.
Hydrogen is the lightest element and does not exist in Earth's atmosphere because it easily bonds with other elements and is not held to the earth by gravity. Any free hydrogen escapes from earth into space. It weighs only 0.0899 kg per cubic meter (0.0056 lb per cubic foot). Since this weight is only 1/14 that of air, it's easy to see why hydrogen is so buoyant. The majority of hydrogen on Earth is bound in water molecules.
Hydrogen is an odorless and colorless gas at atmospheric pressure and temperature, but it becomes a liquid metal when put under very high pressures. Such pressures are found within large planets such as Jupiter and Saturn, and liquid hydrogen exists in the planets' interiors. Jupiter’s magnetic field is thought to be caused by metallic hydrogen, which is electrically conducting, circulating as the planet rotates.
Hydrogen is highly flammable and reactive when exposed to oxidants. This was demonstrated to dramatic effect on May 6, 1937, as the German passenger airship LZ 129 Hindenburg caught fire and was destroyed while attempting to dock at Naval Air Station Lakehurst in Manchester Township, N.J. Amazingly, 62 of the 97 people on board survived the catastrophe.
The first example of man-made hydrogen occurred in the 1500s, when Theophrastus Paracelsus dissolved iron in sulfuric acid and noted a gas (hydrogen) was released. Turquet De Mayerne, a physician in England, duplicated Paracelsus’ research in 1650 and found that the gas was flammable. Neither Paracelsus nor De Mayerne considered that hydrogen could be a new element. In 1670, English scientist Robert Boyle again added iron to sulfuric acid and recognized the resulting (hydrogen) gas burned if air was present.
Hydrogen was first recognized as a distinct element in 1766 by English scientist Henry Cavendish, when he prepared it by reacting hydrochloric acid with zinc. He described hydrogen as “inflammable air from metals” and established that it was the same material (by its reactions and its density) regardless of which metal and which acid he used to produce it. Cavendish also observed that when the substance was burned it produced water. French scientist Antoine Lavoisier in 1783 named the element hydrogen, from the Greek “hydro” meaning water and “genes” meaning forming.
In 1806, English chemist Humphry Davy ran an electric current through purified water and discovered that hydrogen and oxygen were formed. The experiment demonstrated that electricity could split substances into basic elements. When Davy realized that substances were bound together by an electrical phenomenon, he had revealed the principal of chemical bonding.
Here are a few important facts about hydrogen:
Atomic symbol: H
Atomic mass: 1.0079
Density: 0.09 g/L
Melting point: 13.81 K
Boiling point: 20.28 K