Hydride

In chemistry, a hydride is a binary compound of the gas hydrogen with another element of the periodic table of elements. Binary compounds contain only two elements. Hydrogen can bond with almost any other element. Based on the interaction of the elements' electrons, the subatomic particles with negative electrical charges, a hydride bond can be classified as covalent, ionic, or metallic. Covalent hydrides are composed of hydrogen atoms and non-metals. Ionic hydrides consist of hydrogen atoms and active metals, or metals that react quickly with other substances. Metallic hydrides, meanwhile, are composed of hydrogen atoms and transition metals, or those located in blocks three through twelve of the periodic table. Hydride compounds serve a variety of practical purposes. They can be used to store hydrogen itself and separate hydrogen gas from existing chemical compounds. They are also used in the production of heat pumps, rechargeable batteries, and smart glass.

Background

No hydride could exist without the presence of hydrogen. This gas is the most abundant element in the universe, composing about three-quarters of the universe's mass. No life could exist on Earth without hydrogen, as atoms of the element occur in nearly every organism in the world. Hydrogen does not manifest itself in pure gas form. Rather, it appears mostly in compounds of water and air and in plants, petroleum, and coal.

Hydrogen was unknown to humans throughout most of history. The Irish chemist Robert Boyle accidentally produced the gas in 1671 while combining iron and sulfuric acid in a laboratory. However, he did not actually identify hydrogen or attempt to continue producing it, and the element remained officially undiscovered for nearly another century.

In 1766, British scientist Henry Cavendish formally discovered hydrogen by combining iron and acid, just as Boyle had done. The interaction of the substances emitted bubbles that Cavendish collected because he suspected they were not like any other gas that was known at the time. He later proved the bubbles were flammable and produced water when they were burned. Because no other gas identified in this period exhibited these qualities, Cavendish knew for sure that he had discovered a new element. Eighteenth-century French chemist Antoine Lavoisier later named the element hydrogen from the Greek words hydro, meaning water, and genes, meaning forming.

Chemists know a great deal about hydrogen in the twenty-first century. Pure hydrogen does not occur naturally on Earth; rather, it exists in compounds such as water, one molecule of which contains two hydrogen atoms bonded to an atom of oxygen. Pure hydrogen that does enter the planet's atmosphere from outer space quickly leaves it again due to its extremely lightweight and ability to resist Earth's gravity. Scientists can produce hydrogen in a laboratory by using steam to heat natural gas, which is composed mostly of methane. The heated gas will then become a gaseous compound called syngas, which contains hydrogen and carbon monoxide. The hydrogen is then isolated by extracting it from the syngas.

Hydrogen serves many practical purposes. It is used in the ammonia that is an essential component of crop fertilizer. Being flammable, hydrogen may also be used in rocket fuel, welding torches, and hydrochloric acid. Sometimes hydrogen is added to food through the process of hydrogenation. Hydrogenated oils and fats contain trans fat, which is detrimental to cholesterol levels in the human body and can harm overall heart health. In the twenty-first century, engineers were also laboring to produce fully electric vehicles powered by hydrogen fuel cells.

Overview

Like hydrogen itself, hydrides are abundant in the universe due to hydrogen's ability to bond with almost every element on the periodic table. The only elements with which hydrogen cannot bond are some of the noble gases, a small group of chemically similar gases that include helium and neon. The chemical properties of hydrides vary according to several factors. These include the compound's temperature, molecular masses, and the exact ways in which the hydrogen atoms bonded to the other elements.

Hydrides can be one of three types, depending on the substance to which the hydrogen has bonded. One type of hydride is called a covalent hydride. This compound forms from the covalent bonding of hydrogen atoms and the atoms from at least one non-metal element. Covalent bonds are created from the sharing of electron pairs between the atoms of two separate elements. Electrons are subatomic particles with negative electrical charges. A notable covalent hydride is hydrochloric acid, a corrosive acid that forms from a combination of hydrogen and chlorine. Hydrochloric acid is the principal component of human stomach acid.

A second type of hydride is called an ionic hydride. This forms from the bonding of hydrogen atoms and the atoms of active metals such as the alkali metals, found in group one of the periodic table, and alkaline-earth metals, located in group two. These metals are all highly reactive, meaning they energetically interact with other elements. Examples of alkali hydrides include lithium hydride and potassium hydride. Alkaline-earth metal hydrides include calcium hydride and beryllium hydride.

The third type of hydride is a metallic hydride. This compound is created when hydrogen bonds with any of the transition metals, which are located in groups three through twelve on the periodic table and include elements such as iron, copper, and manganese. A defining chemical property of metallic hydrides is that some of them are nonstoichiometric. This means the ratio of hydrogen atoms to the atoms of the other elements is not equal. Metallic hydrides usually display the standard qualities of metals. They are hard, have luster, and conduct heat. At the same time, most metallic hydrides are distinct from pure metals due to their brittleness. Uranium hydride, for instance, appears as a fine powder, while certain yttrium hydrides are dry solids that crumble easily with movement.

One practical use of metallic hydrides is nickel hydride, a compound used to make nickel-metal hydride batteries, a kind of rechargeable battery. Hydrides can also be used to store and transport gaseous hydrogen, make battery electrodes, and create smart glass, which can instantly become transparent or opaque with the application of heat or electricity.

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