Many elements form binary hydrides, which are compounds of hydrogen with another element. Hydrides exist with all the main-group elements except the inert gases (and perhaps indium and thallium) as well as all the lanthanons and actinons that have been studied. There are three basic types, classified according to the type of chemical bond involved: saline, or ionic; metallic (formerly termed interstitial); and covalent. There is also a borderline category. This inexact classification implies that the bonding type changes at different points as the periodic spiral is traversed from right to left, whereas in reality there is a smooth gradation in the bonding in various types of hydrides. The saline hydrides are generally considered those of the alkali metals and the alkaline earth metals (with the possible exception of beryllium hydride, BeH2, and magnesium hydride, MgH2). In saline hydrides, hydrogen is present as a negatively charged ion, H? , and exhibits a family resemblance to such halogens as fluorine and chlorine. Because saline hydrides react vigorously with water, giving off large volumes of gaseous hydrogen, they are useful as light, portable sources of hydrogen. Binary saline hydrides include sodium hydride, NaH2, and calcium hydride, CaH. Complex saline hydrides include the commercial chemicals lithium aluminum hydride, LiAlH4, and sodium borohydride, NaBH4, both of which are employed as reducing agents (substances that provide electrons in oxidation-reduction reactions. Beryllium and magnesium also form stoichiometric MH2 hydrides, but these hydrides are more covalent in nature. Metallic hydrides are alloy-like hydrides that possess some metallic characteristics, such as luster and strong electrical conductivity, but tend to have variable physical properties. Some are more brittle and others sometimes harder than the metals from which they are made. Regarded as intermediate in nature between salts and alloys, metallic hydrides have been portrayed as composed of protons (positive hydrogen ions, H¿ ) and metal atoms in an electron sea. The relative freedom of electronic movement in the hydride is thought to account for the characteristic luster and electrical conductivity. The transition metals and inner transition metals form a large variety of compounds with hydrogen, ranging from stoichiometric compounds to extremely complicated nonstoichiometric systems. (Stoichiometric compounds have a definite composition, while nonstoichiometric compounds have a variable composition.) Examples include titanium hydride, TiH2, and thorium dihydride, ThH2. Most of the lanthanons and some of the actinons, as well as the more electropositive transition metals, also form relatively well-characterized hydrides. In the well-studied titanium family, for example, titanium (Ti), zirconium (Zr), and hafnium (Hf) form nonstoichiometric hydrides when they absorb hydrogen and release heat. These hydrides have a chemical reactivity similar to the finely divided metal itself, being stable in air at ambient temperature but reactive when heated in air or with acidic compounds. They also have the appearance of the metal, being grayish black solids. With the exception of palladium hydride, PdH2, the hydrides of the later transition metals are poorly characterized or appear to be nonexistent. Covalent hydrides are liquids or gases with low melting or boiling points, except in those cases (e.g., water) where their properties are modified by hydrogen bonding. In covalent hydrides, which are primarily compounds of hydrogen and nonmetals, the bonds are evidently electron pairs shared by atoms of comparable electronegativities. This classification includes the hydrides of boron (B), aluminum (Au), and gallium (Ga) of group 13 and all the known hydrides of groups 14–17. As the periodic spiral is traversed from group 13 to group 17, the hydrogen compounds of the nonmetals become more acidic and less hydridic in nature. In other words, they become increasingly less capable of donating H - and more likely to donate H+. Most nonmetal hydrides are volatile compounds, held together in the condensed state by relatively weak van der Waals intermolecular interactions. Although still volatile, ammonia (NH3), water (H2O), and hydrogen fluoride (HF) are held together in the liquid state primarily by the strongest intermolecular force, hydrogen bonding. Other common covalent hydrides are hydrogen sulfide (H2S) and methane (CH4, together with the other hydrocarbons. Ammonia, the most important nonmetal hydride, is consistently among the top five chemicals produced in the United States. A colorless gas with a sharp, penetrating odor, its boiling point is -33.35° C, and its freezing point is -77.7° C. Most of the ammonia that is produced industrially is utilized in one form or another as fertilizer. Myriad other uses exist, including the manufacture of commercial explosives (e.g., trinitrotoluene (TNT), nitroglycerin, and nitrocellulose) and the manufacture of starting materials for fibers and plastics (e.g., nylon, rayon, and polyurethanes). Synthetic ammonia can be considered the starting material for almost all inorganic nitrogen compounds. Two of the more important derivatives of ammonia are hydrazine and hydroxylamine. Because hydrazine burns in oxygen to produce nitrogen gas and water with the liberation of a substantial amount of energy in the form of heat, the major noncommercial use of this compound (and its methyl derivatives) is as a rocket fuel. Hydrazine and its derivatives have been used as fuels in guided missiles, spacecraft (including the space shuttles), and space rockets. The major commercial uses of hydrazine are as a blowing agent, as a reducing agent, in the synthesis of agricultural and medicinal chemicals, as algaecides, fungicides, and insecticides, and as plant growth regulators. Hydroxylamine, a colorless solid that is hygroscopic (rapidly absorbs water) and thermally unstable, is stored at 0 ° C and used as an aqueous solution or as a salt. Additional examples of covalent hydrides are silane (SiH4), arsine (AsH3), and digermane (Ge2H6). Boron forms an extensive series of hydrides that is discussed in paragraph below, and in the menu item Boranes and Carboranes While the neutral hydrogen compounds of aluminum and gallium are elusive species, AlH3 and Ga2H6 have been detected and characterized to some degree. Ionic hydrogen species of both boron (BH4-) and aluminum (AlH4-) are well-known reducing agents and are extensively used as hydride sources. In group 14, carbon forms the most extensive class of hydrogen compounds of any element in the periodic system (see the submenu item hydrocarbons under Organic Compounds. All the other group-14 elements form hydrides that are neither good H¿ nor good H? donors. This is also true for the hydrides of group 15. All the group 16 elements form dihydrides. The hydrogen compounds formed with the elements that follow oxygen—H2S, H2Se, and H2Te—are all volatile, toxic gases with repulsive odors, and are easily prepared by adding dilute acid to the corresponding metal sulfide, selenide, and telluride. All these group 16 dihydrides act as weak acids in water; their acidity increases with atomic number. In group 17, each of the halogens forms a binary compound with hydrogen, HX. At ambient temperature and pressure, these compounds are gases, with hydrogen fluoride having the highest boiling point owing to intermolecular hydrogen bonding. As is true of group 16, the hydrogen halides are proton donors in aqueous solution. However, the halogen compounds are, as a class, much stronger acids. The acid strength of the HX compounds increases with atomic number, with HF being a very weak acid and HI being the strongest proton donor. With the exception of HF, all the hydrogen halides dissolve in water to form strong acids. The difference in the proton-donating ability of HF and the other HX compounds is due to a variety of factors, among them being the strong bond that forms between hydrogen and fluorine. In dimeric or polymeric hydrides, a fourth type of hydride, the identification of which is based on structure, hydrogen is presumed to have formed a connecting bridge between metal or metalloid atoms. The numerous hydrides of boron—e.g., diborane (B2H6), pentaborane (B5H8), and decaborane (B10H14)—are classic examples. The burning of such hydrides gives off considerably more energy than is furnished by carbonaceous fuels and are of interest as high-energy fuels for rockets. Aluminum and, possibly, copper and beryllium hydrides are nonconductors that exist in solid, liquid, or gaseous states. All are thermally unstable, and some explode on contact with air and moisture.