Gaseous covalent hydrides

For some elements such as Hg, atoms (atomic vapor) can also be obtained by chemical reaction. The ions in solution (Hg2+) can be efficiently reduced to Hg, which is a vapor at room temperature. Similarly, other elements with high electronegativities (i.e., electronegativities close to that of hydrogen) can be efficiently converted into vapors by a reduction reaction similar to that used for mercury. This is done with the so-called covalent hydride-forming elements (As, Bi, Pb, Sb, Se, Sn, and Te), which are converted into gaseous hydrides at room temperature. These hydrides are then heated... 

Hydrides are broadly of three types, saline, covalent and metallic. Saline hydrides are formed by the alkali metals (Gp. lA), the alkaline earths (Gp. IIA) and the lanthanides they have ionic lattices, high melting points and, when fused, are electrolytes. Elements of the B Groups from IIIB to VIIB have covalent hydrides, most of them gaseous at room temperature. The metallic hydrides characteristic of some of the transition elements are in effect alloys and usually lack the stoichiometric composition of normal chemical compounds. 

They form gaseous (with the exception of HF, bp 20°C), covalent hydrides with the general formula HX. The thermal stability of these hydrides decreases down the group. The hydrides form acidic aqueous solutions. 

The elements Ge, Sn, Pb, As, Sb, Bi, Te and Se, form covalent gaseous hydrides. A number of these elements are now routinely determined by... 

Gaseous and volatilised analytes can also be easily determined by FAAS and ETAAS. For example, the determination of several elements by the formation of covalent volatile hydrides e.g. arsenic, selenium) and cold vapour generation (mercury and cadmium) is feasible with good analytical sensitivity (see Section 1.4.1.1). 

The main idea of these techniques lies in the interaction of the active hydrogen atom of the alcohols with the anions of metal hydrides, alkyls, acetylides, nitrides, amides, dialkylamides, bis(trialkylsilyl)amides, sulfides, etc., with formation of compounds where an H atom is bonded by a strong covalent bond (usually gaseous HX). Alkaline hydrides of the most active metals (K, Rb, Cs) are used to slow down the reaction of metal with alcohol sometimes it is necessary simply to avoid explosion. 

This technique makes use of a property that these elements exhibit, i.e., the formation of covalent, gaseous hydrides that are not stable at high temperatures. Antimony, arsenic, bismuth, selenium, tellurium, and tin (and to a lesser degree germanium and lead) are volatilised by the addition of a reducing agent like sodium tetrahydroborate(III) to an... 

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