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Covclent or Volatile Hydrides 



In covalent hydrides, the oood between hydrogen and the element la 

 a nonpol^r electron-sharing type where valence electrons are shared on a 

 fairly equal basis becveen tne elements held by the bond. Large differ- 

 ences in electric charges do not exist. In general, molecui.-a of cova- 

 lent hydrides are not strongly attracted to each other, and this absence 

 of strong lnteraolocular forces results in the high degree of volatility 

 and low selting point of the covalent hydrides. These hydrides are 

 generally thenaally unstable, such instability increasing with increas- 

 ing atoaic weight of ihe parent eletsent. For the cost part, these 

 hydrides are cxtresely toxic and burn readily in the air or oxygen with 

 liberation of considerable quantities of heat. Soae of the elements 

 which fora covalent hydrides are: boron, aluminum, galliua, indium, 

 thallium, silicon, and genaaniua. 



? 

 Saline Hycrides ; 



I 

 Saline hydrides are foraed by the reaction of the strongly electro- 

 positive alkali metals and the alkaline-earth tsetals with hydrogen, 

 which becoaes strongly electronegative. The bonds of the saline hydrides 

 result frca the strong electrostatic forces existing between the dioia- 

 Uar electric charges of the two ions. Therefore, saline hydrides are 

 highly polar. These saltlike hydrides are crystalline, exhibit hi{,h 

 heats of formation, and high melting points. The saline hvirides are 

 more dense thar> the parent elements because of strong attraction between 

 the metal and the hydride ions and, for the alkali octal hydride, 

 because of the sore efficient packing of atoas. With the exception of 

 aagnesiua and beryllium, all alkali and alkai<ne-earth metals form 

 saline hydrides. These two exceptions are frequently classified as 

 covalant hydrides because some of their characteristics resemble cova- 

 lent hydrides. Saline hydrides decompose when heated to aoderately high 

 temperature, giving off hydrogen. The saline hydrides are considered 

 important reducing agents, and they are useful in the descaling of 

 metals. They react with water and liberate hydrogen very readily. 

 However, because the dissociation pressure of saline hydrides at tempera- 

 tures to over 800 F is less than atmospheric, they are not useful for 

 the hydrogen storage applications under consideration. 



Metal Hydrides 



Metal hydrides are formed jy the transition metaJs. They generally 

 exhibit metallic properties: high thermal conductivity, high electrical 

 resistivity, hardness, luster, etc. Unlike metals, however, they are 

 quite brittle. 



Metallic hydrides are generally formed by the absorption of hydro- 

 gen by the metal, resulting in compound forsation. Compound formation 

 is evidenced by invariant equilibrium pressures at constant temperature 

 as hydrogen content changes over a icrtain range of compositions. 



