Covalent solids general

The solubility of a given solute in a particular solvent depends on a number of factors. One generalization which can be used for determining solubility is like dissolves like. This means that the more similar the polarity of a solute is to the polarity of the solvent, the more likely the two will form a homogeneous solution. A polar solvent, such as water, will dissolve a polar compound an ionic salt like common table salt, NaCl, will dissolve in water a polar covalent solid like table sugar, sucrose, will dissolve in water. Nonpolar solvents such as naphtha or turpentine will dissolve nonpolar material, such as grease or oil. On the other hand, oil and water do not mix because of their different polar characteristics. 

In valence bond theory, the coordination number of an atom in a molecule or covalent solid is generally Umited to the number of valence orbitals on the atom. Likewise, only... 

The identification of the band gap in ionic crystals with pscudopotentials suggests one other property that may be attributable to ionic crystals. The general insensitivity of to material or structure gives a rationalization to the observation that band gaps in ionic solids and even inert-gas solids vary as cl. However, the point cannot be made as strongly for ionic solids as it can for covalent solids. 

In general, semiconductors can have different types of valence band and conduction band structures. These differences can affect the chemical reactivity of the various types of semiconducting solids. For example, in a covalent solid such as Si, the valence and conduction bands can be considered as crystal orbitals that are either bonding or antibonding combinations of hybridized Si atomic orbitals. This situation is closely related to our polyene example, where the valence band consisted of bonding tt-orbitals and the conduction band consisted of antibonding 7r -orbitals. However, in an ionic crystal such as TiOi, the valence band is composed of crystal orbitals that are derived from the filled O 2p orbitals, while the conduction band is composed of crystal orbitals that are... 

With the exception of the uranyl and vanadyl oxygen atoms all other oxygen atoms are shared between the VO5 pyramids and UOs bipyramids so that the formula of this compound can be written as AU02(V03)3. This is in general agreement with the use of the "metavanadate" name often given for this series. However, this does not really correspond to its crystal structure, which is better described as an extended covalent solid. 

To give a general description of covalent solids and metals, the band theory arising from the infinite polyene chain must be extended to three dimensions. The properties of solids depend largely on the way in which electrons fill the different available bands. 

Generally the covalent solids have comparatively low densities as a result of the low coordination numbers. This effect is intensified in those crystals in which covalently bound structural units are bound in the crystal by van der Waals forces. The distance between two units held by van der Waals forces is significantly greater than that between units held by covalent, ionic, or metallic bonds these large distances result in solids having comparatively low densities. 

The bond between covalent lattices such as silicon and diamond and chemisorbed gases such as hydrogen, nitrogen, and carbon monoxide is covalent. Species that are bound to the surface with covalent bonds generally have low mobilities over the surface. If they have a metallic bond or an ionic or coordination bond with surface atoms they tend to have higher mobilities. The mobility of surface species can be important for the reaction. High surface mobilities result in growth of coarsely crystalline solids, and the reaction rates have conventional kinetics. 

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