Metal lattice covalent

Ranking metal borides as refractory compounds results from the formation of covalent B — B bonds by the electron-deficient B atoms ". As a result the metal lattice may be changed drastically, even for low B contents. 

Besides metallic crystals, covalent crystals also imdergo surface reconstruction as mentioned in Sec. 2.7 and dangling surface atoms reduce the number of their dangling bonds to stabilize the surface energy, thereby forming a reconstructed surface lattice different from the interior lattice. 

The weakness of the covalent bond in dilithium is understandable in terms of the low effective nuclear charge, which allows the 2s orbital to be very diffuse. The addition of an electron to the lithium atom is exothermic only to the extent of 59.8 kJ mol-1, which indicates the weakness of the attraction for the extra electron. By comparison, the exother-micity of electron attachment to the fluorine atom is 333 kJ mol-1. The diffuseness of the 2s orbital of lithium is indicated by the large bond length (267 pm) in the dilithium molecule. The metal exists in the form of a body-centred cubic lattice in which the radius of the lithium atoms is 152 pm again a very high value, indicative of the low cohesiveness of the metallic structure. The metallic lattice is preferred to the diatomic molecule as the more stable state of lithium. 

Why do carbides and nitrides exhibit the properties that make them so useful in industrial applications It is well accepted that these properties are related to the strength of interatomic bonding.2 In transition metal carbides and nitrides, bonding is believed to have both covalent and ionic contributions.3 The carbon or nitrogen atoms occupy interstitial sites in the metal lattice and are believed to promote strong metal-to-nonmetal and metal-to-metal bonds.1 More detailed bonding explanations require... 

Covalent (molecules containing covalently bound hydrogen to nonmetals, with individual molecules held together by intermolecular forces, e.g., CH4, SiHa) Metallic/interstitial (hydrogen molecules are contained in vacant interstitial sites of a transition-metal lattice, e.g., PdHo.e)... 

Coordination numbers. Ionic compounds containing cations and anions tend to show much lower coordination numbers than metallic lattices, but higher than covalently bonded compounds. This is due to the relatively small size of the cations and relatively large size of the anions. It is not geometrically feasible to locate many large anions closely around each cation. 

Solid materials may be classified into three categories according to the bonding characteristics prevailing in the solid metallic solids, ionic solids, and covalent solids. In metallic solids, electrons bind metal ions into a crystalline lattice and are freely moving around all over the metal lattice, the... 

Thinking it Through Elements with low ionization energy readily lose electrons to form positive ions. Elements with high electron affinity readily accept electrons to form negative ions. Ionic bonds, correct choice (D), result when atoms exchange electrons. Covalent bonds, choice (A), result when atoms each contribute an electron to a shared pair. Coordinate covalent bonds, choice (C), are similar except one of the two atoms furnishes both electrons. Metallic bonds, choice (B), result when atoms free one or more valence electrons to the metal lattice. 

Non-epitaxial electrodeposition occurs when the substrate is a semiconductor. The metallic deposit cannot form strong bonds with the substrate lattice, and the stability conferred by co-ordination across the interface would be much less than that lost by straining the lattices. The case is the converse of the metal-metal interface the stable arrangement is that in which each lattice maintains its equilibrium spacing, and there is consequently no epitaxy. The bonding between the metallic lattice of the electrodeposit and the ionic or covalent lattice of the substrate arises only from secondary or van der Waals forces. The force of adhesion is not more than a tenth of that to a metal substrate, and may be much less. 

For an interstitial alloy to form, the solute atoms must have a much smaller bonding atomic radius than the solvent atoms. Typically, the interstitial element is a nonmetal that makes covalent bonds to the neighboring metal atoms. The presence of the extra bonds provided by the interstitial component causes the metal lattice to become harder, stronger, and less ductile. For example, steel, which is much harder and stronger than pure iron, is an alloy of iron that contains up to 3% carbon. Other elements may be added to form alloy steels. Vanadium and chromium may be added to impart strength, for instance, and to increase resistance to fatigue and corrosion. 

Metal atoms can only form weak covalent bonds with each other, and preferentially exist in the solid stale with crystal lattices in which the coordination number (the number of nearest neighbours) of each atom is relatively high (8-14). Collaboration between many atoms Is required for metal stability. Diagrams of the three most common metallic lattices are shown in Figure 5.3. 

A Lithium vapour does contain Li, molecules, but the 2s valence electron in the Li atom is efficiently shielded from the effect of the nuclear charge by the inner Is pair of electrons, so the covalent bond in Li, is long (267 pm) and weak (107 kj moL ). With Li a metallic lattice is formed in which the effects of the assembly of the valence electrons combine to give better stability than in the diatomic molecule. The distance between nearest neighbours in the metal is 304 pm, but there are eight such neighbours, and this leads to a stabilization of 161 kJ moL. ... 

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