Covalent solids, giant

Figure 2.13 Diamond has a giant macroscopic structure in which each atom is held in a rigid three-dimensional array. Other covalent solids include silica and other p-block oxides such as A1203...

Finally, macromolecular covalent solids are unusual in comprising atoms held together in a gigantic three-dimensional array of bonds. Diamond and silica are the simplest examples see Figure 2.13. Giant macroscopic structures are always solid. 

As simple molecular substances, they are usually gases, liquids or solids with low melting and boiling points. The melting points are low because of the weak intermolecular forces of attraction which exist between simple molecules. These are weaker compared to the strong covalent bonds. Giant... 

Covalent solids (or network solids ) can be considered giant molecules that consist of covalently bonded atoms in an extended, rigid crystalline network. Diamond (one crystalline form of carbon) and quartz are examples of covalent solids (Figure 13-32). Because of their rigid, strongly bonded structures, mst covalent solids are very hard and melt at high temperatures. Because electrons are localized in covalent bonds, they are not freely... 

Covalent solid n. A solid in which atoms are bonded covalently to form a giant extended network. 

Covalent solids (or network solids ) can be considered giant molecules that consist of covalendy bonded atoms in an extended, rigid crystalline network. Diamond (one crystalline... 

Even though silicon is metallic in appearance, it is not generally classified as a metal. The electrical conductivity of silicon is so much less than that of ordinary metals it is called a semiconductor. Silicon is an example of a network solid (see Figure 20-1)—it has the same atomic arrangement that occurs in diamond. Each silicon atom is surrounded by, and covalently bonded to, four other silicon atoms. Thus, the silicon crystal can be regarded as one giant molecule. 

In sharp contrast to molecular solids, network solids have very high melting points. Compare the behavior of phosphorus and silicon, third-row neighbors in the periodic table. As listed in Table 11-2. phosphorus melts at 317 K, but silicon melts at 1683 K. Phosphorus is a molecular solid that contains individual P4 molecules, but silicon is a network solid in which covalent bonds among Si atoms connect all the atoms. The vast array of covalent bonds In a network solid makes the entire stmcture behave as one giant molecule. ... 

The connection between polymer chemistry and ceramic science is found in the ways in which linear macromolecules can be converted into giant ultrastructure systems, in which the whole solid material comprises one giant molecule. This transformation can be accomplished in two ways—first by the formation of covalent, ionic, or coordinate crosslinks between polymer chains, and second, by the introduction of crystalline order. In the second approach, strong van der Waals forces within the crystalline domains confer rigidity and strength not unlike that found when covalent crosslinks are present. 

In covalent network solids, covalent bonds join atoms together in the crystal lattice, which is quite large. Graphite, diamond, and silicon dioxide (Si02) are examples of network solids. The crystal is one giant molecule. 

When A and B are both electronegative they form covalent compounds. These may consist of individual molecules (02, H20, etc.) or of giant covalent lattices (polymeric solids) with a... 

On the other hand, diamond, a form of solid carbon, is one of the hardest substances known and has an extremely high melting point (about 3500 °C). The incredible hardness of diamond arises from the very strong covalent carbon-carbon bonds in the crystal, which lead to a giant molecule. In fact, the entire crystal can be viewed as one huge molecule. A small part of the diamond structure is represented below. 

In diamond each carbon atom is bound covalently to four other carbon atoms to produce a very stable solid. Several other elements also form solids whereby the atoms join together covalently to form giant molecules. Silicon and boron are examples. 

Many atomic solids form "giant molecules" that are held together by covalent bonds... 

Only those atoms that form four covalent bonds produce a repetitive three-dimensional structure using only covalent bonds. The diamond structure. Fig. 27.11, is one of several related structures in which only covalent bonds are used to build the solid. The diamond structure is based on a face-centered cubic lattice wherein four out of the eight tetrahedral holes are occupied by carbon atoms. Every atom in this structure is surrounded tetrahedrally by four others. No discrete molecule can be discerned in diamond. The entire crystal is a giant molecule. 

Many atomic solids contain strong directional covalent bonds to form a solid that might best be viewed as a "giant molecule." We call these substances network solids. In contrast to metals, these materials are typically brittle and do not efficiently conduct heat or... 

In a giant atomic solid, such as diamond, the vibrating particles are atoms which are held by strong covalent bonds, and so the solid has a very high melting temperature. 

Covalent carbides, which have giant-molecular structures, as in silicon carbide (SiC) and boron carbide (B4C3). These are hard high-melting solids. Other covalent compounds of carbon (CO2, CS2, CH4, etc.) have covalent molecules. 

Univalent atoms united by covalencies in molecules such as Cla have no bonds left, but polyvalent atoms can form lattices in which each atom is joined to one or more neighbours by a covalency, making the whole array into a giant molecule. The molecules or ions in other lattices are held by van der Waals or Coulomb forces. The physical characteristics of the varied types of solid show very wide variations which refiect these different modes of assemblage. 

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