Network solids phosphorus

Much of the Earth s silicon is found in durable rock formations based on silica. Granite is a common example. This is consistent with a network solid. Phosphorus, in contrast. 

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. ... 

Red phosphorus is less reactive than white phosphorus and is used in match heads because it can be ignited by friction. It is a network solid consisting, most likely, of linked P4 tetrahedral units. Red phosphorus is the most thermodynamically stable form of phosphorus, yet white phosphorus crystallizes from the vapor or from the liquid phase. It is the form chosen to have an enthalpy and free energy of formation equal to 0. For red phosphorus, isH° = -17.6 kJ-moP and AGf° = -12.1 kJ mor at25°C. 

The carbon bonding network around phosphorus centres can be determined in solids by the use of the double cross polarization l C n.m.r. technique as illustrated for the phosphine oxide (29).l Nine of the nineteen possible phosphine sulphides of the pentaphosphine (30) have been prepared (by progressive reaction of (30) with sulphur) and their structures have been investigated by 2D 3lp n.m.r. spectroscopy.20... 

At this point you might be asking yourself, "Why aren t solids such as a crystal of diamond, which is a giant molecule, classified as molecular solids " The answer is that, by convention, a solid is classified as a molecular solid only if (like ice, dry ice, sulfur, and phosphorus) it contains small molecules. Substances like diamond that contain giant molecules are called network solids. 

So far we have considered solids in which atoms occupy the lattice positions. In some of these substances (network solids), the solid can be considered to be one giant molecule. In addition, there are many types of solids that contain discrete molecular units at each lattice position. A common example is ice, where the lattice positions are occupied by water molecules [see Fig. 10.12(c)]. Other examples are dry ice (solid carbon dioxide), some forms of sulfur that contain Ss molecules [Fig. 10.34(a)], and certain forms of phosphorus that contain P4 molecules [Fig. 10.34(b)]. These substances are characterized by strong... 

The chemical differences between nitrogen and phosphorus are apparent in their elemental forms. In contrast to the diatomic form of elemental nitrogen, which is stabilized by strong tt bonds, there are several solid forms of phosphorus that all contain aggregates of atoms. White phosphorus, which contains discrete tetrahedral P4 molecules [see Fig. 20.18(a)], is very reactive it bursts into flames on contact with air (it is said to be pyrophoric). Consequently, white phosphorus is commonly stored under water. White phosphorus is quite toxic the P4 molecules are very damaging to tissue, particularly the cartilage and bones of the nose and jaw. The much less reactive forms, called black phosphorus and red phosphorus, are network solids (see Section 10.5). Black phosphorus... 

Consider the structures of elements of the third period (Na—Ar). Argon with eight valence electrons has a complete octet as a result it does not form any bonds. Chlorine, sulfur, and phosphorus form molecules (CI2, Sg, and P4) in which the atoms make one, two, and three bonds, respectively ( FIGURE 12.20). Silicon forms an extended network solid in which each atom is bonded to four equidistant neighbors. Each of these elements forms 8—N bonds, where N is the number of valence electrons. This behavior can easily be understood through application of the octet rule. 

Red phosphorus is less reactive than white phosphorus and is used in match heads because it can be ignited by fiiction. It is a network solid consisting, most likely, of linked P4 tetrahedral units. 

The element phosphorus forms a variety of allotropic forms in the solid state. In the chemistry stockroom, you are likely to find red phosphorus and possibly white phosphorus (Figure B). As you can see, white phosphorus has the molecular formula P4, whereas red phosphorus might be represented as Px, where x is a very large number. The difference in properties between the two allotropes reflects the difference in their bonding patterns, molecular versus network covalent ... 

The tetrahedral phosphate ion PO43-, like the silicate ion with which it is isoelectronic, can form chains and networks, and indeed its parent oxide, a white, extremely hygroscopic solid commonly called phosphorus pentoxide (P2O5), consists of discrete P4O10 molecular units in which four PO4 units each share three corner oxygens such that the four P centers themselves form a tetrahedron. We customarily distinguish the following classes of phosphates ... 

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