Nonmetallic elemental solids

Several nonmetallic elements and metalloids have a network covalent structure. The most important of these is carbon, which has two different crystalline forms of the network covalent type. Both graphite and diamond have high melting points, above 3500°C. However, the bonding patterns in the two solids are quite different... 

The ionic model, the description of bonding in terms of ions, is particularly appropriate for describing binary compounds formed from a metallic element, especially an s-block metal, and a nonmetallic element. An ionic solid is an assembly of cations and anions stacked together in a regular array. In sodium chloride, sodium ions alternate with chloride ions, and large numbers of oppositely charged ions are lined up in all three dimensions (Fig. 2.1). Ionic solids are examples of crystalline... 

Elemental sulfur is a yellow, tasteless, almost odorless, insoluble, nonmetallic molecular solid of crownlike S8 rings (9). The two common crystal forms of sulfur are monoclinic sulfur and rhombic sulfur. The more stable form under normal conditions is rhombic sulfur, which forms beautiful yellow crystals (Fig. 15.12). At low temperatures, sulfur vapor consists mainly of S8 molecules. At temperatures above 720°C, the vapor has a blue tint from the S, molecules that form. The latter are paramagnetic, like O,. 

The (compositionally) simplest mineral class comprises the native elements, that is, those elements, either metals or nonmetals that occur naturally in the native state, uncombined with others. Native gold, silver, and copper, for example, are metals that naturally occur in a ductile and malleable condition, while carbon - in the form of either graphite or diamond -and sulfur are examples of nonmetallic native elements. Next in compositional complexity are the binary minerals composed of two elements a metal or nonmetallic element combined with oxygen in the oxides, with a halogen - either fluorine, chlorine bromine, or iodine - in the halides, or sulfur, in the sulfides. The oxide minerals, for example, are solids that occur either in a somewhat hard, dense, and compact form in mineral ores and in rocks, or as relatively soft, unconsolidated sediments that melt at moderate to... 

Uranium reacts with most nonmetallic elements to form a variety of compounds, all of which are radioactive. It reacts with hot water and dissolves in acids, but not in alkalis (bases). Uranium is unique in that it can form solid solutions with other metals, such as molybdenum, titanium, zirconium, and niobium. 

It is important to realize that temperature markedly affects the solubility of most substances. For almost all salts, which are solid compounds composed of positive and negative ions (most often composed of both metallic and nonmetallic elements), an increase in temperature leads to an increase in the amount of the salt that will dissolve. Figure 9-2 shows the solubilities of potassium chloride (KCl) and potassium nitrate (KNO3) as a function of temperature. 

Carbon is a solid and one of the eighteen nonmetallic elements. The nonmetals have a number of common properties ... 

The nonmetallic elements, with the exception of hydrogen, are on the right of the periodic table. Nonmetals are very poor conductors of electricity and heat and may also be transparent. Solid nonmetals are neither malleable nor ductile. Rather, they are brittle and shatter when hammered. At 30°C (86°F), some non-metals are solid (carbon, C), others are liquid (bromine, Br), and still others are gaseous (helium, He). 

If you were asked to produce at ordinary temperatures a sample consisting of the free atoms of an element, your only choice would be a Group 18 element, one of the noble gases. All the other elements occur with their atoms linked together in some way. The nonmetallic elements exist as molecules, such as the diatomic species H2, N2, 02, F2, Cl2, Br2, and I2, and the polyatomic species P4 and S8. Elements near the border between metals and nonmetals can form solids with an extended network of atoms, such as the graphite or diamond forms of carbon and crystalline silicon. There are also countless examples of diatomic, polyatomic, and extended network compounds between different nonmetallic elements, including the millions of organic compounds. 

Of the predominantly nonmetallic elements, seven are solids under ordinary atmospheric conditions (B, C, Si, P, S, Se, and I), only one is a liquid (Br), and the remainder are gases (H, N, 0, F, Cl, He, Ne, A, Kr, and Xe). The physical properties of these elements present far more striking contrasts than do those of the metals. Thus, among the nonmetals one encounters the extremely volatile helium, which boils at -267°C (i.e., just 5°C above absolute zero), and the nonvolatile element carbon, which melts at about 3500°C. Similarly, the densities and other physical properties of these elements differ tremendously, as is made more evident by an inspection of a table of physical properties of the elements. 

The chemical elements provide examples of three of the four classes of crystalline solids described in this section. Only ionic solids are excluded, because a single element cannot have the two types of atoms of different electronegativities needed to form an ionic material. We have already discussed some of the structures formed by metallic elements, which are sufficiently electropositive that their atoms readily give up electrons to form the electron sea of metallic bonding. The nonmetallic elements are more complex in their structures, reflecting a competition between intermolecular and intramolecular bonding and producing molecular or covalent solids with varied properties. 

The structural and bonding trends in halides follow similar patterns to those in oxides (see Topics B2 and F7). Most nonmetallic elements form simple molecular compounds in which halogen atoms each have a single bond to the other element. This is true also for metals in high oxidation states (e.g. TiCl4 and UF6). The compounds may be solids, liquids or gases, with volatility in the order... 

Elements respond to these driving forces in ways that depend on their chemical characteristics. Volatile molecules formed by nonmetallic elements enter the atmosphere from volcanic emissions, as waste products of life, and from human energy use and industry. Some volatile compounds are rapidly oxidized by photochemical processes, and some are quickly washed out by dissolving in rainfall. Elements (especially metallic ones) that do not form volatile compounds under normal conditions are confined to the solid and liquid parts of the environment. Soluble ions (e.g. Na+, Cl-) are removed from rocks in weathering processes and end up in sea water. Other elements (e.g. Al,... 

Objective 30 The nonmetallic elements exhibit more variety in the types of particles their solids... 

Given the name or chemical formula for a substance, (1) categorize the substance as a metallic element, carbon in the diamond form, another nonmetallic element, an ionic compound, a polar molecular compound with hydrogen bonds, a polar molecular compound without hydrogen bonds, or a nonpolar molecular compound (2) identify the type of particle that forms its fundamental structure and (3) identify the type of attraction holding its particles in the solid and liquid form. 

Consult a handbook of chemical and physical data (ask your instructor where you can locate a copy of the handbook) to find (a) two metals less dense than water, (b) two metals more dense than mercury, (c) the densest known solid metallic element, (d) the densest known solid nonmetallic element. 

Properties of nonmetals are more varied than those of metals. A number of nonmetals are gases in the elemental state hydrogen, oxygen, nitrogen, fluorine, chlorine, and the noble gases. Only one, bromine, is a liquid. All the remaining nonmetals are solids at room temperature. Unlike metals, nonmetallic elements are poor conductors of heat and electricity they exhibit both positive and negative oxidation numbers. 

Sulfur is a tasteless, odorless, nonmetallic element. Sulfur along with selenium (Se) and tellurium (Te) are called chalcogens. The valences of sulfur are 2, 4, and 6, which can be represented by compounds such as hydrogen sulfide (H2S), sulfur dioxide (SO2), and barium sulfate (BaS04), respectively. Pure sulfur is insoluble in water. The most stable variety of sulfur, rhombic sulfur, is a yellow crystalline solid. 

The structures of solid nonmetallic elements are more varied than those of metals. In fact, different forms of the same element often occur. For example, solid carbon occurs in three forms. Different forms of a given element are called allotropes. The three allotropes of carbon are the familiar diamond and graphite forms and a form that has only recently been discovered—buckminsterfullerene. These elemental forms have very different properties because of their different structures (see Figure 3.14). 

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