Electrical conductors, crystalline solids

Unlike metals, which are good electrical and heat conductors, crystalline solids such as diamond and seinicoriductors such as silicon are good heat conductors but poor electrical conductors. As a result, such materials find widespread use in the electronics industry. Despite their higher price, diamond heat sinks are used in the cooling of sensitive electronic components because of the... 

Approximately 75% of all elements found on and in the Earth are metals. They are crystalline solids that at room temperature range from hard to butter-like soft to liquid (mercury). They are generally good conductors of heat and electricity as a result of the swarm of relatively free electrons in their outer shell that move without much resistance to other elements, particularly those with a dearth of electrons in their outer shells. In pure states, most metals have a shiny luster when cut. Those located at the far left of the table have only one electron in their outer shell. Therefore, they are very reactive and are not usually found in pure form. Instead, they are found in compounds, minerals, or ores that must be processed to extract the pure metal from the other elements in the compounds. 

Molecular solids are crystalline solids in which molecules are held together by weak electrical forces between molecules. These bonds between molecules are usually weaker than bonds within molecules. Molecular solids dissolved in water are not good electrical conductors. Examples of molecular solids are sulfur (S8) phosphorus (P4) and water, or ice (H20). 

The nature of the bonds between the structural units of crystalline solids impart other physical properties to these solids. Metals are good conductors of electricity because metallic bonds allow a free flow of electrons. Covalent network, molecular, and ionic solids do not conduct electricity because their bonds do not provide for mobile electrons. Remember, however, that ionic solids in a water solution have free electrons and are good conductors of electricity. Metallic solids are malleable and ductile covalent network solids are brittle and hard. These differences in physical properties are caused by the chemical bonds between the units It is all in the bonds ... 

Sodium chloride is a typical ionic compound. Like most ionic compounds, it is a crystalline solid at room temperature. It melts at a very high temperature, at 801°C. As well, it dissolves easily in water. A solution of sodium chloride in water is a good conductor of electricity. Liquid sodium chloride is also a good electrical conductor. 

Tetrachlorides of S, Se, Te and Po are formed by direct chlorination. SCI4 is a yellow liquid, stable only at low temperatures SeCl4 is a colourless, crystalline solid subliming at 196°, and TeCl4 a white, hygroscopic solid, m.p. 224°. Fused TeCl4 is an even better conductor of electricity than BrFg probably because... 

All but 40 of the known elements ore metals. Metals are elements whose atoms can lose one or more electrons to form electrically positive ions. Most metals are good conductors of heat and electricity. They are malleable (can be beaten or rolled into a new shape) and ductile (can be pulled out into long wires). All metals are shiny, crystalline solids, except mercury, which is a liquid. 

When white phosphorus is heated at 200° under a pressure of 12,000 kgm. per sq. cm., transformation takes place into another allotropic modification known as black phosphorus. This forms a black crystalline solid, insoluble in carbon disulphide. It can be ignited with difficulty with a match, its ignition temperature in air being about 400°. When heated in a closed tube it vaporises and condenses to violet and white phosphorus. It differs from the other forms of phosphorus in being a conductor of electricity. Its density is 2 691, The question of the relative stability of violet and black phosphorus has perhaps not yet been definitely settled but the results obtained point to violet phosphorus being the more stable form, ... 

The initial product obtained in the reaction of mercury with AsFs was a crystalline solid with a distinct golden metallic lustre. X-ray crystallography (5) demonstrated that the compound could be formulated as Hg2 86AsFe, and it revealed the presence of infinite linear chains of mercury atoms in two mutually perpendicular directions. It was realized that the electrical properties of such a compound would be of considerable interest because the compound might be a highly anisotropic metallic conductor. 

Of the solids given, ionically bonded sodium chloride is expected to be crystalline, a poor electrical conductor in the solid form, and a good conductor when fused. Diamond, formed of covalently bonded carbon atoms, is a network substance that does not form cubic crystalline patterns, and does not conduct electricity either when solid or fused. None of the allotropic forms of sulfur is expected to conduct electricity. Choice (D), the metal chromium, could possibly form a cubic solid crystalline form, but can be eliminated because it is expected to conduct electricity both when a solid and when fused. The correct choice is (A), because sodium chloride is a crystalline solid that is a poor conductor in the solid state and a good conductor when fused. ... 

Finally, ionic bonds are formed between elements from opposite sides of the periodic table (typically between a metal and a nonmetal), where there is a large difference in electronegativity between the atoms. Because metals have very low lEs and nonmetals have large EAs, an ionic bond is characterized by the transfer of one or more electrons from one atom to another to form a cation-anion pair. As a general rule, the nondirectional nature of the electrostatic attraction between the ions leads to fairly high melting and boiling points. Most ionic solids are insulators because the ions are fixed in place in the crystalline lattice however, they become electrical conductors when molten or dissolved in aqueous solution. 

Based on the following properties of elemental boron, classify it as one of the crystalline solids discussed in Section 11.6 high melting point (2300°C), poor conductor of heat and electricity, insoluble in water, very hard substance. 

Nonmetallic (ionic) crystalline solids such as salt are nonconductors of electricity and fair conductors of heat. They vary widely in... 

Metals share several common properties. All metals are opaque (we cannot see through them), and they are good conductors of heat and electricity. They generally have high malleability (the ability to be bent or hammered into desired forms) and ductihty (the ability to be drawn into wires). We can explain these properties by the bonding theories that we have already examined for metals the electron sea model and band theory. In the electron sea model, discussed in Section 9.11, each metal atom within a metal sample donates one or more electrons to an electron sea, which flows within the metal. In band theory, discussed in Section 11.13, the atomic orbitals of the metal atoms are combined, forming bands that are delocalized over the entire crystalline solid electrons move freely within these bands. The mobile electrons in both of these models endow metals with many of their shared properties. 

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