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Types of Materials, Electron Energy Bands, and Chemical Bonds

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Types of Materials, Electron Energy Bands, and Chemical Bonds Chapter 1 Types of Materials, Electron Energy Bands, and Chemical Bonds Look around you; you will easily distinguish three classes of materials: metals, ceramics, and poly- mers. Metals can be bent to shape but are strong; they conduct electricity and are never transparent. Ceramics, like stones, china, and glass, cannot be bent but fracture; they are electric insulators and some are transparent. Polymers (plastics) are lighter than the other materials and relatively soft; they do not keep the new shape when bent; they are used as electric insulators and can be transparent (most eyeglasses are plastic). Semiconductors are selected ceramics that are highly purifi ed and pro- cessed to achieve unique electronic properties. Fiberglass and other composites are artifi cial mixtures of two or three of the material classes mentioned above. Can we understand where these different properties come from? By studying the structure of atoms and the behavior of electrons when atoms are assembled to make materials, the differences between these classes become clear. In fact, these differences have much to do with Pauli’s exclusion principle; at the end of the book, you will be able to answer the following challenge: “ Draw a stone bridge and a steel bridge and explain their different shapes in terms of Pauli’s exclusion principle. ” We can also clas- sify the materials according to their utilization. One distinguishes structural materials, which are used for their strength in the construction of buildings and machines; functional materials, which include the conductors, insulators, semiconductors, optical materials, and magnets; and biomaterials, which are selected and processed for their compatibility with the human body and are used in the fabrication of prostheses and medical devices. In all these classes of utilization, one fi nds metals, ceramics, poly- mers, and composites. LEARNING OBJECTIVES After studying this chapter, the student will be able to: 1. Identify the main classes of materials and describe their distinguishing properties. 2. Describe the electronic structure of an atom in terms of nucleus and electrons. 004_P373587_Ch01.indd4_P373587_Ch01.indd 3 99/18/2008/18/2008 77:31:20:31:20 PPMM 4 CHAPTER 1 Types of Materials, Electron Energy Bands, and Chemical Bonds 3. Describe a chemical bond in terms of the lowering of the energy of shared valence electrons. 4. Defi ne electron energy levels and energy bands. 5. Distinguish between an energy band and a chemical bond. 6. Relate the electrical, optical, and mechanical properties of metals and ceramics to the behavior of the valence electrons in terms of Pauli’s exclusion principle. 7. Describe metallic, covalent, and ionic bonds. 8. Describe secondary bonds and distinguish between van der Waals and permanent dipole bonds. 9. Relate the mechanical, optical, and electrical properties of polymers to the nature of the sec- ondary bonds. 10. Distinguish between thermoplastic and thermoset polymers in terms of the bonds between molecular chains. 11. Relate elasticity and thermal expansion to the force potential of the chemical bond. 1.1 THE CLASSES OF MATERIALS Take a paper clip; it is made of metal . It is solid, but you can bend it and it will keep its new shape. Most of our tools are made of metals because they are strong, hard, and do not break readily under high stress. Let us observe how most metallic objects are fabricated: many consist of sheets or bars of metal that have been bent, stamped, or forged into their fi nal shapes; they have also been cut, sawed, or drilled and can be fabricated to high precision. Now look at a porcelain dish, which is a ceramic . You would not think of trying to bend it: it is very strong but brittle. A knife does not scratch a porcelain plate: porcelain is harder than steel but it will break when dropped to the fl oor. Ceramics remain strong at very high temperatures: ceramic bricks line the inside of furnaces and form the crucibles used in melting metals. These materials can only be machined by abrasion with diamond. Consider a plastic soda bottle. First, notice how light the plastic materials, scientifi cally known as poly- mers , are; this is because they are hydrocarbons, consisting mostly of carbon and hydrogen atoms, which are very light. The plastic is much softer than a metal or a ceramic; you can bend it, but it will not keep the new shape the way a metal does. Most plastics cannot be used at high temperatures; they soften or liquefy at temperatures not much higher than that of boiling water. Let us examine an electric cord. Its core is copper, a metal, which is an excellent conductor of electric- ity. In fact, all metals are good electric conductors. This wire is surrounded by a polymer insulator that can withstand high voltages without passage of any measurable current. Now look at a spark plug. The sparks are generated by a high voltage applied between two metallic electrodes. These are surrounded 004_P373587_Ch01.indd4_P373587_Ch01.indd 4 99/18/2008/18/2008 77:31:21:31:21 PPMM 1.1 The Classes of Materials 5 Table 1.1 Distinctive Properties of Metals, Ceramics and Polymers . Metals Ceramics Polymers Mechanical Properties Strong Hard Soft Deformable Not deformable Cannot be shaped Impact resistant Brittle like metals Electrical Properties Electric conductors Insulators Insulators Optical Properties Always opaque Can be transparent Can be transparent Composition Elements of columns 1 to 3 Diamond, Si, Ge and Hydrocarbons: molecules of the periodic table and all compounds: oxides, of C with H and some other transition metals and rare carbides, nitrides, sulfi des, elements. earths. selenides of metals. by a white ceramic insulator that can withstand a high voltage and tolerates the high temperatures of the engine. Ceramic insulators are also used to insulate high-voltage power lines from the metallic towers. Metals are good electric conductors; ceramics and polymers are electric insulators . Windows, glasses, diamond, sapphire (Al2 O3 ), quartz (SiO2 ), and many other ceramics are transpar- ent. Other ceramics are translucent, which means that light passes through them but is scattered by internal defects; some ceramics are colored; they are transparent to some wavelengths of light but not to others. Plastics can be transparent, translucent, or colored as well. The lenses of eyeglasses are now most often made of a polymer because of its low weight. Plastic soda bottles and many other polymers are trans- parent or translucent. No metal is transparent. Light of all wavelengths is absorbed and refl ected by metals. Metals are used as mirrors. Our observations are recapitulated in Table 1.1 . What about semiconductors and composites such as fi berglass or graphite tennis rackets? These mate- rials do not constitute different classes by nature. Semiconductors are ceramics (mostly silicon and compounds such as GaAs, GaP, etc.) or polymers; they are highly refi ned and processed to obtain their remarkable electronic properties. Composites are artifi cial mixtures of different materials that combine the properties of their constituents (fi berglass composites combine the strength of glass fi bers with the light weight of polymers; metal-ceramic composites possess the hardness of ceramics and the fracture resistance of metals). These will be discussed in later chapters. Can we understand what makes a material a metal, a ceramic, or a polymer? The major properties of the classes of materials are a direct consequence of the behavior of their valence electrons. In order to show that, we start by reviewing the structure of atoms and the periodic table, then we examine the behavior of the electrons when atoms join to form a molecule, and fi nally 004_P373587_Ch01.indd4_P373587_Ch01.indd 5 99/18/2008/18/2008 77:31:21:31:21 PPMM 6 CHAPTER 1 Types of Materials, Electron Energy Bands, and Chemical Bonds we examine the behavior of the electrons in a solid consisting of a very large number of atoms. With this knowledge, we analyze how the behavior of these electrons generates the distinctive properties of metals, ceramics, and polymers. 1.2 THE STRUCTURE OF ATOMS All matter is made of atoms. We know that every atom is formed of a small, positively charged nucleus that is surrounded by electrons. The nucleus is composed of protons and neutrons. Each pro- ton carries the electric charge q ϭ ϩ 1.6 ϫ 10 Ϫ 19 C; the neutron carries no charge. Each electron car- ries the negative charge q ϭ Ϫ 1.6 ϫ 10 Ϫ 19 C. A neutral atom contains the same number of protons and electrons. If this number is not equal, the atom carries a positive or negative charge and is called an ion. The number of protons in its nucleus characterizes every element; this is the atomic number . The proton and the neutron are about 1,840 times heavier than the electron; the number of protons and neutrons in the nucleus of an atom determines the atomic mass of an element . Not all atoms of a given element have the same number of neutrons. Atoms of a particular element that have different numbers of neutrons are referred to as isotopes . Because most elements are mixtures of different iso- topes, the atomic mass characterizing a given element is not necessarily an integer. Iron, for instance, has atomic number 26 and atomic mass 55.85. Iron atoms contain 26 protons and 26 electrons. About 85% of iron atoms contain 30 neutrons and 15% of them have 29 neutrons. The molecular weight of a compound is the sum of the atomic masses of the elements it contains. The molecu- ϭ ϩ ϫ lar weight of water is 18.016 16.00 (O) 2 1.008 (H); the molecular weight of alumina (Al2 O3 ) is ϭ 2 ϫ 26.98 (Al) ϩ 3 ϫ 16 (O) ϭ 101.98.
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