section%2030.2-3.4 - Chapter 3: Bonding 3.2 Primary Bonds...

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Chapter 3: Bonding 3.2 Primary Bonds Objectives Predict the type of bonds formed for different atoms. Predict properties from knowledge of bonding. Materials are not made of individual atoms, they are made of groups of atoms. Sometimes these are groups of all the same kind of atom (like a piece of iron), and sometimes these are groups of different atoms (like polyethylene, which is made of carbon and hydrogen). While this is probably obvious, it is important because the bonds that form between the atoms has a significant influence on the material’s properties. For example, melting point, stiffness, and thermal expansion are all directly related to the strength of the bonds. This relationship between structure and properties is one of the major themes of materials science and engineering, and so understanding the types of bonds that are present in different materials is very important. One way to define chemical compounds is that they are groups of atoms held together by primary bonds . Primary bonds involve the sharing of electrons between different atoms. Electrons are shared to satisfy the octet rule . This rule states that atoms are most stable when they have a filled valence shell, which for many atoms means 8 electrons. (One important exception to this is hydrogen, which has 2 electrons in a filled valence shell.) There are three basic types of primary bonds, each of them formed by different ways of sharing the electrons to satisfy the octet rule. These three types are ionic, covalent, and metallic bonds. Schematic illustrations of each of these bonds are shown in Figures 3.2.1-3.2.3 and the strengths of the different types of primary bonds are shown in Table 3.2.1. Electrons are shared between atoms in each of these bonds as follows: Ionic bond : Bond formed when an electron is transferred from one atom to another to satisfy the octet rule for each of them, resulting in positive and negative ions. These ions are then attracted to each other through electrostatic interactions. As shown in Table 3.2.1, ionic bonds are the strongest bonds and the melting temperatures of ionic compounds are the highest of any material. Covalent bond
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This note was uploaded on 03/30/2010 for the course EMA 3010 taught by Professor Unknown during the Spring '08 term at University of Florida.

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section%2030.2-3.4 - Chapter 3: Bonding 3.2 Primary Bonds...

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