COURSE The Structure of Metals Lesson:Tvpes of Atomic Bonds
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Introduction Why are some metals hard and others soft? why are some metals brittle, while others are ductile and thus can be shaped easily without rature? why is it that some metals can withstand high temperatures, while others cannot? Why is it that a sheet metal may behave differently when stretched in one direction versus another? These and numerous other questions can be answered by studying the atomic structure of metals-that is, the arrangement of the atoms within the metals-because atomic structure greatly influences the properties and behavior of those metals. This knowledge then guides us in controlling and predicting the behavior and performance of metals in various manufacturing processes. Understanding the structure of metals also allows us to predict and evaluate their properties, thus allowing us to make appropriate selections for specific applications under various conditions.
Types of Atomic Bonds All matter is made up of atoms containing a nucleus of protons and neutrons and surrounding clouds, or orbits, of electrons. In recent decades, a large number of subatomic particles have been identified, with additional complexities in the nucleus. The number of protons in the nucleus determines whether an atom will be metallic, non metallic, or semi metallic. An atom with a balanced charge has the same number of electrons as protons; when there are too many or too few electrons, the atom is called an ion. An excess of electrons results in a negatively charged atom, referred to as an anion, while too few electrons results in a positively charged atom, called a cation.
Types of Atomic Bonds The number of electrons in the outermost orbit of an atom determines the chemical affinity of that atom for other atoms. Atoms can transfer or share electrons; in doing so, multiple atoms combine to form molecules. Molecules are held together by attractive forces called bonds through electron interaction The basic types of atomic attraction associated with electron transfer, called primary or strong bonds, are as follows: lonic bonds Covalent bonds Metallic bonds
Tvpes of Atomic Bonds The basic types of atomic attraction associated with electron transfer, called primary or strong bonds, are as follows: lonic bonds: When one or more electrons from an outer orbit are transferred from one material to another, a strong attractive force develops between the two ions. An example is that of sodium (Na) and chlorine (CI) in common table salt; such salt consists of Na+ and Cl ions (hence the term ionic bond), which are strongly attracted to each other. Also, the attraction is between all adjacent ions, allowing crystalline structures to be formed. Molecules with ionic bonds generally have poor ductility and low thermal and electrical conductivity Covalent bonds: In a covalent bond, the electrons in outer orbits are shared by atoms to form molecules. The number of electrons shared is reflected by terms such as "single bond," "double bond," etc. Polymers consist of large molecules that are covalantly bonded together; water (H2O) and nitrogen gas (N2) are additional commorn examples of molecules formed from covalent bonds. Solids formed by covalent bonding typically have low electrical conductivity and can have high hardness. (Diamond, a form of covalently bonded carbon, is an example.)
Types of Atomic Bonds Metallic bonds: Metals have relatively few electrons in their outer orbits; thus, they cannot complete the outer shell of other self-mated atoms. Instead, metals and alloys form metallic bonds, whereby the available electrons are shared by all atoms in contact. The resultant electron cloud provides attractive forces to hold the atoms together and results in generally high thermal and electrical conductivity. In addition to the strong attractive forces associated with electrons, weak or secondary attractions occur between molecules. Also referred to as van der Waals forces, these forces arise from the attraction of opposite charges without electron transfer.
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