MATERIALS OF ENGR FO
MATERIALS OF ENGR FO ME 2723
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This 2 page Class Notes was uploaded by Dolly Koepp on Tuesday October 13, 2015. The Class Notes belongs to ME 2723 at Louisiana State University taught by A. Raman in Fall. Since its upload, it has received 27 views. For similar materials see /class/222913/me-2723-louisiana-state-university in Mechanical Engineering at Louisiana State University.
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Date Created: 10/13/15
Types of Inter atomic Bonds There are basically four different types of bonds prevailing between atoms in solids These are 1 Metallic 2 Ionic 3 Covalent and 4 H bond or Dipole bond These are brie y described below 1 Metallic Bond found in all metals and alloys Metallic materials have atoms of elements that have excess number of free electrons than required to ll up the last outermost closed shell The atoms do not have neighbors with which electronic interactions take place to provide the outermost sp octets completed The excess valence electrons move freely in outer orbits of adjacent bonded atoms and appear as a free moving uid The atomic core with the inert gas complete shell equivalent con guration appears to have a net charge surrounded by the sea of free electrons or the electron cloud This gives rise to coulombian attractive forces between the positive atom cores and the negative electron cloud pervading in between Valency number of electrons that can be used for bonding comes from free electrons Metals form ions They shrink in the process of giving up electrons They are good conductors of electricity and heat because of the presence of free electrons 0 They have pliability plastic deformability again because of the presence of free elctrons They corrode because they can easily give up the free electrons past the nearby closed shell and become ions Equot Ionic Bond The ionic bond prevails in all naturally occurring ceramic solids Ceramic materials are made up of and 7 ions The ions are from metal atoms that give up electrons The 7 ions are from metalloid or non metallic atoms or atom groups such as OH or 04 etc Oppositely charged ions attract each other and a type of ion gets surrounded and contacted by ions of the opposite kind Thus ions are surrounded and touched by 7 ions as rst neighbors and Viceversa Like ions do not touch each other Unlike ions are in contact The bonding is coulombian in nature much stronger than in metals The strong bond facilitates higher melting temperatures for ceramics There are no free electrons present normally This gives rise to electrical insulation or at best semiconduction ions are smaller in size than the 7 ions and often get themselves located in the interstices between the 7 ions The ionic radius ratio dictates what kind of coordination would occur ions are called cations eg Fey Fe Al etc the 7 ions are referred to as anions e g Cl39 0239 OH39 etc 3 Covalent Bond found in plastic and polymers rubber etc carbonaceous materials also in Si Ge etc semiconductor elements and compounds Atoms in bond share the outer valence electrons beyond the nearest closed shell such that each acquires closed shell con guration The sp octet is lled using the electrons in common shared pool No excess electron is present after such sharing in common pool Electrons in bond are held in localized directional bonds A single bond has one electron from each of the two bonded atoms thus there will be two electrons in common pool common for both atoms in bond Since no free electron is present the material comes out to be an insulator Some are semiconductors Semiconductors form spatial network of atoms arranged in an orderly manner which enables crystal formation basic semiconductors have the diamond cubic structure Cbased organic materials form molecules distinct groups of atoms small and large polymersplastics within which covalent bond prevails Outside of these molecules the attraction between the molecules is ofthe dipole bond type described in the next section The materials do not generally crystallize Under stress some may form crystalline domains and the plastic becomes partially crystalline 4 Dipole Bond Hydrogen Bond In plastic and polymers dipole bond prevails 0 Neutral molecules form electric dipoles due to nonsymmetric nature of the molecules Adjacent molecules line up such that opposite poles of the electric dipoles face each other and are attracted by coulombian forces The poles are weak and hence the force of attraction is also very weak and can be broken easily The bond energy is very low usually less than 1 eV In hydrogen bond the charge of the H nucleus attached to the other atoms in bond enables the H nucleus to get attached to different negatively charged atoms such as O 39 ions Thus in water the charge center of the H ion will be attached to two 0239 ions in two different H20 molecules enabling weak hydrogen bond But hydrogen bond is stronger than the dipole bond discussed earlier The bonding between two HF molecules is similarly due to the strong charge on H ion getting attracted to two F39 ions on two adjacent molecules Note that the bond strength is given in energy units usually in eV per bonded unit The bond strength is high in ionic and covalent bonds but weak in metallic and very weak in dipole bonds The highest bond energy could be as much as 7 to 8 eV per bond
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