Lecture 02 - Bonding and crystal structure

Lecture 02 - Bonding and crystal structure - MATE1412:...

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MATE1412 2011 MATE1412: Materials Engineering I Lecture 2 Callister : 8 th Ed -Chapter 2, pages 28-39, & Chapter 3, pages 46-55, 72-73 Prof. Tim Sercombe Room : 2.12 Phone : 6488 3124 email :: tim.sercombe@uwa.edu.au
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MATE1412 2011 Reminders • Tutorials start next week, Labs in Week 3 • Closed in shoes are required for ALL labs. • Use OLCR to sign up for labs • Text book: can buy text and WileyPlus access code at a discount • Can also purchase online text through Wiley Plus • All relevant information on unit website ( http://www.mech.uwa.edu.au/unit/MATE1412/ ).
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MATE1412 2011 Learning Objectives At the end of this lecture you should be able to: – Understand the factors that promotes bonding – Recognise difference between the types of bonding – Infer material properties from the type of bonding – Understand how atoms assemble into solid structures (for now, focused on metals) – Understand how the density of a material depends on its structure – Calculate the theoretical density of an element – Understand the difference between a single crystal and polycrystalline material
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MATE1412 2011 Bonding Forces and Energies • Many physical properties are determined by the interatomic forces that bind two atoms together. • Consider the simple case of bringing 2 atoms close to each other.
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As gap decreases, attractive forces dominate At some point electron shells overlap => repulsive forces At large separations, negligible forces exerted At equilibrium, the attractive force, F A equals the repulsive force, F R
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Bonding Force Equilibrium point, r O , typically ~3Å = 3x10 -10 m Adapted from Fig. 2.8(a), Callister and Rethwisch 8e.
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Bonding Energy Bonding energy , E O , is the energy required to separate the two atoms to infinite distance Equilibrium point Adapted from Fig. 2.8(b), Callister and Rethwisch 8e.
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Bonding Energy and Properties • Materials with a high bonding energy have a: High or low melting point? Elements Bonding Energy (kJ/mol) Melting Point (ºC) Hg 68 -38 Al 324 660 Fe 406 1538 W 849 3410 C 713 ~4440
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MATE1412 2011 Bonding Energy and Properties • At room temperature : – Solids have high bonding energy (eg Al 324kJ/mol) – Liquids intermediate bonding energy, (eg Hg 68kJ/mol) – Gases low bonding energy (eg Ar 7.7kJ/mol). • Also can determine – Stiffness – Thermal expansion co-efficient – Strength
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MATE1412 2011 Primary Interatomic Bonds • Three types – Ionic – Covalent – Metallic • Nature of bond depends on electron structure of the atoms (ie. the valence e - ) • Atoms are more stable with full outermost electron shell (like noble gases) and will give or take e - to get there.
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MATE1412 2011 Ionic bond metal + nonmetal donates accepts electrons electrons Dissimilar electronegativities ex: Mg O Mg 1 s 2 2 s 2 2 p 6 3 s 2 O 1 s 2 2 s 2 2 p 4 [Ne] 3 s 2 Mg 2+ 1 s 2 2 s 2 2 p 6 O 2- 1 s 2 2 s 2 2 p 6 [Ne] [Ne]
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MATE1412 2011 Ionic Bonding • Occurs between + and - ions. • Requires
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This note was uploaded on 09/06/2011 for the course ENGINEERIN 1 taught by Professor Tim during the Spring '11 term at American University of Kuwait.

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Lecture 02 - Bonding and crystal structure - MATE1412:...

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