MAE381 2 2010

MAE381 2 2010 - Chapter 2: Atomic bonding Reading...

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Unformatted text preview: Chapter 2: Atomic bonding Reading assignment Ch. 2 and 3 of textbook Effect of atomic bonding Example: carbon exists as graphite (soft with greasy feeling) or diamond (hardest known material) Atomic & electronic configuration Bonding BOND STRENGTH Mechanical & Physical Properties graphite diamond Group 2A of the Periodic Table Primary bonding types Ionic bonding Covalent bonding Metallic bonding Ionic bonding Non-directional bond due to electrostatic forces Coulombic force F is proportional to F > 0 Attractive force F < 0 Repulsive force- Interionic force Force of attraction between Na + and Cl- ions Z 1 = +1 for Na + , Z 2 = -1 for Cl- e = 1.60 x 10-19 C , = 8.85 x 10-12 C 2 /Nm 2 a = Sum of Radii of Na + and Cl- ions = 0.095 nm + 0.181 nm = 2.76 x 10-10 m ( 29 N C a e Z Z F attraction 9 10- 2 12- 2 19 2 2 2 1 10 02 . 3 m) 10 x /Nm2)(2.76 C 10 x 8.85 ( 4 ) 10 60 . 1 )( 1 )( 1 ( 4-- + = - +- =- = + 2-18 Bond energy Bonding force and Energy curves for a Na + & Cl- pair. Since F = dE/da, the equilibrium bond length (a o ) occurs @ F=0 and E is a minimum. Interatomic spacing The equilibrium distance between atoms is caused by a balance between repulsive and attractive forces. Equilibrium separation occurs when no net force acts to either attract or separate the atoms or the total energy of the pair of atoms is at a minimum. For a solid metal the interatomic spacing is equal to the atomic diameter or 2r. For ionically bonded materials, the spacing is the sum of the two different ionic radii. Coordination Coordination number (C.N.) = No. of nearest neighbors (radius R) around (touching) a particular atom/ion (radius r). C.N. depends on r/R ratio. C.N. = 4 (in 2 dimensions) Stable Stable Unstable (critical case) r/R > min r/R = min r/R < min C.N. = 2 Schematic drawing with nearest neighbors not in contact C.N. = 3 Schematic drawing with nearest neighbors not in contact C.N. = 4 (3 dimensions) C.N. = 4 (3 dimensions) Tetrahedral coordination C.N. = 4 (3 dimensions)...
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MAE381 2 2010 - Chapter 2: Atomic bonding Reading...

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