Final_Review_-_Quick_answer - 1 Why are the atomic weights...

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1. Why are the atomic weights of the elements generally not integers? Give two reasons. (1) the atomic masses of the atoms are not integers (except for carbon 12), and (2) the atomic weight is taken as the weighted average of all naturally occurring isotopes. 2. Give electron configurations for a nitrogen atom and Mg 2+ ion. N: 1s 2 2s 2 2p 3 Mg 2+ : 1s 2 2s 2 2p 6 3. Why are covalently bonded materials generally less dense than ionically or metallically bonded ones? Covalent bonds are directional in nature whereas metallic and ionic ones are not. When bonds are directional, the atoms cannot pack together in as dense a manner, yielding lower mass density. 4. Name the three most common metallic crystal structures. Which ones are close- packed? FCC, BCC and HCP. FCC and HCP are close-packed. 5. What is the atomic packing factor (APF)? The APF is the sum of the sphere volumes of all atoms within a unit cell (assuming the atomic hard sphere model) divided by the unit cell volume. 6. Do noncrystalline (amorphous) materials display the phenomenon of allotropy (or polymorphism)? No, since noncrystalline materials do not have a defined crystal structure, they cannot have more than on crystal structure, which is the definition of allotropy. 7. What are two types of point defects? Vacancies, interstitials, impurities (interstitial or substitutional) 8. Name the three types of dislocations (hint, one is a combination of the other two)? For each type, is the Burgers vector parallel or perpendicular to the dislocation line? Edge (perpendicular), screw (parallel), mixed (it depends on the location, can be parallel, perpendicular, or neither) 9. Name 2 types of interfacial defects, and 2 types of bulk/volume defects. Interfacial: grain boundaries, external surfaces, twin boundaries, stacking faults, phase boundaries, domain walls. Bulk/volume: pores, cracks, foreign inclusions, other phases. 10. Name two methods that atoms can diffuse through a bulk material. Vacancy diffusion and interstitial diffusion. 11. What is Fick’s fist law? What is Fick’s second law?
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First law (in 1D): dC J D dx = - Second law (in 1D): 2 2 dC d C D Dt dx = 12. Draw a stress-strain curve for a ductile and brittle metal. See figure 6.13 on page 147 of the textbook 13. Describe plastic deformation in one short sentence. Deformation that is permanent or nonrecoverable after release of the applied load. 14. What is slip? What is a slip system. Slip is the plastic deformation as the result of dislocation motion; also, the adjacent shear displacement of two planes of atoms. A slip system is the combination of a crystallographic plane and, within that plane, a crystallographic direction along which slip occurs. 15. Name three ways to strengthen a material.
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Final_Review_-_Quick_answer - 1 Why are the atomic weights...

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