Name: ________________

First Exam 1 hour

Winter 1997 Prof. M. Barsoum

1) Answer the following questions:

5 pts i) The first three terms of the Madelung constant for the NaCl structure______________

5 pts ii) The creation of vacancies in a solid is an endothermic/endothermic (choose one) process because __________________________________________________________.

5 pts iii) The band gap of a material is the minimum energy required to create ___________ and/or ____________

5 pts iv) The surface energy of a solid increases/decreases (choose one) as the melting point of the solid increases because ________________________________________.

6 pts v) Complete the following reactions and identify them as stoichiometric, non-stoichiometric or extrinsic reactions. For the first two, i.e. stoichiometric and non-stoichiometric, make sure you write down the mass action expression as well.

Dissolution of Al₂O₃ in MgO

5 pts. vi) The wave function of an electron is useful because once it is known one can say something about ______________________________________________

2 pts. vi) Was Bohr boring? Was Boltzmann insane? What are You doing here? ________________________________________________________

2) The antifluorite structure is one in which the anions are in a cubic close-packed arrangement and the cations occupy all the tetrahedral sites.

5 pts a) Draw the unit cell.

10 pts b) If the cations are Li and the anions are oxygen, calculate the theoretical density of lithium oxide. Information you need will be given in class.

5 pts c) Write down two possible defect reactions one stoichiometric and the other nonstoichiometric, and their mass action expressions for lithium oxide.

5 pts d) Based on this structure what is the most likely stoichiometric defect you would expect? Explain.

5 pts f) This structure is more likely to occur if the cations are large/small (choose one) with respect to the anions because __________________________________________.

____________________________________________________________________________

4 pts. 3) a) The energy of a bond is usually made up of two terms; one repulsive and the other attractive. For an ionic bond write down the two terms and plot each one on the same curve.

15 pts.b) On the same curve also plot the net energy curve and explain how from that curve it is possible, in principle, to determine Young’s modulus and thermal expansion point of a solid.

8 pts. c) Starting with the two expressions in part a, prove that for an ionic bond:

Define all terms.

4 pts. d) If the ionic bond in parts a and b was that between Na and Cl, what is the relationship, if any, between r_o and the lattice parameter of NaCl?

6 pts. e) What factors favor the formation of ionic bonds in ceramics and which favor the formation of covalent bonds.