SOLUTIONS TO PROBLEMS PREFACE This section of instructor's resource materials contains solutions and answers to all problems and questions that appear in the textbook. My penmanship leaves something to be desired; therefore, I generated these solutions/answers using computer software so that the resulting product would be "readable." Furthermore, I endeavored to provide complete and detailed solutions in order that: (1) the instructor, without having to take time to solve a problem, will understand what principles/skills are to be learned by its solution; and (2) to facilitate student understanding/learning when the solution is posted. I would recommend that the course instructor consult these solutions/answers before assigning problems and questions. In doing so, he or she ensures that the students will be drilled in the intended principles and concepts. In addition, the instructor may provide appropriate hints for some of the more difficult problems. With regard to symbols, in the text material I elected to boldface those symbols that are italicized in the textbook. Furthermore, I also endeavored to be consistent relative to symbol style. However, in several instances, symbols that appear in the textbook were not available, and it was necessary to make appropriate substitutions. These include the following: the letter a (unit cell edge length, crack length) is used in place of the cursive a. And Roman E and F replace script E (electric field in Chapter 18) and script F (Faraday's constant in Chapter 17), respectively. I have exercised extreme care in designing these problems/questions, and then in solving them. However, no matter how careful one is with the preparation of a work such as this, errors will always remain in the final product. Therefore, corrections, suggestions, and comments from instructors who use the textbook (as well as their teaching assistants) pertaining to homework problems/solutions are welcomed. These may be sent to me in care of the publisher. 1 CHAPTER 2 ATOMIC STRUCTURE AND INTERATOMIC BONDING PROBLEM SOLUTIONS 2.1 (a) When two or more atoms of an element have different atomic masses, each is termed an isotope. (b) The atomic weights of the elements ordinarily are not integers because: (1) the atomic masses 12 of the atoms generally are not integers (except for C), and (2) the atomic weight is taken as the weighted average of the atomic masses of an atom's naturally occurring isotopes. 2.2 Atomic mass is the mass of an individual atom, whereas atomic weight is the average (weighted) of the atomic masses of an atom's naturally occurring isotopes. 2.3 (a) In order to determine the number of grams in one amu of material, appropriate manipulation of the amu/atom, g/mol, and atom/mol relationships is all that is necessary, as ⎛ 1 mol ⎞ ⎛ 1 g/mol ⎞ #g/amu = ⎜ ⎟ ⎜ ⎟ ⎝ 6.023 x 1023 atoms⎠ ⎝ 1 amu/atom⎠ -24 = 1.66 x 10 g/amu (b) Since there are 453.6 g/lb , m ( )( 23 ) 1 lb-mol = 453.6 g/lb 6.023 x 10 atoms/g-mol m 26 = 2.73 x 10 atoms/lb-mol 2.4 (a) Two important quantum-mechanical concepts associated with the Bohr model of the atom are that electrons are particles moving in discrete orbitals, and electron energy is quantized into shells. (b) Two important refinements resulting from the wave-mechanical atomic model are that electron position is described in terms of a probability distribution, and electron energy is quantized into both shells and subshells--each electron is characterized by four quantum numbers. 2.5 The n quantum number designates the electron shell. 2 The l quantum number designates the electron subshell. The m quantum number designates the number of electron states in each electron subshell. l The m quantum number designates the spin moment on each electron. s 2.6 For the L state, n = 2, and eight electron states are possible. Possible l values are 0 and 1, while 1 possible m values are 0 and ±1. Therefore, for the s states, the quantum numbers are 200( ) and l 2 1 1 1 1 1 200(− ). For the p states, the quantum numbers are 210( ), 210(− ), 211( ), 211(− ), 2 2 2 2 2 1 1 21(-1)( ), and 21(-1)(− ). 2 2 For the M state, n = 3, and 18 states are possible. Possible l values are 0, 1, and 2; 1 possible m values are 0, ±1, and ±2; and possible m values are ± . Therefore, for the s states, l s 2 1 1 1 1 1 the quantum numbers are 300( ), 300(− ), for the p states they are 310( ), 310(− ), 311( ), 2 2 2 2 2 1 1 1 1 1 1 311(− ), 31(-1)( ), and 31(-1)(− ); for the d states they are 320( ), 320(− ), 321( ), 2 2 2 2 2 2 1 1 1 1 1 1 1 321(− ), 32(-1)( ), 32(-1)(− ), 322 ( ), 322 (− ), 32(-2)( ), and 32(-2)(− ). 2 2 2 2 2 2 2 2.7 The electron configurations of the ions are determined using Table 2.2. 2+ 2 2 6 2 6 6 Fe - 1s 2s 2p 3s 3p 3d 3+ 2 2 6 2 6 5 Fe - 1s 2s 2p 3s 3p 3d + 2 2 6 2 6 10 Cu - 1s 2s 2p 3s 3p 3d 2+ 2 2 6 2 6 10 2 6 10 2 6 Ba - 1s 2s 2p 3s 3p 3d 4s 4p 4d 5s 5p - 2 2 6 2 6 10 2 6 Br - 1s 2s 2p 3s 3p 3d 4s 4p 2- 2 2 6 2 6 S - 1s 2s 2p 3s 3p + 2.8 The Na ion is just a sodium atom that has lost one electron; therefore, it has an electron configuration the same as neon (Figure 2.6). - The Cl ion is a chlorine atom that has acquired one extra electron; therefore, it has an electron configuration the same as argon. 2.9 Each of the elements in Group IIA has two s electrons. 2 2 6 2 6 7 2 2.10 (a) The 1s 2s 2p 3s 3p 3d 4s electron configuration is that of a transition metal because of an incomplete d subshell. 3 2 2 6 2 6 (b) The 1s 2s 2p 3s 3p electron configuration is that of an inert gas because of filled 3s and 3p subshells. 2 2 5 (c) The 1s 2s 2p electron configuration is that of a halogen because it is one electron deficient from having a filled L shell. 2 2 6 2 (d) The 1s 2s 2p 3s electron configuration is that of an alkaline earth metal because of two s electrons. 2 2 6 2 6 2 2 (e) The 1s 2s 2p 3s 3p 3d 4s electron configuration is that of a transition metal because of an incomplete d subshell. 2 2 6 2 6 1 (f) The 1s 2s 2p 3s 3p 4s electron configuration is that of an alkali metal because of a single s electron. 2.11 (a) The 4f subshell is being filled for the rare earth series of elements. (b) The 5f subshell is being filled for the actinide series of elements. 2.12 The attractive force between two ions F is just the derivative with respect to the interatomic A separation of the attractive energy expression, Equation (2.8), which is just ⎛ A⎞ d⎜ − ⎟ dEA ⎝ r⎠ A F = = = A dr dr r2 2+ 2- The constant A in this expression is defined in footnote 3. Since the valences of the Ca and O ions (Z and Z ) are both 2, then 1 2 ( )( ) Ze Z e 1 2 F = A 2 4πε r o ( −19 )2 (2)(2)1.6 x 10 C = ( −12 )( −9 )2 (4)(π) 8.85 x 10 F/m) 1.25 x 10 m -10 = 5.89 x 10 N 2.13 (a) Differentiation of Equation (2.11) yields dEN A nB = − = 0 dr r(1 + 1) r(n + 1) 4 (b) Now, solving for r (= r ) o A nB = 2 (n + 1) r r o o or 1/(1 - n) ⎛ A ⎞ r = ⎜ ⎟ o ⎝ nB⎠ (c) Substitution for r into Equation (2.11) and solving for E (= E ) o o A B E = − + o r r n o o A B = − + 1/(1 - n) n/(1 - n) ⎛ A ⎞ ⎛ A ⎞ ⎜ ⎟ ⎜ ⎟ ⎝ nB⎠ ⎝ nB⎠ 2.14 (a) Curves of E , E , and E are shown on the plot below. A R N 5 (b) From this plot r = 0.24 nm o E = -5.3 eV o (c) From Equation (2.11) for E N A = 1.436 -6 B = 7.32 x 10 n = 8 Thus, ⎛ A ⎞ 1/(1 - n) r = ⎜ ⎟ o ⎝ nB⎠ 1/(1 - 8) ⎡ ⎤ ⎢ 1.436 ⎥ = = 0.236 nm ⎢ ( -6)⎥ (8)7.32 x 10 ⎣⎢ ⎦⎥ and −6 1.436 7.32 x 10 E = − + o 1/(1 − 8) 8/(1 − 8) ⎡ ⎤ ⎡ ⎤ 1.436 1.436 ⎢ ⎥ ⎢ ⎥ ⎢ (8)⎛ 7.32 x 10−6⎞ ⎥ ⎢ (8)⎛ 7.32 x 10−6⎞ ⎥ ⎣ ⎝ ⎠ ⎦ ⎣ ⎝ ⎠ ⎦ = - 5.32 eV + - 2.15 This problem gives us, for a hypothetical X -Y ion pair, values for r (0.35 nm), E (-6.13 eV), and o o n (10), and asks that we determine explicit expressions for attractive and repulsive energies of Equations 2.8 and 2.9. In essence, it is necessary to compute the values of A and B in these equations. Expressions for r and E in terms of n, A, and B were determined in Problem 2.13, o o which are as follows: 1/(1 - n) ⎛ A ⎞ r = ⎜ ⎟ o ⎝ nB⎠ 6 A B E = − + o 1/(1 - n) n/(1 - n) ⎛ A ⎞ ⎛ A ⎞ ⎜ ⎟ ⎜ ⎟ ⎝ nB⎠ ⎝ nB⎠ Thus, we have two simultaneous equations with two unknowns (viz. A and B). Upon substitution of values for r and E in terms of n, these equations take the forms o o 1/(1 - 10) ⎛ A ⎞ 0.35 nm = ⎜ ⎟ ⎝ 10B⎠ A B −6.13 eV = − + 1/(1 − 10) 10/(1 − 10) ⎛ A ⎞ ⎛ A ⎞ ⎜ ⎟ ⎜ ⎟ ⎝ 10B⎠ ⎝ 10B⎠ -5 Simultaneous solution of these two equations leads to A = 2.38 and B = 1.88 x 10 . Thus, Equations (2.8) and (2.9) become 2.38 E = − A r −5 1.88 x 10 E = R 10 r Of course these expressions are valid for r and E in units of nanometers and electron volts, respectively. 2.16 (a) Differentiating Equation (2.12) with respect to r yields −r/ρ dE C De = − dr r2 ρ At r = r , dE/dr = 0, and o −(r /ρ) C De o = (2.12b) r 2 ρ o Solving for C and substitution into Equation (2.12) yields an expression for E as o 7 −(r /ρ) ⎛ ro⎞ E = De o ⎜ 1 − ⎟ o ⎝ ρ⎠ (b) Now solving for D from Equation (2.12b) above yields (r /ρ) Cρe o D = 2 r o Substitution of this expression for D into Equation (2.12) yields an expression for E as o C⎛ ρ ⎞ E = ⎜ − 1⎟ o r ⎜ r ⎟ o⎝ o ⎠ 2.17 (a) The main differences between the various forms of primary bonding are: Ionic--there is electrostatic attraction between oppositely charged ions. Covalent--there is electron sharing between two adjacent atoms such that each atom assumes a stable electron configuration. Metallic--the positively charged ion cores are shielded from one another, and also "glued" together by the sea of valence electrons. (b) The Pauli exclusion principle states that each electron state can hold no more than two electrons, which must have opposite spins. 2.18 Covalently bonded materials are less dense than metallic or ionically bonded ones because covalent bonds are directional in nature whereas metallic and ionic are not; when bonds are directional, the atoms cannot pack together in as dense a manner, yielding a lower mass density. 2.19 The percent ionic character is a function of the electron negativities of the ions X and X A B according to Equation (2.10). The electronegativities of the elements are found in Figure 2.7. For MgO, X = 1.2 and X = 3.5, and therefore, Mg O ⎡ (−0.25)(3.5−1.2)2⎤ %IC = 1 − e x 100 = 73.4% ⎢ ⎥ ⎣ ⎦ For GaP, X = 1.6 and X = 2.1, and therefore, Ga P 8 ⎡ (−0.25)(2.1−1.6)2⎤ %IC = 1 − e x 100 = 6.1% ⎢ ⎥ ⎣ ⎦ For CsF, X = 0.7 and X = 4.0, and therefore, Cs F ⎡ (−0.25)(4.0−0.7)2⎤ %IC = 1 − e x 100 = 93.4% ⎢ ⎥ ⎣ ⎦ For CdS, X = 1.7 and X = 2.5, and therefore, Cd S ⎡ (−0.25)(2.5−1.7)2⎤ %IC = 1 − e x 100 = 14.8% ⎢ ⎥ ⎣ ⎦ For FeO, X = 1.8 and X = 3.5, and therefore, Fe O ⎡ (−0.25)(3.5−1.8)2⎤ %IC = 1 − e x 100 = 51.4% ⎢ ⎥ ⎣ ⎦ 2.20 Below is plotted the bonding energy versus melting temperature for these four metals. From this plot, the bonding energy for copper (melting temperature of 1084°C) should be approximately 3.6 eV. The experimental value is 3.5 eV. 9 2 2 2.21 For silicon, having the valence electron structure 3s 3p , N' = 4; thus, there are 8 - N' = 4 covalent bonds per atom. 2 5 For bromine, having the valence electron structure 4s 4p , N' = 7; thus, there is 8 - N' = 1 covalent bond per atom. 2 3 For nitrogen, having the valence electron structure 2s 2p , N' = 5; thus, there are 8 - N' = 3 covalent bonds per atom. 2 4 For sulfur, having the valence electron structure 3s 3p , N' = 6; thus, there are 8 - N' = 2 covalent bonds per atom. 2.22 For brass, the bonding is metallic since it is a metal alloy. For rubber, the bonding is covalent with some van der Waals. (Rubber is composed primarily of carbon and hydrogen atoms.) For BaS, the bonding is predominantly ionic (but with some covalent character) on the basis of the relative positions of Ba and S in the periodic table. For solid xenon, the bonding is van der Waals since xenon is an inert gas. For bronze, the bonding is metallic since it is a metal alloy (composed of copper and tin). For nylon, the bonding is covalent with perhaps some van der Waals. (Nylon is composed primarily of carbon and hydrogen.) For AlP the bonding is predominantly covalent (but with some ionic character) on the basis of the relative positions of Al and P in the periodic table. 2.23 The intermolecular bonding for HF is hydrogen, whereas for HCl, the intermolecular bonding is van der Waals. Since the hydrogen bond is stronger than van der Waals, HF will have a higher melting temperature. 2.24 The geometry of the H O molecules, which are hydrogen bonded to one another, is more restricted 2 in the solid phase than for the liquid. This results in a more open molecular structure in the solid, and a less dense solid phase. 10