CHAPTER 2
ATOMIC STRUCTURE AND INTERATOMIC BONDING
PROBLEM SOLUTIONS
2.1 (a) When two or more atoms of an element have have different atomic masses, each is termed termed an isotope. isotope. (b)
The atomic weights of the elements ordinarily are not integers because: (1) the atomic 12 masses 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 of grams in one amu of material, appropriate appropriate manipulation manipulation of the amu/atom, g/mol, and atom/mol relationships is all that is necessary, as
#g/amu =
1mol ( 1g/mol ) 6.023x10 23 atoms 1amu/atom
= 1.66 x 10
-24
g/amu
(b) Since there are 453.6 g/lb , m 1 lb-mol = (453.6 g/lb m )(6.023 x 10
= 2.73 x 10
26
23
atoms/g-mol)
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 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. 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 possible m values are 0 and ±1. Therefore, for the s states, the quantum numbers are l 1 1 1 1 1 200( ) and 200(- ). For the p states, the quantum numbers are 210( ), 210(- ), 211( ), 211(2 2 2 2 2 1 1 1 ), 21(-1)( ), and 21(-1)(- ). 2 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 l s 2 1 1 1 1 states, the quantum numbers are 300( ), 300(- ), for the p states they are 310( ), 310(- ), 2 2 2 2 311(\F(1,2)), 311(-\F(1,2)), 31(-1)(\F(1,2)), and 31(-1)(-\F(1,2)); for the d states they are 1 1 1 1 1 1 1 1 1 1 320( ), 320(- ), 321( ), 321(- ), 32(-1)( ), 32(-1)(- ), 322( ), 322(- ), 32(-2)( ), and 32(-2)(- ). 2 2 2 2 2 2 2 2 2 2
2.7 The electron configurations of the ions are determined using Table 2.2. 2 2 6 2 6 6 - 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 22 2 6 2 6 S - 1s 2s 2p 3s 3p Fe
2.8 The Cs
+
2+
ion is just a cesium atom that has lost one electron; therefore, it has an electron
configuration the same as xenon (Figure 2.6). The Br ion is a bromine atom that has acquired one extra electron; therefore, it has an electron configuration the same as krypton.
2.9 Each of the elements in Group VIIA has five p 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.
2 2 6 2 6 (b) The 1s 2s 2p 3s 3p electron configuration is that of an inert gas because of filled 3 s 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
A
is just the derivative with respect to the interatomic
separation of the attractive energy expression, Equation (2.8), which is just
( )= A
A d - dE r A F = = A dr dr
2 r
The constant A in this expression is defined in footnote 3 on page 21. Since the valences of + 2the K and O ions are +1 and -2, respectively, Z = 1 and Z = 2, then 1
(Z F = A
=
1
2
e )(Z e )
4 πε
o
2 2 r
-19 2 (1)(2)(1.6x10 C) -9 2 (4)(π)(8.85x10 -12 F/m)(1.5x10 m)
= 2.05 x 10
-10
N
2.13 (a) Differentiation of Equation (2.11) yields dE
A nB N = (1+1) - (n+1) = 0 dr r r
(b) Now, solving for r (= r ) o A nB = (n+1) 2 r r o o or
r = o
(c) Substitution for r
o
A 1/(1 - n) nB
( )
into Equation (2.11) and solving for E (= E ) o A B E =+ n o r o ro
=-
A
( ) A nB
1/(1-n)
+
B
( )
A n/(1-n) nB
2.14 (a) Curves of E , E , and E are shown on the plot below. A R N 2 1
E
R
0 EN ) V e ( y g r e n E g n i d n o B
-1 r = 0.28 nm o
-2 -3 -4 E o = -4.6 eV
-5 EA
-6 -7 0.0
0.2
0.4
0. 6
Interatomic Separation (nm)
0. 8
1. 0
(b) From this plot r
= 0.28 nm o E = -4.6 eV o (c) From Equation (2.11) for E
N
A = 1.436 B = 5.86 x 10
-6
n= 9 Thus, A 1/(1 - n) nB
( )
r = o
=
1.436 1/(1 - 9) (9)(5.86x10-6)
= 0.279 nm
an d -6 1.436 5.86x10 E =+ o 1.436 1.436 1/(1-9) 9/(1-9) (9)(5.86x10-6) (9)(5.86x10-6)
= - 4.57 eV + 2.15 This problem gives us, for a hypothetical X -Y ion pair, values for r
o
(0.35 nm), E
o
(-6.13 eV),
and 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 o
o
Problem 2.13, which are as follows: A 1/(1 - n) nB
( )
r = o
E =o
A
(nBA )
1/(1-n)
+
B
(nBA )
n/(1-n)
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
A 1/(1 - 10) 10 B
( )
0.35 nm =
-6.13 eV = -
A
( )
1/(1-10)
A 10 B
+
B
( )
A 10/(1-10) 10 B
Simultaneous solution of these two equations leads to A = 2.38 and B = 1.88 x 10
-5
. Thus,
Equations (2.8) and (2.9) become
E =A
E
R
=
2.38 r
1.88x10
-5
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 ρ r o Solving for C and substitution into Equation (2.12) yields an expression for E r -r / ρ o E = D e o 1- o ρ
(b) Now solving for D from Equation (2.12b) above yields r / ρ Cρe o D= 2 r o
(2.12b)
o
as
Substitution of this expression for D into Equation (2.12) yields an expression for E
o
as
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
A
and X
according to Equation (2.10). The electronegativities of the elements are found in Figure 2.7. For TiO , XTi = 1.5 and X = 3.5, and therefore, 2 O
[
%IC = 1-e(-0.25)(3.5-1.5)
2
] x 100 = 63.2%
For ZnTe, XZn = 1.6 and X Te = 2.1, and therefore,
[
%IC = 1-e(-0.25)(2.1-1.6)
2
] x 100 = 6.1%
For CsCl, X Cs = 0.7 and XCl = 3.0, and therefore,
[
%IC = 1-e(-0.25)(3.0-0.7)
For InSb, X In = 1.7 and X Sb = 1.9, and therefore,
2
] x 100 = 73.4%
B
[
%IC = 1-e(-0.25)(1.9-1.7)
2
] x 100 = 1.0%
For MgCl , X Mg = 1.2 and X Cl = 3.0, and therefore, 2
[
%IC = 1-e(-0.25)(3.0-1.2)
2
] x 100 = 55.5%
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. 10
6
y g r e n E g n i d n o B
W
8
) V e (
4
Fe
3.6 eV
Al
2
0 -1000
Hg
0
1000
2000
3000
4000
Melting Temperature (C)
2 2 2.21 For germanium, having the valence electron structure 4s 4p , N' = 4; thus, there are 8 - N' = 4 covalent bonds per atom. 2 3 For phosphorus, having the valence electron structure 3s 3p , N' = 5; thus, there are 8 - N' = 3 covalent bonds per atom. 2 4 For selenium, having the valence electron structure 4s 4p , N' = 6; thus, there are 8 N' = 2 covalent bonds per atom. 2 5 For chlorine, having the valence electron structure 3s 3p , N' = 7; thus, there is 8 - N' = 1 covalent bond 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 2 restricted 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.
