Chapter 2 / Atomic Structure and Interatomic Bonding

More documents

Recommendations

Info

18 ● Chapter 2 / Atomic Structure and Interatomic Bonding IA 1 H 2.1 3 4 Li Be 1.0 1.5 11 12 Na Mg 0.9 1.2 19 20 K Ca 0.8 1.0 37 Rb 0.8 IIA 38 Sr 1.0 21 Sc 1.3 39 Y 1.2 55 56 57 71 Cs Ba La Lu 0.7 0.9 1.1 1.2 87 88 89 102 Fr Ra Ac No 0.7 0.9 1.1 1.7 IIIB IVB VB VIB VIIB 22 Ti 1.5 40 Zr 1.4 72 Hf 1.3 23 V 1.6 41 Nb 1.6 73 Ta 1.5 24 Cr 1.6 42 Mo 1.8 74 W 1.7 25 Mn 1.5 43 Tc 1.9 75 Re 1.9 26 Fe 1.8 44 Ru 2.2 76 Os 2.2 27 Co 1.8 45 Rh 2.2 77 Ir 2.2 As may be noted from the periodic table, most of the elements really come under the metal classification. These are sometimes termed electropositive elements, indicating that they are capable of giving up their few valence electrons to become positively charged ions. Furthermore, the elements situated on the right-hand side of the table are electronegative; that is, they readily accept electrons to form negatively charged ions, or sometimes they share electrons with other atoms. Figure 2.7 displays electronegativity values that have been assigned to the various elements arranged in the periodic table. As a general rule, electronegativity increases in moving from left to right and from bottom to top. Atoms are more likely to accept electrons if their outer shells are almost full, and if they are less ‘‘shielded’’ from (i.e., closer to) the nucleus. ATOMIC BONDING IN SOLIDS 2.5 BONDING FORCES AND ENERGIES VIII 28 Ni 1.8 46 Pd 2.2 78 Pt 2.2 IB IIB 29 Cu 1.9 47 Ag 1.9 79 Au 2.4 An understanding of many of the physical properties of materials is predicated on a knowledge of the interatomic forces that bind the atoms together. Perhaps the principles of atomic bonding are best illustrated by considering the interaction between two isolated atoms as they are brought into close proximity from an infinite separation. At large distances, the interactions are negligible; but as the atoms approach, each exerts forces on the other. These forces are of two types, attractive and repulsive, and the magnitude of each is a function of the separation or interatomic distance. The origin of an attractive force FA depends on the particular type of bonding that exists between the two atoms. Its magnitude varies with the distance, as represented schematically in Figure 2.8a. Ultimately, the outer electron shells of the two atoms begin to overlap, and a strong repulsive force FR comes into play. The net force FN between the two atoms is just the sum of both attractive and repulsive components; that is, FN FA FR (2.2) 30 Zn 1.6 48 Cd 1.7 80 Hg 1.9 IIIA IVA VA VIA VIIA FIGURE 2.7 The electronegativity values for the elements. (Adapted from Linus Pauling, The Nature of the Chemical Bond, 3rd edition. Copyright 1939 and 1940, 3rd edition copyright © 1960, by Cornell University. Used by permission of the publisher, Cornell University Press.) 5 B 2.0 13 Al 1.5 31 Ga 1.6 49 In 1.7 81 Tl 1.8 6 C 2.5 14 Si 1.8 32 Ge 1.8 50 Sn 1.8 82 Pb 1.8 7 N 3.0 15 P 2.1 33 As 2.0 51 Sb 1.9 83 Bi 1.9 8 O 3.5 16 S 2.5 34 Se 2.4 52 Te 2.1 84 Po 2.0 9 F 4.0 17 Cl 3.0 35 Br 2.8 53 I 2.5 85 At 2.2 0 2 He 10 Ne 18 Ar 36 Kr 54 Xe 86 Rn
Force F Attraction Repulsion Attraction Repulsion Potential energy E + 0 + 0 E 0 r 0 2.5 Bonding Forces and Energies ● 19 which is also a function of the interatomic separation, as also plotted in Figure 2.8a. When FA and FR balance, or become equal, there is no net force; that is, FA FR 0 (2.3) Then a state of equilibrium exists. The centers of the two atoms will remain separated by the equilibrium spacing r0, as indicated in Figure 2.8a. For many atoms, r0 is approximately 0.3 nm (3 A˚ ). Once in this position, the two atoms will counteract any attempt to separate them by an attractive force, or to push them together by a repulsive action. Sometimes it is more convenient to work with the potential energies between two atoms instead of forces. Mathematically, energy (E) and force (F) are related as Or, for atomic systems, Attractive force F A Repulsive force F R Net force F N (a) Repulsive energy E R Net energy E N Attractive energy E A (b) Interatomic separation r Interatomic separation r EN r r E Fdr (2.4) F N dr (2.5) FA dr r FR dr (2.6) EA ER (2.7) in which EN, EA, and ER are respectively the net, attractive, and repulsive energies for two isolated and adjacent atoms. FIGURE 2.8 (a) The dependence of repulsive, attractive, and net forces on interatomic separation for two isolated atoms. (b) The dependence of repulsive, attractive, and net potential energies on interatomic separation for two isolated atoms.
Page 1 and 2: This micrograph, which represents t
Page 3 and 4: 2.3 Electrons in Atoms ● 11 the m
Page 5 and 6: Probability 1.0 Orbital electron Nu
Page 7 and 8: ELECTRON CONFIGURATIONS 2.3 Electro
Page 9: 2.4 THE PERIODIC TABLE IA 1 H 1.008
Page 13 and 14: Na + Cl Na + Cl Cl Na + Cl Na +
Page 15 and 16: 2.6 Primary Interatomic Bonds ● 2
Page 17 and 18: + Atomic or molecular dipoles + 2.
Page 19 and 20: Important Terms and Concepts ● 27
Page 21: Calculate the bonding energy E0 in

Chapter 2 / Atomic Structure and Interatomic Bonding

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?