3 Mechanisms 3.1.The Atomic Structure of Condensed Matter This chapter strives to explain the fundamental mechanical prop- erties of materials by relating them to the atomistic structure of solids and by discussing the interactions which atoms have with each other. Atoms!that make up condensed matter are often arranged in a three-dimensional periodic array,that is,in an ordered man- ner called a crystal.The periodic arrangement may differ from case to case,leading to different crystal structures.This fact has been known since the mid-nineteenth century through the work of A.Bravais(1811-1863),a French physicist at the Ecole Poly- technique.His concepts were confirmed (1912),particularly af- ter X-ray diffraction techniques were invented,by Laue and Bragg,and routinely utilized by Debye-Scherrer,Guinier,and others.(Periodic arrays of atoms act as three-dimensional grat- ings which cause X-rays to undergo interference and thus pro- duce highly symmetric diffraction patterns from which the peri- odic arrangement of atoms can be inferred.)Since about a decade ago,the arrangement of atoms also can be made visible utilizing high-resolution electron microscopy (or other analytical instru- ments).Specifically,a very thin foil of the material under inves- tigation is irradiated by electrons,yielding a two-dimensional projection pattern of the three-dimensional atomic arrangement, as seen in the lower part of Figure 3.1. Atomos (Greek)=indivisible.This term is based on a philosophical con- cept promulgated by Democritus (about 460-370 B.c.)postulating that matter is composed of small particles that cannot be further divided.3 This chapter strives to explain the fundamental mechanical prop￾erties of materials by relating them to the atomistic structure of solids and by discussing the interactions which atoms have with each other. Atoms1 that make up condensed matter are often arranged in a three-dimensional periodic array, that is, in an ordered man￾ner called a crystal. The periodic arrangement may differ from case to case, leading to different crystal structures. This fact has been known since the mid-nineteenth century through the work of A. Bravais (1811–1863), a French physicist at the École Poly￾technique. His concepts were confirmed (1912), particularly af￾ter X-ray diffraction techniques were invented, by Laue and Bragg, and routinely utilized by Debye–Scherrer, Guinier, and others. (Periodic arrays of atoms act as three-dimensional grat￾ings which cause X-rays to undergo interference and thus pro￾duce highly symmetric diffraction patterns from which the peri￾odic arrangement of atoms can be inferred.) Since about a decade ago, the arrangement of atoms also can be made visible utilizing high-resolution electron microscopy (or other analytical instru￾ments). Specifically, a very thin foil of the material under inves￾tigation is irradiated by electrons, yielding a two-dimensional projection pattern of the three-dimensional atomic arrangement, as seen in the lower part of Figure 3.1. Mechanisms 3.1 • The Atomic Structure of Condensed Matter 1Atomos (Greek)
indivisible. This term is based on a philosophical con￾cept promulgated by Democritus (about 460–370 B.C.) postulating that matter is composed of small particles that cannot be further divided