Fundamentals of Materials Science and Engineering - An Integrated Approach

Fundamentals of Materials Science and Engineering - An Integrated Approach

(Parte 7 de 7)

First,thevalenceelectronsarethosethatoccupytheoutermostshell.Theseelectronsvalence electron are extremely important; as will be seen, they participate in the bonding between atoms to form atomic and molecular aggregates. Furthermore, many of the physical and chemical properties of solids are based on these valence electrons.

In addition, some atoms have what are termed “stable electron configurations”; that is, the states within the outermost or valence electron shell are completely filled. Normally this corresponds to the occupation of just the s and p states for the outermost shell by a total of eight electrons, as in neon, argon, and krypton; one exception is helium, which contains only two 1s electrons. These elements (Ne, Ar, Kr, and He) are the inert, or noble, gases, which are virtually unreactive chemically. Some atoms of the elements that have unfilled valence shells assume stable electron

Increasing energy

Figure 2.5 Schematic representation of the filled and lowest unfilled energy states for a sodium atom.

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2 • Chapter 2 / Atomic Structure and Interatomic Bonding

Table 2.2 A Listing of the Expected Electron Configurations for Some of the Common Elementsa

Element Symbol Atomic Number Electron Configuration a When some elements covalently bond, they form sp hybrid bonds. This is especially true for C, Si, and Ge.

configurations by gaining or losing electrons to form charged ions, or by sharing electrons with other atoms. This is the basis for some chemical reactions and also for atomic bonding in solids, as explained in Section 2.6.

Under special circumstances, the s and p orbitals combine to form hybrid spn orbitals, where n indicates the number of p orbitals involved, which may have a value of 1, 2, or 3. The 3A, 4A, and 5A group elements of the periodic table (Figure 2.6) are those that most often form these hybrids. The driving force for the formation of hybrid orbitals is a lower energy state for the valence electrons. For carbon the sp3 hybrid is of primary importance in organic and polymer chemistries. The shape

(Parte 7 de 7)