LEWIS SYMBOLS AND THE OCTET RULE

LEWIS SYMBOLS AND THE OCTET RULE We begin with descriptions of the three main types of chemical bonds ionic, covaient, and metaiiic. In evaluating bonding, Lewis symbois provide a useful shorthand for keeping track of valence electrons. 

CHEMICAL BONDS, LEWIS SYMBOLS, AND THE OCTET RULE (INTRODUCTION AND SECTION 8.1) In this chapter we have focused on the interactions that lead to the formation of chemical bonds. We classify these bonds into three broad groups ionic bonds, which result from the electrostatic forces that exist between ions of opposite charge covalent bonds, which result from the sharing of electrons by two atoms and metallic bonds, which result from a delocalized sharing of electrons in metals. The formation of bonds involves interactions of the outermost electrons of atoms, their valence electrons. The valence electrons of an atom can be represented by electron-dot symbols, called Lewis symbols. The tendencies of atoms to gain, lose, or share their valence electrons often follow the octet rule, which says that the atoms in molecules or ions (usually) have eight valence electrons. 

Chemical Bonds, Lewis Symbols, and the Octet Rule... 

Three Types of Bonding Lewis Symbols and the Octet Rule The Ionic Bonding Model... 

Types of Bonding Three Ways Metals and Nonmetals Combine 277 Lewis Symbols and the Octet Rule 278 The Ionic Bonding Model 280 Why Ionic Compounds Form The Importance of Lattice Energy 280 Periodic Trends in Lattice Energy 281 How the Model Explains the Properties of Ionic Corrpounds 283... 

Lewis recognized that certain molecules such a PCI5 and SF6 are exceptions to the octet rule because their Lewis structures indicate that the central atom has more than eight electrons in its valence shell 10 for the P atom in PCI5 and the S atom in SF4, and 12 for the S atom in SFg (Figure 1.17). Such molecules are called hypervalent because the valence of the central atom is greater than its principal valence. To write a Lewis structure for such molecules, the Lewis symbol for the hypervalent atom must be modified to show the correct number of unpaired electrons. For the molecules in Figure 1.17 we would need to write the Lewis symbols as follows ... 

The first step toward visualizing what a molecule looks like is to convert its molecular formula to its Lewis structure (or Lewis formula). This two-dimensional structural formula consists of electron-dot symbols that depict each atom and its neighbors, the bonding pairs that hold them together, and the lone pairs that fill each atom s outer level (valence shell). In many cases, the octet rule (Section 9.1) guides us in allotting electrons to the atoms in a Lewis structure in many other cases, however, we set the rule aside. 

The symbol of the element represents the nucleus and all of the electrons except the outer valence shell. The valence electrons are represented by dots, and sometimes by crosses or circles. A key concept in building Lewis structural formulas is to satisfy the octet rule which states that many elements achieve stability by forming covalent bonds in order to fill their outer shell with eight electrons. 

An octet of electrons consists of full s and p subshells in an atom. In a Lewis symbol, an octet is shown as four pairs of valence electrons arranged around the element symbol, as in the Lewis symbols for Ne and Ar in Table 8.1. There are exceptions to the octet rule, but it provides a useful framework for introducing many important concepts of bonding. 

Lewis symbols predict bonding behavior and show how atoms obey the octet rule. (Section 9.1)... 

The octet rule states that the number of electrons associated with bond pairs and lone pairs of electrons for each of the Lewis symbols (except H) in a Lewis structure will be eight (an octet). 

Definitions. Define and illustrate the following terms (a) octet rule, (b) Lewis symbol, (c) ionic bond, (d) covalent bond, (e) crystal lattice energy, (f) Iree radical, (g) network covalent compound, (h) electronegativity, (i) polar molecule, (j) dipole moment, (k) formal charge, (1) oxidation number, (m) hydrogen bond, (n) dipole-dipole attraction, (o) London forces. 

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