Chapter 8 11

Chapter 8 Basic Concepts of Chemical

Bonding

CHEMISTRY The Central Science

9th Edition

Chapter 8 21

• Chemical bond: attractive force holding two or more atoms together• Covalent bond: electrons are shared

• Usually found between nonmetals• Ionic bond results from the transfer of electrons from

a metal to a nonmetal• Metallic bond: attractive force holding pure metals

together

8.1: Chemical Bonds, Lewis Symbols, and the Octet

Rule

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Text, P. 277

•Valence electrons are represented as dots around the symbol for the element•Electrons available for bonding are indicated by unpaired dots

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The Octet Rule• All noble gases except He have an ns2np6 configuration • Octet rule: atoms tend to gain, lose, or share electrons

until they are surrounded by 8 valence electrons (4 electron pairs)

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Consider the reaction between sodium and chlorine:Na(s) + ½Cl2(g) NaCl(s) DHºf = -410.9 kJ

8.2: Ionic Bonding

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•NaCl forms a very regular structure •Regular arrangement of Na+ and Cl- in 3D

• Ions are packed as closely as possible

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Energetics of Ionic Bond Formation• Lattice energy: the energy required to completely

separate an ionic solid into its gaseous ions

NaCl(s) Na+(g) + Cl-

(g) is endothermic (DH = +788 kJ/mol)

• Lattice energy depends on the charges on the ions and the sizes of the ions

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• Lattice energy (and thus stability) increases as• The charges on the ions increase• The distance between the ions decreases

• High lattice energies make ionic compounds hard and brittle with high melting points

Text, P. 279

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8.3: Covalent Bonding

• When two similar atoms bond, neither of them wants to lose or gain an electron to form an octet• Similar electron affinities• They share pairs of electrons to each obtain an octet

• Example: H + H H2

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Lewis Structures• Covalent bonds can be represented by the Lewis symbols

of the elements:

• In Lewis structures, each pair of electrons in a bond is represented by a single line:

Cl + Cl Cl Cl

Cl Cl H FH O

HH N H

HCHH

HH

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Multiple Bonds• It is possible for more than one pair of electrons to be

shared between two atoms (multiple bonds):• One shared pair of electrons = single bond• Two shared pairs = double bond • Three shared pairs = triple bond

• Bond distances decrease from single triple bondsH H O O N N