Chapter 11: Chemical Bonding I: Basic Concepts Chemistry 140 Fall 2005 General Chemistry Principles and Modern Applications Petrucci • Harwood • Herring 8th Edition Chapter 11: Chemical Bonding I: Basic Concepts Philip Dutton University of Windsor, Canada N9B 3P4 Prentice-Hall © 2002 (modified 2003 by Dr. Paul Root and 2005 by Dr. David Tramontozzi) Thermochemistry branch of chemistry concerned with heat effects accompanying chemical reactions. Direct and indirect measurement of heat. Answer practical questions: why is natural gas a better fuel than coal, and why do fats have higher energy value than carbohydrates and protiens. Prentice-Hall © 2002 General Chemistry: Chapter 9

General Chemistry: Chapter 9 Contents 11-1 Lewis Theory: An Overview 11-2 Covalent Bonding: An Introduction 11-3 Polar Covalent Bonds 11-4 Writing Lewis Structures 11-5 Resonance 11-6 Exceptions to the Octet Rule 11-7 The Shapes of Molecules 11-8 Bond Order and Bond Lengths 11-9 Bond Energies Focus on Polymers— Macromolecular Substances Prentice-Hall © 2002 General Chemistry: Chapter 9

11-1 Lewis Theory: An Overview Valence e- play a fundamental role in chemical bonding. e- transfer leads to ionic bonds. Sharing of e- leads to covalent bonds. e- are transferred or shared to give each atom a noble gas configuration the OCTET RULE. Prentice-Hall © 2002 General Chemistry: Chapter 9

General Chemistry: Chapter 9 Lewis Symbols A chemical symbol represents the nucleus and the core e-. Dots around the symbol represent valence e-. • Si • • • N •• • P As Sb Bi •• •• •• • Al • • Se • •• I •• Ar • • •• • • •• •• Prentice-Hall © 2002 General Chemistry: Chapter 9

Lewis Structures for Ionic Compounds Chemistry 140 Fall 2005 Lewis Structures for Ionic Compounds Ba • O •• 2+ •• 2- BaO Ba •• O •• •• Mg • Cl •• 2+ •• - MgCl2 Mg 2 •• Cl •• •• Binary ionic compounds. Note the types of arrows used to move electrons – fishhooks for single e-. Write the Lewis symbol for each atom Determine how many e- each atom must gain or lose. Use multiples of one or both ions to balance the number of electrons. Prentice-Hall © 2002 General Chemistry: Chapter 9

General Chemistry: Chapter 9 Chemistry 140 Fall 2005 11-2 Covalent Bonding Notice the octets of electrons attained by each atom through covalent bond formation. One covalent bond represents two electrons – one being donated from each atom Lone pairs are pairs of electrons not used in bonding – they play a role in molecular shape Hydrogen is an exception since it only requires 2 electrons to fill its outer shell. Prentice-Hall © 2002 General Chemistry: Chapter 9

Coordinate Covalent Bonds Chemistry 140 Fall 2005 Coordinate Covalent Bonds A covalent bond in which a single atom contributes both of the electrons to be shared N H + N •• H •• Cl - H Cl Note double headed arrow for two electron movement Coordinate covalent bond is a shared pair of electrons in which both electrons used for the bond come from one of the atoms. Once formed a coordinate covalent bond is indistinguishable from a regular covalent bond. Prentice-Hall © 2002 General Chemistry: Chapter 9

Multiple Covalent Bonds Chemistry 140 Fall 2005 Multiple Covalent Bonds Single covalent bond formation results in a situation where both oxygen and carbon do not have complete octets of electrons. • • • C O • •• • • O • C • O • • • • • • • • • Shifting the unpaired electrons to form additional bonds (a double bond in this case) gives each atom a complete octet. Double bonds involve two pairs, or four electrons • C O •• C O •• Prentice-Hall © 2002 General Chemistry: Chapter 9

Multiple Covalent Bonds Chemistry 140 Fall 2005 Multiple Covalent Bonds We see for N2 that the formation of a triple bond is required to satisfy the octet of each nitrogen atom. • • N • •• •• N • • N •• • • N • •• N •• Triple bonds involve three pairs, or six electrons Multiple bond lengths are shorter than single bonds and release more energy when broken Prentice-Hall © 2002 General Chemistry: Chapter 9

General Chemistry: Chapter 9 Chemistry 140 Fall 2005 11-3 Polar Covalent Bonds δ+ δ- H Cl Nonpolar covalent bonds involve equa sharing of a pair of electrons. This occurs when the two atoms involved have similar electronegativities Polar covalent bonds involve the unequal sharing of a pair of electrons The electrons spend more time around the atom with stronger electron attracting ability, higher electronegativity This gives the more electronegative atom a partial negative charge and the lesser electronegative atom a partial positive charge. Prentice-Hall © 2002 General Chemistry: Chapter 9

Electronegativity An atoms ability to compete for shared electrons. Chemistry 140 Fall 2005 Electronegativity An atoms ability to compete for shared electrons. Related to ionization energy and electron affinity Electronegativity follows the same trend as electron affinity and ionization energy. Flourine has the highest electronegativity, Fancium has the least electronegativiy The polarity of a bond is determined by the difference in electronegativity Prentice-Hall © 2002 General Chemistry: Chapter 9

Percent Ionic Character Chemistry 140 Fall 2005 Percent Ionic Character The larger the difference in electronegativity between two bonded atoms, the higher the % ionic character of the bond. The greater the difference in electronegativity, the more polar the bond. Ionic bonds are the extreme polar bond, ΔEN greater than 1.7 Prentice-Hall © 2002 General Chemistry: Chapter 9

Writing Lewis Structures Chemistry 140 Fall 2005 Writing Lewis Structures All the valence e- of atoms must appear. Usually, the e- are paired. Usually, each atom requires an octet. H only requires 2 e-. Multiple bonds may be needed. Readily formed by C, N, O, S, and P. Reintroduce Lewis structures Prentice-Hall © 2002 General Chemistry: Chapter 9

General Chemistry: Chapter 9 Skeletal Structure Identify central and terminal atoms. H H H C C O H H H Many times this becomes apparent with the way that the molecular formula is written (CH3CH2OH) and using some intuition. Prentice-Hall © 2002 General Chemistry: Chapter 9

Some general trends and rules Chemistry 140 Fall 2005 Skeletal Structure Some general trends and rules Hydrogen atoms are always terminal atoms. Central atoms are generally those with the lowest electronegativity. Carbon atoms are always central atoms. Generally structures are compact and symmetrical. H can only accommodate two electrons H and O are common exceptions to rule 2 Organic compounds are not compact nor symmetrical. Prentice-Hall © 2002 General Chemistry: Chapter 9

Strategy for Writing Lewis Structures Prentice-Hall © 2002 General Chemistry: Chapter 9