Chapter Eight BONDING GENERAL CONCEPTS Questions to Consider

Chapter Eight BONDING: GENERAL CONCEPTS

Questions to Consider • • • What is meant by the term “chemical bond”? Why do atoms bond with each other to form compounds? How do atoms bond with each other to form compounds? Chapter 8 | Slide 2 Copyright © Houghton Mifflin Company. All rights reserved.

A Chemical Bond • • • No simple way to define this. Forces that hold groups of atoms together and make them function as a unit. A bond will form if the energy of the aggregate is lower than that of the separated atoms. 8. 1 Chapter 8 | Slide 3 Copyright © Houghton Mifflin Company. All rights reserved.

The Interaction of Two Hydrogen Atoms 8. 1 Chapter 8 | Slide 4 Copyright

The Interaction of Two Hydrogen Atoms 8. 1 Chapter 8 | Slide 5 Copyright

Key Ideas in Bonding • • • Ionic Bonding – electrons are transferred Covalent Bonding – electrons are shared equally What about intermediate cases? 8. 1 Chapter 8 | Slide 6 Copyright © Houghton Mifflin Company. All rights reserved.

Polar Covalent Bond • • Unequal sharing of electrons between atoms in a molecule. Results in a charge separation in the bond (partial positive and partial negative charge). 8. 1 Chapter 8 | Slide 7 Copyright © Houghton Mifflin Company. All rights reserved.

The Effect of an Electric Field on Hydrogen Fluoride Molecules Chapter 8 | Slide

Polar Molecules 8. 1 Chapter 8 | Slide 9 Copyright © Houghton Mifflin Company.

Concept Check • What is meant by the term “chemical bond? ” • Why do atoms bond with each other to form molecules? • How do atoms bond with each other to form molecules? 8. 1 Chapter 8 | Slide 10 Copyright © Houghton Mifflin Company. All rights reserved.

Electronegativity • • The ability of an atom in a molecule to attract shared electrons to itself. On the periodic table, electronegativity generally increases across a period and decreases down a group. 8. 2 Chapter 8 | Slide 11 Copyright © Houghton Mifflin Company. All rights reserved.

The Pauling Electronegativity Values 8. 2 Chapter 8 | Slide 12 Copyright © Houghton

Concept Check • If lithium and fluorine react, which has more attraction for an electron? Why? • In a bond between fluorine and iodine, which has more attraction for an electron? Why? 8. 2 Chapter 8 | Slide 13 Copyright © Houghton Mifflin Company. All rights reserved.

Concept Check • What is the general trend for electronegativity across rows and down columns on the periodic table? • Explain the trend. 8. 2 Chapter 8 | Slide 14 Copyright © Houghton Mifflin Company. All rights reserved.

The Relationship Between Electronegativity and Bond Type 8. 2 Chapter 8 | Slide 15

Exercise Arrange the following bonds from most to least polar: a) N-F O-F C-F

Concept Check Which of the following bonds would be the least polar yet still be considered polar covalent? Mg-O C-O O-O Si-O N-O 8. 2 Chapter 8 | Slide 17 Copyright © Houghton Mifflin Company. All rights reserved.

Concept Check Which of the following bonds would be the most polar without being considered ionic? Mg-O C-O O-O Si-O N-O 8. 2 Chapter 8 | Slide 18 Copyright © Houghton Mifflin Company. All rights reserved.

Dipole Moment • • Property of a molecule whose charge distribution can be represented by a center of positive charge and a center of negative charge. Use an arrow to represent a dipole moment. – Point to the negative charge center with the tail of the arrow indicating the positive center of charge. 8. 3 Chapter 8 | Slide 19 Copyright © Houghton Mifflin Company. All rights reserved.

Dipole Moment 8. 3 Chapter 8 | Slide 20 Copyright © Houghton Mifflin Company.

No Net Dipole Moment (Dipoles Cancel) 8. 3 Chapter 8 | Slide 21 Copyright

Stable Compounds • Atoms in stable compounds usually have a noble gas electron configuration.

Isoelectronic Series • A series of ions/atoms containing the same number of electrons. O 2 -, F-, Ne, Na+, Mg 2+, and Al 3+ 8. 4 Chapter 8 | Slide 23 Copyright © Houghton Mifflin Company. All rights reserved.

Concept Check Choose an alkali metal, an alkaline metal, a noble gas, and a halogen so that they constitute an isoelectronic series when the metals and halogen are written as their most stable ions. – – – What is the electron configuration for each species? Determine the number of electrons for each species. Determine the number of protons for each species. Rank the species according to increasing radius. Rank the species according to increasing ionization energy. 8. 4 Chapter 8 | Slide 24 Copyright © Houghton Mifflin Company. All rights reserved.

Ionic Radii 8. 4 Chapter 8 | Slide 25 Copyright © Houghton Mifflin Company.

We can “read” from the periodic table: • Trends for: – Atomic size, ion radius, ionization energy, electronegativity • Electron configurations • Formula prediction for ionic compounds • Covalent bond polarity ranking 8. 4 Chapter 8 | Slide 26 Copyright © Houghton Mifflin Company. All rights reserved.

Lattice Energy • The change in energy that takes place when separated gaseous ions are packed together to form an ionic solid. Lattice energy = k(Q 1 Q 2/r) 8. 5 Chapter 8 | Slide 27 Copyright © Houghton Mifflin Company. All rights reserved.

Born-Haber Cycle for Na. Cl 8. 5 Chapter 8 | Slide 28 Copyright ©

Formation of an Ionic Solid 1. Sublimation of the solid metal • M(s) M(g) [endothermic] 2. Ionization of the metal atoms • M(g) M+(g) + e [endothermic] 3. Dissociation of the nonmetal • 1/2 X (g) X(g) [endothermic] 2 8. 5 Chapter 8 | Slide 29 Copyright © Houghton Mifflin Company. All rights reserved.

Formation of an Ionic Solid (continued) 4. Formation of X ions in the gas phase: • X(g) + e X (g) [exothermic] 5. Formation of the solid MX • M+(g) + X (g) MX(s) [quite exothermic] 8. 5 Chapter 8 | Slide 30 Copyright © Houghton Mifflin Company. All rights reserved.

Comparing Energy Changes Chapter 8 | Slide 31 Copyright © Houghton Mifflin Company. All

Partial Ionic Character of Covalent Bonds • None of the bonds reaches 100% ionic character even with compounds that have a large electronegativity difference (when tested in the gas phase). 8. 6 Chapter 8 | Slide 32 Copyright © Houghton Mifflin Company. All rights reserved.

The relationship between the ionic character of a covalent bond and the electronegativity difference of the bonded atoms 8. 6 Chapter 8 | Slide 33 Copyright © Houghton Mifflin Company. All rights reserved.

Ionic Compound • Any compound that conducts an electric current when melted will be

Models • Models are attempts to explain how nature operates on the microscopic level based on experiences in the macroscopic world. 8. 7 Chapter 8 | Slide 35 Copyright © Houghton Mifflin Company. All rights reserved.

Fundamental Properties of Models 1. A model does not equal reality. 2. Models are oversimplifications, and are therefore often wrong. 3. Models become more complicated and are modified as they age. 4. We must understand the underlying assumptions in a model so that we don’t misuse it. 8. 7 Chapter 8 | Slide 36 Copyright © Houghton Mifflin Company. All rights reserved.

Bond Energies • To break bonds, energy must be added to the system (endothermic). • To form bonds, energy is released (exothermic). 8. 8 Chapter 8 | Slide 37 Copyright © Houghton Mifflin Company. All rights reserved.

Bond Energies ΔH = ΣD(bonds broken) – ΣD(bonds formed) D represents the bond energy per mole of bonds (always has a positive sign). 8. 8 Chapter 8 | Slide 38 Copyright © Houghton Mifflin Company. All rights reserved.

Localized Electron Model • A molecule is composed of atoms that are bound together by sharing pairs of electrons using the atomic orbitals of the bound atoms. 8. 9 Chapter 8 | Slide 39 Copyright © Houghton Mifflin Company. All rights reserved.

Localized Electron Model • Electron pairs are assumed to be localized on a particular atom or in the space between two atoms: § Lone pairs – pairs of electrons localized on an atom § Bonding pairs – pairs of electrons found in the space between the atoms 8. 9 Chapter 8 | Slide 40 Copyright © Houghton Mifflin Company. All rights reserved.

Localized Electron Model 1. Description of valence electron arrangement (Lewis structure). 2. Prediction of geometry (VSEPR model). 3. Description of atomic orbital types used to share electrons or hold lone pairs. 8. 9 Chapter 8 | Slide 41 Copyright © Houghton Mifflin Company. All rights reserved.

Lewis Structure • Shows how valence electrons are arranged among atoms in a molecule. • Reflects central idea that stability of a compound relates to noble gas electron configuration. 8. 10 Chapter 8 | Slide 42 Copyright © Houghton Mifflin Company. All rights reserved.

Writing Lewis Structures 1. Sum the valence electrons. 2. Place bonding electrons between pairs of atoms. 3. Atoms usually have noble gas configurations. Arrange remaining electrons to satisfy the octet rule (or duet rule for hydrogen). 8. 10 Chapter 8 | Slide 43 Copyright © Houghton Mifflin Company. All rights reserved.

Concept Check Draw a Lewis structure for each of the following molecules: H 2

Exceptions • When it is necessary to exceed the octet rule for one of several third-row (or higher) elements, place the extra electrons on the central atom. 8. 11 Chapter 8 | Slide 46 Copyright © Houghton Mifflin Company. All rights reserved.

Resonance • More than one valid Lewis structure can be written for a particular molecule. • Actual structure is an average of the resonance structures. 8. 12 Chapter 8 | Slide 47 Copyright © Houghton Mifflin Company. All rights reserved.

Concept Check Draw a Lewis structure for each of the following molecules: CO CH 3 OH PCl 5 SF 6 CO 2 BF 3 NO 3 - 8. 10 -8. 12 Chapter 8 | Slide 48 Copyright © Houghton Mifflin Company. All rights reserved.

Formal Charge • Nonequivalent Lewis structures. • Atoms in molecules try to achieve formal charges as close to zero as possible. • Any negative formal charges are expected to reside on the most electronegative atoms. 8. 12 Chapter 8 | Slide 49 Copyright © Houghton Mifflin Company. All rights reserved.

Formal Charge • Formal charge = # valence e– on free atom – # valence e– assigned to the atom in the molecule. • Assume: – Lone pair electrons belong entirely to the atom in question – Shared electrons are divided equally between the two sharing atoms 8. 12 Chapter 8 | Slide 50 Copyright © Houghton Mifflin Company. All rights reserved.

VSEPR Model • VSEPR: Valence Shell Electron-Pair Repulsion. • The structure around a given atom is determined principally by minimizing electron pair repulsions. 8. 13 Chapter 8 | Slide 51 Copyright © Houghton Mifflin Company. All rights reserved.

Predicting a VSEPR Structure 1. Draw Lewis structure. 2. Put electron pairs as far apart as possible. 3. Determine positions of atoms from the way electron pairs are shared. 4. Determine the name of molecular structure from positions of the atoms. 8. 13 Chapter 8 | Slide 52 Copyright © Houghton Mifflin Company. All rights reserved.

VSEPR 8. 13 Chapter 8 | Slide 53 Copyright © Houghton Mifflin Company. All

VSEPR: Two Electron Pairs 8. 13 Chapter 8 | Slide 54 Copyright © Houghton

VSEPR: Three Electron Pairs 8. 13 Chapter 8 | Slide 55 Copyright © Houghton

VSEPR: Four Electron Pairs 8. 13 Chapter 8 | Slide 56 Copyright © Houghton

VSEPR: Iodine Pentafluoride 8. 13 Chapter 8 | Slide 57 Copyright © Houghton Mifflin

Concept Check Determine the shape for each of the following molecules, and include bond angles: HCN PH 3 SF 4 O 3 Kr. F 4 8. 13 Chapter 8 | Slide 58 Copyright © Houghton Mifflin Company. All rights reserved.

Concept Check True or false: A molecule that has polar bonds will always be polar. -If true, explain why. -If false, provide a counter-example. 8. 13 Chapter 8 | Slide 59 Copyright © Houghton Mifflin Company. All rights reserved.

Concept Check True or false: Lone pairs make a molecule polar. -If true, explain why. -If false, provide a counter-example. 8. 13 Chapter 8 | Slide 60 Copyright © Houghton Mifflin Company. All rights reserved.