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Chapter Eightp328

Bonding:

General Concepts

Contents

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8-1 Types of Chemical Bonds

))(1031.2( 2119

rQQ

nmJE

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Coulomb’s law

The energy of interaction between a pair of ionscan be calculated using Coulomb’s law:

where E has units joules, r is the distancebetween the ion centers in nanometers, andQ1 and Q2 are the numerical ion charges.

For example, the distance between the centers ofthe Na+ and Cl- ions is 0.276 nm, and the ionicenergy pair of ions is

Jnm

nmJE 1919 1037.8]276.0

)1)(1([)1031.2(

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Questions to Consider

What is meant by the term “chemical

bond?”

Why do atoms bond with each other to

form molecules?

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Figure 8.1 (a)

The interaction of two

hydrogen atoms.

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Figure 8.1(b) Energy profiles as a function between the

hydrogen atoms. As the atoms approach each other (right side

of graph), the energy decreases until the distances reaches

0.074 nm and then begins to increase again due to repulsions.

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Bondlength

Key ideas in bonding

Ionic Bonding: Electrons are transferred

Covalent Bonding: Electrons are shared

equally

What about intermediate cases?

Polar covalent bond:

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H F

FH F

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Chemical bond

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?

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8-2 Electronegativityp333

Expected H-X bonding energy= ½ (H-H bond energy + X-X bond energy)

△ = (H-X)act - (H-X)exp

The greater is the difference in the electronegativities of the atoms,the greater is the ionic compound and the greater is the value of △.

Linus Pauling(1901-1995)

If X has a greater electronegativity than H, the shares electron(s)will tend to be closer to the X atom. The molecule will be polar,with charge distribution.

Electronegativity: the ability of an atom in amolecule to attract shared electrons to itself.

Figure 8.2 The effect of an electric field on hydrogen fluoride molecules

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Polar molecules

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The Pauling electronegativity values

Figure 8-3

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The Pauling electronegativity values. Electronegativity generally

increases across a period and decreases down a group.

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The general trend for electronegativity

What is the general trend for

electronegativity across rows and down

columns on the periodic table?

Explain the trend.

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Ex 8.1 Relative Bond PolaritiesP335

Order the following bonds according to polarity:

H–H, O–H, Cl–H, S–H, and F–H.

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Table 8.1 The Relationship BetweenElectronegativity and Bond Type

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8-3 Bond polarity and dipole momentsp335

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Ex 8.2 Bond Polarity and DipoleMoment

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For each of the following molecules, show the direction

of the bond polarities and indicate which ones have a

dipole moment: HCl, Cl2, SO3(a planar molecule with the

oxygen atoms spaced evenly around the central sulfur

atom), CH4 [trtrahedral(see Table 8.2) with the carbon

atom at the center], and H2S (V-shaped with the sulfur

atom at the point).

Solution:

(a) HCl

(b) Cl2

(c) SO3

p338(d) CH4

(e) H2S

8-4 Ions: Electronconfigurations and sizes

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Predicting Formulas of Ionic Compounds

Sizes of ions p340

Table 8.3 Common ions with noble gas configuration inionic compounds

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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.

React 3

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Ionic radii

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What we can “read”from theperiodic table:

Trends for Atomic size Ion radius Ionization energy Electronegativity

Electron configurations Predicting formulas for ionic

compounds Ranking polarity of bonds

Ex 8.4 Relative Lon Size IIP342

Choose the largest ion in each of the followinggroups.

a.Li+, Na+, K+, Rb+, Cs+

b.Ba2+, Cs+, I-, Te2-

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Ex 8.3 Relative Lon Size IP342

Arrange the ions Se2-, Br-, Rb+, and Sr2+ in order of

decreasing size.

8-5 Energy effects in binaryionic compounds

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Formation of an ionic solid

1. Sublimation of the solid metal

•M(s) M(g) [endothermic] (For Li(s) is +161 kJ.)

2. Ionization of the metal atoms

•M(g) M+(g) + e[endothermic] (For Li(g) is +520 kJ)

3. Dissociation of the nonmetal

•1/2X2(g) X(g) [endothermic] (For F is +½ (154 kJ)

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4. Formation of Xions in the gas phase:X(g) + e X(g) [exothermic] (For F- is -328 kJ/mole)

5. Formation of the solid MX:

M+(g) + X(g) MX(s) [quite exothermic]

(Corresponding to the lattice energy for LiF, which is -1047

kJ./mole)

Formation of an ionic solid (continued)

Figure 8.11

Comparingenergy changes

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Born-Haber cycle for NaCl

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Lattice Energy Calculations p344

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8.6 Partial ionic character ofcovalent bonds

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The relationship between the ionic character

of a covalent bond and the electronegativity

difference of the bonded atoms.

Figure 8.13

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8-7 The Covalent Chemical Bond:A Model

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Models

Models are attempts to explain how natureoperates on the microscopic level based onexperiences in the macroscopic world.

The Localized Electron Bonding Model

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Fundamental Properties of Models

1. A model does not equal reality.

2. Models are oversimplifications, and aretherefore often wrong.

3. Models become more complicated as they

age.

4. We must understand the underlyingassumptions in a model so that we don’tmisuse it.

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8-8 Covalent Bond Energies andChemical Reactions

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Ex 8.5 △H from Bond Energies

Using the bond energies listed in Table 8.4, calculate △H

for the reaction of methane with chlorine and fluorine to

give Freon-12(CF2Cl2).

)(2)(2)()(2)(2)( 22224 gHClgHFgClCFgFgClgCH

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8-9 The Localized ElectronBonding Model

A molecule is composed of atoms that are

bound together by sharing pairs of electrons

using the atomic orbitals of the bound atoms.

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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 long pairs.

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8-10 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.

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Lewis Structures

1. Sum the valence electrons.

2. Place bonding electrons between pairs of

atoms.

3. Atoms usually have noble gas

configurations.

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Ex 8.6 Writing Lewis Structures P357

Give the Lewis structure for each of the following.

a. HF, b. N2, c. NH3, d. CH4, e. CF4, f. NO+

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8-11 Exceptions to the Octet Rule p358

Ex 8.7 Lewis Structures for MoleculesThat Violate the Octet Rule I

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Write the Lewis structure for PCl5.

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Ex 8.8 Lewis Structures for MoleculesThat Violate the Octet Rule II

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Write the Lewis structure for each molecule or ion.

a. ClF3 b. XeO3 c. RnCl2 d. BeCl2 e. ICl4-

8-12 Resonance p362

Ex 8.9 Resonance StructuresP363

Describe the electron arrangement in the nitrite

anion (NO2-) using the localized electron model.

Rules Governing Formal Charge p366

Ex 8.10 Formal ChargesP366

Give possible Lewis structures for XeO3 , an explosive

compound of xenon. Which Lewis structure or structures

are most appropriate according to the formal charges?

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8-13 Molecular Structure:The VSEPR Model

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VSEPR Model

The structure around a given atom is

determined principally by minimizing

electron pair repulsions.

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Predicting a VSEPR Structure

1. Draw Lewis structure.

2. Put 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.

P369Ex 8.11 Prediction of MolecularStructure I

Describe the molecular structure of the water molecule.

The Lewis structure for water is

There are four pairs of electrons: two bonding pairs and twononbonding pairs. To minimize repulsions, these bestarrangement in a tetrahedral array, as shown in Fig. 8.17.

Solution

Figure 8.17

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Ex 8.12 Prediction of Molecular Structure II

When phosphorus reacts with excess chlorine gas, the

compound phosphorus pentachloride (PCl5) is formed. In

the gaseous and liquid states, this substance consists of

PCl5 molecules, but in the solid state it consists of a 1 : 1

mixture of PCl4+ and PCl6

- ions. Predict the geometric

structures of PCl5, PCl4+, and PCl6

-.

SolutionThe Lewis structure for PCl5 is shown. Five pairs of electrons aroundthe phosphorous atom require a trigonal bipyramidal arrangement(see Table 8.6).

The Lewis structure for the PCl4+ ions (5+4(7) -1 = 32 valence

electrons) is shown. There are four pairs of electrons surroundingthe phosphorus atom in the PCl4

+ ion, which requires a tetrahedralarrangement of the pairs.

The Lewis structure for PCl6- (5 + 6(7) + 1 = 48 valence electrons)

is shown. Since each electron pair is shared with a chlorine atom, anoctahedral PCl6

- anion is predicted.

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Ex 8.13 Prediction of MolecularStructure III

Because the noble gases have filled s and p valence orbitals,they were not expected to be chemically reactive. In fact,for many years these elements were called insert gasesbecause of this supposed inability to form any compounds.However. In the early 1960s several compounds of krypton,xenon, and radon were synthesized. For example, a team atthe Argonne National Laboratory produced the stablecolorless compound xenon tetrafluoride (XeF4). Predict itsstructure and whether it has a dipole moment.

Solution

The Lewis structure for XeF4 is

The xenon atom in this molecule is surrounded bysix pairs of electrons, which means an octahedralarrangement.

The arrangement in Fig. 8.20(b) is preferred, and themolecular structure is predicted to be square planar. Thereis an octahedral arrangement of electron pairs, but theatoms form a square planar structure. Although each Xe-Fbond is polar, their structure causes the polarities to cancel.Thus XeF4 has no dipole moment as shown in the margin.

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P376Ex 8.14 Structures of Molecules withMultiple Bonds

Predict the molecular structure of the sulfur dioxide

molecule. Is this molecule expected to have a dipole moment?Solution

We must determine the Lewis structure for the SO2molecule, which has 18 valence electrons. The expectedresonance structures are

The structure of the SO2 molecule expected to be V-

shaped, with a 120-degree bond angle. The molecule has a

dipole moment as shown:

Molecules Containing No Single Central Atomp377

The VSEPR Model- How Well Does It Work?p379

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VSEPR

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VSEPR: Two Electron Pairs

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VSEPR: Three Electron Pairs

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VSEPR: Four Electron Pairs

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VSEPR: Iodine Pentafluoride