Chapter 13case.ntu.edu.tw/CASTUDIO/Files/speech/Ref/CS0099S1B02_13.pdf · Bond Energies • Bond-dissociation energy (D) is the quantity of energy required to break one mole of covalent

Chapter 13 Slide 1 of 60

Chapter 13

Bonding: General Concepts


Chemical Bonds: A Preview

• Forces called chemical bonds hold atoms together in molecules and keep ions in place in solid ionic compounds.

• Chemical bonds are electrical forces; they reflect a balance in the forces of attraction and repulsion between electrically charged particles.

• Through appropriate measurements, scientists can determine the internuclear distances that correspond to the lowest energy states of molecules.

• Quantum mechanical calculations can then be used to develop a theoretical model that fits the experimental measurements.


Electrostatic Attractions and Repulsions


Energy of Interaction

A B

B

B

B

A

A

A


Electronegativity

• Electronegativity (EN, expressed as χ), is a measure of the ability of an atom to attract bonding electrons to itself when the atom is in a molecule.

• Mulliken’s EN

Absolute EN, χ = (IE + EA)/2• Pauling’s EN

define χH = 2.2∆ χ = χA - χB = [∆ΑΒ(kJ)/96.49]1/2 = [∆ΑΒ(kcal)/23.06]1/2

∆ΑΒ = D(A-B) - 1/2 [D(A-A) + D(B-B)]

Bond dissociation energy of A-B


Pauling’s Electronegativities

2.2

Electronegativity Differenceand Bond Type

• Two identical atoms have the same electronegativity and share a bonding electron pair equally. This is called a non-polar covalent bond. (For χA ~ χB, or lχA - χBl < 0.3)

• In covalent bonds between atoms with somewhat larger electronegativity differences (0.3 < lχA - χBl < 1.8) , electron pairs are shared unequally. The electrons are drawn closer to the atom of higher electronegativity, and the bond is called a polar covalent bond.

• With still larger differences in electronegativity (lχA - χBl > 1.8) , electrons may be completely transferred from metal to nonmetal atoms to form ionic bonds.


Electronegativity and Bond Type


Ionic Bonds and Ionic Crystals

• When atoms lose or gain electrons, they acquire a noble gas configuration, but do not become noble gases.

• Because the two ions formed in a reaction between a metal and a non-metal have opposite charges, they are strongly attracted to one another and form an ion pair.

• The net attractive electrostatic forces that hold the cations and anions together are ionic bonds.

• The highly ordered solid collection of ions is called an ionic crystal.

Interionic Forces of Attraction- Coulombic Force

E = (Z+Z-)/4πεr


Formation of a Crystalof Sodium Chloride

Unit Cell of Sodium Chloride


Energy Changes inIonic Compound Formation

• The enthalpy of formation of the ionic compound is more important than first ionization energy and electron affinity.

• The overall enthalpy change can be calculated using a step-wise procedure called the Born-Haber cycle.

• The sum of the enthalpy change values for the individual steps is in accordance with Hess’s Law.

• The large negative value of the lattice energy is the major factor that makes ionic compound formation an energetically favorable process.


A Born-Haber Cycle Example

- EA(Cl)IE(Na)

1/2D(Cl-Cl)

∆Ηsublimation (Na) ∆H5 = - UU : lattice energy


A Born-Haber Cycle ExampleU

1/2D(Cl-Cl)

∆Ηsublimation (Na)

IE(Na) -EA(Cl)

lattice energy U = - ∆Ηf0(NaCl) + ∆Ηsublimation (Na) + 1/2D(Cl-Cl) + IE(Na) - EA(Cl)

= (+411 +107 +122 + 496 –349) kJ/mol

= +787 kJ/mol

∆Ηf0(NaCl)

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

Na(g) + Cl(g)Na(s) + 1/2Cl2(g)


The Lewis Theory ofChemical Bonding

• Electrons, particularly valence electrons, play a fundamental role in chemical bonding.

• When metals and non-metals combine, valence electrons usually are transferred from the metal to the non-metal atoms giving rise to ionic bonds.

• In combinations involving only non-metals, one or more pairs of valence electrons are shared between the bonded atoms producing covalent bonds.

• In losing, gaining, or sharing electrons to form chemical bonds, atoms tend to acquire the electron configurations of noble gases.


Lewis Symbols

• In a Lewis symbol, the chemical symbol for the element represents the nucleus and core electrons of the atom, and dots around the symbol represent the valence electrons.


Using Lewis Symbolsto Represent Ionic Bonding

• Instead of using complete electron configurations to represent the loss and gain of electrons, Lewis symbols can be used for ionic bond.

·· ··Na· + ·Cl: → Na1+ :Cl: 1-

·· ··


Lewis Structures OfSimple Molecules

• A Lewis structure is a combination of Lewis symbols that represents the formation of covalent bonds between atoms.

• In most cases, a Lewis structure shows the bonded atoms with the electron configuration of a noble gas; that is, the atoms obey the octet rule. (H obeys the duet rule.)


Lewis Structures (continued)

• The shared pairs of electrons in a molecule are called bonding pairs.

• In common practice, the bonding pair is represented by a dash (-).

or• The other electron pairs, which are not shared, are

called non-bonding pairs, or lone pairs.


2nd period elements follow octet rule

The non-metals of the second period (except boron?) tend to form a number of covalent bonds equal to eight minus the group number.


Coordinate Covalent Bonds

For example:• hydronium ions

H3O+

• F3B-NH3

In some cases, one atom provides both electrons of the shared pair to form a bond called a coordinate covalent bond (or dative bond).


Multiple Covalent Bonds

• The covalent bond in which one pair of electrons is shared is called a single bond.

• Multiple bonds can also form:– Double bonds have two shared pairs of

electrons.– Triple bonds have three shared pairs of

electrons.• A double bond is represented by two dashes (=).• A triple bond is represented by three dashes (≡).


Polar and Non-polarCovalent Bonds


Formal Charge

Formal charge = # of valence electrons - [# of lone pair electrons + ½ (bonding

electrons)]• Usually, the most plausible Lewis structure is one with

no formal charges.• When formal charges are required, they should be as

small as possible.• Negative formal charges should appear on the most

electronegative atoms.• Adjacent atoms in a structure should not carry formal

charges of the same sign.


Formal Charge Illustrated


Lewis Structures & Formal Charge

CO FC(C) = 4 - 2 - 3 = -1

FC(O) = 6 - 2 - 3 = +1

CO2 FC(C) = 4 - 0 - 4 = 0

FC(O) = 6 - 4 - 2 = 0

N2O ↔ ↔

(-1) (+1)

(0) (0) (0)

(-1) (0)(+1) (0) (+1) (-1) (-2) (+1) (+1)

(+2) (-1)(-1)

resonance structures


Resonance: Delocalized Bonding

• Resonance theory states that whenever a molecule or ion can be represented by two or more plausible Lewis structures that differ only in the distribution of electrons, the true structure is a composite, or hybrid, of them.

• The different plausible structures are called resonance structures.

↔

• The actual molecule or ion that is a hybrid of the resonance structures is called a resonance hybrid.

• Electrons that are part of the resonance hybrid are spread out over several atoms and are referred to as being delocalized.


Resonance

ClO42-

(+3) (+2) (+1)

(0) (-1)

(-1)

(-1)(-1)

(-1)

(-1) (-1)

(-1)

(0)

↔↔ ↔

↔

Oxoacids: HNO3, H2SO4, HClO4


Molecules that Don’t Followthe Octet Rule

• Molecules with an odd number of valence electrons have at least one of them unpaired and are called free radicals.

• Some molecules have incomplete octets. These are usually compounds of Be, B, and Al, generally have some unusual bonding characteristics, and are often quite reactive.


Molecules that Don’t Followthe Octet Rule (continued)

• Some compounds have expanded valence shells, which means that the central atom has more than eight electrons around it.e.g. SF6, PCl5


Bond Lengths

• The term bond order indicates whether a covalent bond is single (b.o. = 1), double (b.o. = 2), or triple (b.o. = 3).

• Bond length is the distance between the nuclei of two atoms joined by a covalent bond.

• Bond length depends on the particular atoms in the bond and on the bond order.

• The length of the covalent bond joining unlike atoms is the sum of the covalent radii of the two atoms.

• Bond lengths are usually measured in picometers(10-12 meter).


A Visualization of Bond Dissociation Energy


Bond Energies

• Bond-dissociation energy (D) is the quantity of energy required to break one mole of covalent bonds between atoms in a molecule in the gas phase.

• An average bond energy is the average of the bond-dissociation energies for a number of different molecules containing the particular bond.

average bond energy of O-H= (499 + 428)/2 = 464 J/mol

bond-dissociation energy of H-H= 436 kJ/mol


Bond Energies


Representative Bond Lengths & Average Bond Energies

??

?


Bond Dissociation Energies & Hreaction

•The sum of the enthalpy changes for breaking the old bonds and forming the new bonds is the enthalpy change for the reaction.

C2H6 + Cl2 → C2H5Cl + HCl

∆Hreaction = ∆Hbond broken + ∆Hbond formed

= Σ D(bonds broken) - Σ D(bonds formed)

∆H = [D(C-H) + D(Cl-Cl)] - [D(C-Cl) + D(H-Cl)]

= [414 +243] – [339 + 431] = -113 kJ/mol

> 0 < 0

∆ H


Molecular Geometry

• The molecular geometry, or the shape of a molecule is described by the geometric figure formed when the atomic nuclei are imagined to be joined by the appropriate straight lines.

• Determination of molecular geometry1. Valence-Shell Electron-Pair Repulsion (VSEPR)- Pairs of valence electrons in bonded atoms repel one another.- The mutual repulsions push electron pairs as far from one another

as possible.

2. Ligand Field Stabilization Energy- Mainly for compounds with the central atoms as transition metals


Molecular Geometry of Water


Electron-Group Geometries

• An electron group is any collection of valence electrons, localized in a region around a central atom, that repels other groups of valence electrons.

• The mutual repulsions among electron groups lead to an orientation of the groups that are called electron-group geometry.


Electron-Group Geometries

#. ofElectron Pair

Electron-groupGeometry

2 linear

3 trigonal planar

4 tetrahedral

5 Trigonalbipyramidal

6 octahedral


A Balloon Analogy


Geometries of Methane#. of electron group on C = 1/2 (4 + 4) = 4

Electron group geometry: tetrahedral

VSEPR notation AX4

Molecular geometry: tetrahedral

109.50


Molecular Geometry of Water

#. of electron group on O = 1/2 (6 + 2) = 4

Electron group geometry: tetrahedral

VSEPR notation AX2E2

Molecular geometry: bent

104.50 103.20

A: the central atom in a structureX: terminal atomsE: the lone pairs of electrons


Electron Repulsion

• Lone-pair – Lone-pair > Lone-pair – Bonding-pair> Bonding-pair – Bonding-pair

• Triple bond > Double bond > Single bond

Electron Repulsion


Molecular Geometry forIodine Pentafluoride (IF5)

#. of electron group on I = 1/2 (7 + 5) = 6

Electron group geometry: octahedral

VSEPR notation AX5E

Molecular geometry: Square pyramidal


Molecular Geometry forSulfur Tetrafluoride (SF4)

#. of electron group on S = 1/2 (6 + 4) = 5

Electron group geometry: trigonal bipyramidal

VSEPR notation AX4E

Molecular geometry: trigonal pyramidal or Seesaw

9001200

900


Molecular Geometry forIodine Trifluoride (IF3)

#. of electron group on I = 1/2 (7 + 3) = 5

Electron group geometry: trigonal bipyramidal


Molecular geometry: triangular or T-shaped

9001200

1200


Molecular Geometry forIodine Tetrafluoride Ion (IF4

-)

#. of electron group on I = 1/2 (7 + 4 +1) = 6

Electron group geometry: octahedral


Molecular geometry: Square planar


A VSEPR Summary


A VSEPR Summary (continued)






Polar Moleculesand Dipole Moments

• A molecule with separate centers of positive and negative charge is called a polar molecule.

• The dipole moment (µ) of a molecule is the product of the magnitude of the charge (δ) and the distance (d) that separates the centers of positive and negative charge.

µ = δ d



• Dipole moments are generally expressed in a quantity

called a debye.

• 1 debye (D) = 3.34 x 10-30 C m = 10-18 esu

• charge of electron = 1.602 x 10-19 C = 4.80 x 10-10 esu



Calculation of percentage ionic character of HCl

Η−Cl, µ = 1.08 D, r H-Cl = 127pm

δ = µ /d = 1.08 x 3.34 x 10-30 /(127 x 10-12 )

= 2.84 x10-20 C

percentage ionic character = [2.84 x10-20 / (1.602 x 10-19)]

x 100% = 17.7%


Dipole-Dipole Interactions

Dipole momentµ = δ ·d

E = - 2(µ1 ·µ2)/4πεr3

d

r

µ2

µ1


How Polar Molecules Behave in an Electric Field


Bond Dipoles andMolecular Dipoles

• All polar covalent bonds have a bond dipole; a separation of positive and negative charge centers in an individual bond.

• Bond dipoles have both a magnitude and a direction.

• molecular dipole = vector sum of bond dipolesCO2, linear with no dipole moment (µ = 0 D) water is bent (bond angle = 104.5o) and µ = 1.84 D.


Molecular Shapesand Dipole Moments

• Molecules can be predicted to be polar or non-polar based on the following three-step approach:– Use electronegativity values to predict bond

dipoles.– Use the VSEPR method to predict the molecular

shape.– From the molecular shape, molecular dipole =

vector sum of bond dipoles• Lone-pair electrons can also make a contribution to

dipole moments. e.g. NH3 µ = 5.0 x 10-30 C.mNF3 µ = 0.7 x 10-30 C.m

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Chapter 13case.ntu.edu.tw/CASTUDIO/Files/speech/Ref/CS0099S1B02_13.pdf · Bond Energies • Bond-dissociation energy (D) is the quantity of energy required to break one mole of covalent