Bonding – General ConceptsBonding – General Concepts

What is a Bond?• A force that holds atoms together.• We will look at it in terms of energy.

– Bond energy - the energy required to break a bond.

• Why are compounds formed?– Because it gives the system the lowest

energy.

Ionic Bonding• An atom with a low ionization energy

reacts with an atom with high electron affinity.

• The electron moves.• Opposite charges hold the atoms together.

Electronegativity• The ability of an electron to attract shared

electrons to itself.• Pauling method• Imaginary molecule HX• Expected H-X energy =

H-H energy + X-X energy2

• = (H-X) actual - (H-X)expected

Electronegativity is known for almost every element• Gives us relative electronegativities of all

elements.• Tends to increase left to right.• decreases as you go down a group.• Most Noble gases do not have values.• Difference in electronegativity between

atoms tells us how polar.

Electronegativity: The ability of anThe ability of anatom in a molecule to attract shared electrons to atom in a molecule to attract shared electrons to itself.itself.

Electronegativity difference

Bond Type

Zero

Intermediate

Large

Covalent

Polar Covalent

Ionic

Covalent C

haracter decreasesIonic C

haracter increases

Ionic BondsIonic Bonds Electrons are transferred

Electronegativity differences are generally greater than 1.7 The formation of ionic bonds is always exothermic!

Determination Determination of Ionic of Ionic

CharacterCharacter

Compounds are ionic if they conduct electricity in their molten state

Electronegativity difference is not the final determination of ionic character

• Q is the charge.• r is the distance between the centers.• If charges are opposite, E is negative

– exothermic• Same charge, positive E, requires energy

to bring them together.– endothermic

19 1 2(2.31 10 ) QQE x J nmr

1 2QQEr

Coulomb’s LawCoulomb’s Law

Size of ions• Ion size increases down a group.• Cations are smaller than the atoms they

came from.• Anions are larger.• across a row they get smaller, and then

suddenly larger.• First half are cations.• Second half are anions.

Periodic Trends• Across the period nuclear charge

increases so they get smaller.• Energy level changes between anions and

cations.

Li+1

Be+2

B+3

C+4

N-3O-2 F-1

Table of Table of Ion Ion

SizesSizes

Size of Isoelectronic ions• Iso - same• Iso electronic ions have the same # of

electrons• Al+3 Mg+2 Na+1 Ne F-1 O-2 and N-3 • All have 10 electrons.• All have the configuration 1s22s22p6

Size of Isoelectronic ions• Positive ions have more protons so they

are smaller.

Al+3

Mg+2

Na+1 Ne F-1 O-2 N-3

Ionic Compounds• We mean the solid crystal.• Ions align themselves to maximize

attractions between opposite charges,• and to minimize repulsion between like

ions.• Can stabilize ions that would be unstable

as a gas.• React to achieve noble gas configuration

List the following atoms in order of increasing ionization energy: Li, Na, C,

O, F.

Li < Na < C < O

< F

Na < Li < C < O

< F

F < O < C < Li

< Na

Na < Li < F <

O < C

Na < Li < C < F

< O

6%

82%

0%0%

12%

1. Li < Na < C < O < F2. Na < Li < C < O < F3. F < O < C < Li < Na4. Na < Li < F < O < C5. Na < Li < C < F < O

Table

Sodium losing an electron is an ________ process and fluorine losing an electron is an

_______ process.

endotherm

ic, exo

thermic

exotherm

ic, endotherm

ic

endotherm

ic, endotherm

ic

exotherm

ic, exo

thermic

more in

formati

on needed

15%20%

5%

20%

40%

1. endothermic, exothermic2. exothermic, endothermic3. endothermic, endothermic4. exothermic, exothermic5. more information needed

Table

Which of the following statements is true about the ionization energy

of Mg+?

It will

be equal to th

e io...

It will

be equal to and o...

It will

be equal to and o...

It will

be equal to and o...

none of the above

9%

18%

9%

36%

27%

1. It will be equal to the ionization energy of Li.

2. It will be equal to and opposite in sign to the electron affinity of Mg.

3. It will be equal to and opposite in sign to the electron affinity of Mg+.

4. It will be equal to and opposite in sign to the electron affinity of Mg2+.

5. none of the aboveTable

Choose the compound with the most ionic bond.

LiCl

KF NaCl

LiF KCl

5%

74%

5%

16%

0%

1. LiCl2. KF3. NaCl4. LiF5. KCl

Table

In which pair do both compounds exhibit predominantly ionic

bonding?

PCl5 and HF

Na2SO3 and BH3

KI and O

3

NaF and H2O

RbCl and CaO

0% 0%

100%

0%0%

1. PCl5 and HF2. Na2SO3 and BH3

3. KI and O3

4. NaF and H2O5. RbCl and CaO

Table

Which of the following arrangements is in order of increasing size?

Ga3+ > Ca2+ > K+ > Cl– >...

S2– > Cl– > K+ > Ca2+ > ...

Ga3+ > S2– > Ca2+ > Cl– ...

Ga3+ > Ca2+ > S2– > Cl– .

Ga3+ > Ca2+ > S2– > K+ >...

67%

33%

0%0%0%

1. Ga3+ > Ca2+ > K+ > Cl– > S2–

2. S2– > Cl– > K+ > Ca2+ > Ga3+

3. Ga3+ > S2– > Ca2+ > Cl– > K+

4. Ga3+ > Ca2+ > S2– > Cl– > K+

5. Ga3+ > Ca2+ > S2– > K+ > Cl–

Table

Which of the following species would be expected to have the

lowest ionization energy?

F- Ne O2-

Mg2

+ Na+

0%

50%

0%

50%

0%

1. F-

2. Ne3. O2-

4. Mg2+

5. Na+

Table

Sodium Sodium Chloride Chloride Crystal Crystal LatticeLattice

Ionic compounds form solids at ordinary Ionic compounds form solids at ordinary temperatures.temperatures.Ionic compounds organize in a characteristic Ionic compounds organize in a characteristic crystal lattice of alternating positive and crystal lattice of alternating positive and negative ions.negative ions.

Forming Ionic Compounds• Lattice energy - the energy associated with

making a solid ionic compound from its gaseous ions.

• M+(g) + X-(g) MX(s)• This is the energy that “pays” for making

ionic compounds.• Energy is a state function so we can get

from reactants to products in a round about way.

Calculating Lattice Energy• Lattice Energy = k(Q1Q2 / r)• k is a constant that depends on the

structure of the crystal.• Q’s are charges.• r is internuclear distance.• Lattice energy is greater with more highly

charged ions.• This bigger lattice energy “pays” for the

extra ionization energy.• Also “pays” for unfavorable electron

affinity.

Estimate Hf for Sodium ChlorideNa(s) + ½ Cl2(g) NaCl(s)

Lattice Energy: Na+(g) + Cl-(g) NaCl(s) -786 kJ/mol

Ionization Energy for Na: Na(g) Na+(g) + e- 495 kJ/molElectron Affinity for Cl: Cl(g) + e- Cl-(g) -349

kJ/molBond energy of Cl2: Cl2(g) Cl(g) 239 kJ/molEnthalpy of sublimation for Na: Na(s) Na(g)

109 kJ/mol

Estimate Hf for Sodium ChlorideNa(s) + ½ Cl2(g) NaCl(s) Lattice Energy -786 kJ/mol

Ionization Energy for Na 495 kJ/molElectron Affinity for Cl -349 kJ/molBond energy of Cl2 239 kJ/molEnthalpy of sublimation for Na

109 kJ/mol

Na(s) Na(g) + 109 kJ

Na(g) Na+(g) + e- + 495 kJ ½ Cl2(g) Cl(g) + ½(239 kJ) Cl(g) + e- Cl-(g) - 349

kJ

Na+(g) + Cl-(g) NaCl(s) -786 kJ

Na(s) + ½ Cl2(g) NaCl(s) -412 kJ/mol

F- Cl- Br- I-

Li+ 1036 853 807 757Na+ 923 787 747 704K+ 821 715 682 649Rb+ 785 689 660 630Cs+ 740 659 631 604

Lattice Energies of Alkali Metals Halides (kJ/mol)

OH- O2-

Na+ 900 2481Mg2+ 3006 3791Al3+ 5627 15,916

Lattice Energies of Salts of the OH- and O2- Ions (kJ/mol)

What about covalent compounds?

• The electrons in each atom are attracted to the nucleus of the other.

• The electrons repel each other,• The nuclei repel each other.• They reach a distance with the lowest

possible energy.• The distance between is the bond length.

Polar-Covalent bonds

Nonpolar-Covalent bonds

Covalent BondsCovalent Bonds

Electrons are unequally shared Electronegativity difference between .3 and 1.7

Electrons are equally shared Electronegativity difference of 0 to 0.3

Covalent Bonding Forces

Electron – Electron – electron electron repulsive forcesrepulsive forces Proton – proton repulsive forces Electron – proton attractive forces

Bond Length Diagram

Fundamental Properties of ModelsFundamental Properties of ModelsA model does not equal reality. Models are oversimplifications, and

are therefore often wrong. Models become more complicated

as they age. We must understand the

underlying assumptions in a model so that we don’t misuse it.

The The OctetOctet Rule RuleCombinations of elements tend to form so that each atom, by gaining, losing, or sharing electrons, has an octet of electrons in its highest occupied energy level. Diatomic Fluorine

Comments About the Octet RuleComments About the Octet Rule 2nd row elements C, N, O, F observe the

octet rule. 2nd row elements B and Be often have

fewer than 8 electrons around themselves - they are very reactive.

3rd row and heavier elements CAN exceed the octet rule using empty valence d orbitals.

When writing Lewis structures, satisfy octets first, then place electrons around elements having available d orbitals.

  Shows how valence electrons are Shows how valence electrons are arranged among atoms in a molecule. arranged among atoms in a molecule.   Reflects central idea that stability of Reflects central idea that stability of a compound relates to noble gas a compound relates to noble gas electron configuration.electron configuration.

Lewis StructuresLewis Structures

ResonanceResonance  Resonance is invoked when more than one valid Lewis structure can be written for a particular molecule.

  The actual structure is an average of the resonance structures.

H

H

H

H

HH

H

H

H

H

H

HBenzene, C6H6

  The bond lengths in the ring are identical, and between those of single and double bonds.

Resonance Bond Length and Bond EnergyResonance Bond Length and Bond Energy

 Resonance bonds are shorter and stronger than single bonds.

  Resonance bonds are longer and weaker than double bonds.

H

H

H

H

HH

H

H

H

H

H

H

Resonance in a carbonate ion:

Resonance in an acetate ion:

Resonance in Polyatomic Ions

Localized Electron ModelLocalized Electron ModelLewis structures are an application of the “Localized Electron Model”

L.E.M. says: Electron pairs can be thought of as “belonging” to pairs of atoms when bondingResonance points out a weakness in the Localized Electron Model.

VSEPR – Valence Shell Electron Pair VSEPR – Valence Shell Electron Pair Repulsion RepulsionX + EX + E Overall StructureOverall Structure FormsForms

2 Linear AX2

3 Trigonal Planar AX3, AX2E4 Tetrahedral AX4, AX3E, AX2E2

5 Trigonal bipyramidal AX5, AX4E, AX3E2, AX2E3

6 Octahedral AX6, AX5E, AX4E2

A = central atomA = central atomX =X = atoms bonded to Aatoms bonded to A

E = nonbonding electron pairs on AE = nonbonding electron pairs on A

VSEPR: LinearVSEPR: Linear

AXAX22 COCO22

VSEPR: Trigonal PlanarVSEPR: Trigonal Planar

AXAX33

AXAX22EE

BFBF33

SnClSnCl22

VSEPR: TetrahedralVSEPR: Tetrahedral

AXAX44

AXAX33EE

AXAX22EE22

CClCCl44

PClPCl33

ClCl22OO

VSEPR: Trigonal Bi-pyramidalVSEPR: Trigonal Bi-pyramidal

AXAX55

AXAX44EE

AXAX33EE22

AXAX22EE33

PClPCl55

SFSF44

ClFClF33

II33--

VSEPR: OctahedralVSEPR: Octahedral

AXAX66

AXAX55EE

AXAX44EE22

SFSF66

IClICl44--

BrFBrF55