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T esT esT esT es Covalent

Ionic

Metallic

Prof. M.L. Weaver

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Examples of Covalent BondingExamples of Covalent Bonding

H2OH2 I

VA

He-

NeFLi

H2.1

BeSiC

C(diamond)

C

Cl2O

colu

m

-

Ar-

Kr-

4.0

Cl3.0

Br2.8

1.0

Na0.9

K0.8

1.5

Mg1.2

Ca1.0

Ti1.5

Cr1.6

Fe1.8

Ni1.8

Zn1.8

As2.0

2.5

Si1.8

Ga1.6

Ge1.8

2.0

Xe-

Rn-

I

2.5

At

2.2

Rb

0.8

Cs0.7

Sr1.0

Ba

0.9

Sn1.8

Pb1.8

Molecules with nonmetals, e.g. Cl2, F2, O2

0.7

0.9 a sAdapted from Fig. 2.7, Callister 6e. (Fig. 2.7 isadapted from Linus Pauling, The Nature of the Chemical Bond, 3rd edition, Copyright1939 and 1940, 3rd edition. Copyright 1960 by Cornell University.

Chapter 2 -

Molecules with metals and nonmetals

Elemental solids (RHS of Periodic Table) Compound solids (about column IVA)

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Simple Bonding ModelsSimple Bonding Models Covalent (continued)Covalent (continued)

The molecular solid has covalent

bonds (dark lines) only within

individual molecules. Thus, there is

no covalently bonded path between

the atom labeled 1 and the atom

There is a covalently bonded path

between an two atoms.

-

labeled 2; the molecules are bonded

to one another only by weaksecondary forces.

,SiC, BN, etc.

All atoms are linked by covalent bonds, i.e.,

there is a covalently bonded path between any

crystalline materials C60, H2O, and

macromolecular solids polyethylene.

Atoms within each molecule are linked by

weak interactions known collectively as intermolecular forces orsecondary bonds (including van

der Waals VDW , di olar, and h dro en bond .

a oms n e so . cova ent on s, u e mo ecu es a ma e up

the crystal are held together only by the

In such solids, not all atoms are connected by a path of strong covalent bonds.

Rule of Thumb: if more than two thirds of the components in a covalently bonded compound are

Prof. M.L. Weaver

, , , , , .

However, diamond is a noteworthy example illustrating that this guideline should be applied

with caution.

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Covalent Bonding Model (continued)Covalent Bonding Model (continued)Simple model assumes that electrons are shared

between atoms and that electron charge density

accumulates between relatively positive atomic cores.

Defining characteristic of a covalent bond is the

existence of a local maximum in the valence electron

density in the regions between the atomic cores. For

Fig.Valenceelectrondensitymapinthe{110}planeofSi.Contoursareat0.1e/3.The

shape

of

the

peaks

are

theoretically

redicted and also found in man IIIV

example, experimentally measured charged density in

Si is shown:

The eak in electron densit at the mid ointsemiconductors,e.g.GaAs.connecting the two Si nuclei is signature of the

covalent bond.

between the atomic cores is clearly distinct from the

ionic bonding model, where the valence electrons are

centered on the anion ositions and the metallic

Prof. M.L. Weaver

bonding modelwhere the valence electrons are

uniformly distributed in the free electron sea.

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Ionic bondIonic bond metalmetal + nonmetal+ nonmetalIonic bondIonic bond metalmetal + nonmetal+ nonmetal

donates accepts

electrons electrons

ss m ar e ec ronega v es

ex: M O M 1s2 2s2 2 6 3s2 O 1s2 2s2 2 4

[Ne] 3s2

Mg2+ 1s2 2s2 2p6 O2 1s2 2s2 2p6

[Ne] [Ne]

Prof. M.L. Weaver

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Ionic BondingIonic BondingIonic BondingIonic Bonding Occursbetween+and ions.

Requireselectrontransfer.

arge erence ne ec ronega v yrequ re .

Example:

NaCl

Na(metal)

unstable

Cl(nonmetal)

unstableelectron

-CoulombicAttraction

stable an onstable

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Ionic BondingIonic BondingIonic BondingIonic Bonding Energy minimum energy most stable

r

Anr

BEN =EA +ER =

RepulsiveenergyER

Interatomicseparationr

N

AdaptedfromFig.2.8(b),Callister&Rethwisch8e.

Prof. M.L. Weaver

93

AttractiveenergyEA

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Ionic Bonding Model (continued)Ionic Bonding Model (continued)

Calculate force of attraction (FA) between Ca2+ and O2- ions with their centers separated by1.25 nm

salts and ceramicsare ionically bound.

Non-directional, Na+ will

Na: 1s2

2s2

2p6

3s1

Na+

:give up 1 e-Cl: 1s22s22p63s23p5Cl-:picks it up

attract any adjacent Cl-

equally in all directions.

Ionic bonding occurswhen >0.5 lar e EN

This transferresults in a long-range

coulombic attraction between oppositely

charged ions.

difference (far L and R

columns on Periodic table.) Why is melting point of CaF2 > CaCl2 > CaBr2?

The lattice energy, similar to bond energy, is the energy required to separate all of the ions(cation and anion) in a crystal to infinity. Thus, its a measure of the crystals bond strength.

Why would LiCl (ro=2.57) and SrO (ro=2.58) have approximately the same interionic

Prof. M.L. Weaver

spacing and the same crystal structure (rocksalt), but have different lattice energies ofE

o=9eV and E

o=33eV, respectively?

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Relationship between Melting Point andRelationship between Melting Point and

13.3. The melting points of AB ionic crystals increase withz2/d. 13.4. The correlation betweenz2/dand the elastic modulus C44, forvarious AB com ounds with a NaCl structure. Data from J.J.

Prof. M.L. Weaver

FromMaterialsScience anIntermediateTextbyWilliamF.Hosford

Gilman,Progress in Ceramic Science 1 (1961) 146-194.

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Examples of Ionic BondingExamples of Ionic Bonding

Predominant bonding in CeramicsNaCl

He-

NeFLi

H2.1

Be

g

CaF2O

-

Ar-

Kr-

.

Cl

3.0

Br2.8

.

Na0.9

K0.8

.

Mg

1.2

Ca1.0 Ti1.5 Cr1.6 Fe1.8 Ni1.8 Zn1.8 As2.0

.

Xe-

Rn

-

I2.5

At2.2

Rb0.8

Cs0.7

Sr1.0

Ba0.9

0.7

0.9

Chapter 2 -

Adapted from Linus Pauling, The Nature of the Chemical Bond, 3rd edition, Copyright 1939 and

1940, 3rd edition. Copyright 1960 by Cornell University.

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Take Ionic Bonding Model a Step FurtherTake Ionic Bonding Model a Step Further

and A l it to Cr stal Structuresand A l it to Cr stal StructuresCompute the total electrostatic contribution to the lattice energy,E. Sum both the attractive andrepulsive interactions between all of the ions of nearest neighbor distance (ro):

where is the Madelun constant, which is a relationshi of the distance of

the ions from one another due to a specific type of crystal.It depends on the

geometric arrangement of the constituent ions in the crystal structure. See

class handout and Table 7.9 in Rohrer for values.

o

A

r

keME

2M(1)

or nary s ruc ures, s common o use a re uce a e ung cons an , :

where n1 and n2 are stoichiometry of cation and anion. For NaCl, n1=n2=Z1=Z2=1

The reduced Madelung constant leads to a convenient expression for the total electrostatic

energy which separates the chemical parameters such as charge (Z), stoichiometry (n) and ionic

(2)

distance/sizes (ro) from the structural information in the Madelung

constant:o

A

r

ZZnnkeE

2

)( 21212

(3). . , , . .

CsCl > NaCl > ZnS. Also, compounds with layered structures (more directional bonding),

e.g. CdCl2 and V2O5 have lowers which implies the electrostatic

contribution to the bonding is diminished while the covalent

Prof. M.L. Weaver

contribution is increased.

We still need to include the repulsive energy contribution, we will return to this later since we

need to discuss the Lennard-Jones portion of the energy.

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MetallicBondsMetallicBonds Valenceatomsinelectrons

behaveasadelocalizedseaofelectrons

Thebehavior,illustratedin,allowsforhighconductivity

Aslightlymoredetailedexplanationcanbefoundonthenext a e.

ElectronSea

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Metallic - Metallic materials have one two or at most

Metallic Bonding ModelMetallic Bonding Model

three valence electrons.

With this scheme, these electrons are not bound to any

articular atom in the solid and are more or less free to drift

throughout the entire metal sharing electrons.

The remaining non-valence electrons and atomic nuclei

form ion cores which ossess a net ositive char e e ual

in magnitude to total valence electron charge per atom.

The ion cores arranged periodically are shielded from one

another, and also " lued" to ether b the sea of valence

(free) electrons or electron clouds.

In other words, the free electrons shield the positively

char ed ion cores from mutuall re ulsive electrostatic

METALLIC

forces, which they would otherwise exert upon one other,thus metallic bond is non-directional.

Due to lar e number of freel movin electrons, metals are

Prof. M.L. Weaver

good thermal (conduction of heat by free electrons) and

electrical conductors.