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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
91
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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
Prof. M.L. Weaver
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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.