Chapter 2:Chapter 2: Atomic Structure and Interactive Bonding Atomic Structure and Interactive Bonding

Why Study Atomic Structure and Interactive Why Study Atomic Structure and Interactive Bonding?Bonding?

An important reason to have an understanding of An important reason to have an understanding of inter-atomic bonding in solids is that, in some inter-atomic bonding in solids is that, in some instances, the type of bond allows us to explain a instances, the type of bond allows us to explain a material’s properties. material’s properties.

For example, consider carbon, which may exist as For example, consider carbon, which may exist as both graphite and diamond. Whereas graphite is both graphite and diamond. Whereas graphite is relatively soft and has a greasy feel to it, relatively soft and has a greasy feel to it, diamond is the hardest known material. This diamond is the hardest known material. This dramatic disparity in properties is directly dramatic disparity in properties is directly attributable to a type of interatomic bonding attributable to a type of interatomic bonding found in graphite that does not exist in diamond.found in graphite that does not exist in diamond.

2.4 Periodic Table2.4 Periodic Table All the elements have been classified All the elements have been classified

according to electron configuration.according to electron configuration. Elements are situated with increasing Elements are situated with increasing

atomic number.atomic number. Seven horizontal rows are called Seven horizontal rows are called

periods.periods. All elements that are arrayed in a All elements that are arrayed in a

given column or group have similar given column or group have similar valence electron structures, as well valence electron structures, as well as chemical and physical properties.as chemical and physical properties.

2.4 Periodic Table (Contd.)2.4 Periodic Table (Contd.)

2.4 Periodic Table (Contd.)2.4 Periodic Table (Contd.) Electropositive elements Electropositive elements capable of giving up capable of giving up

their few valence electrons to become positively their few valence electrons to become positively charged ions. charged ions.

Electronegative elements Electronegative elements readily accept readily accept electrons to form negatively charged ions, or electrons to form negatively charged ions, or sometimes they share electrons with other sometimes they share electrons with other atoms.atoms.

Figure 2.7 displays electronegativity values Figure 2.7 displays electronegativity values assigned to various elements.assigned to various elements.

As a general rule, electronegativity increases in As a general rule, electronegativity increases in moving from left to right and from bottom to top. moving from left to right and from bottom to top.

2.4 Periodic Table (Contd.)2.4 Periodic Table (Contd.)

Ch. 2: ATOMIC BONDING IN SOLIDSCh. 2: ATOMIC BONDING IN SOLIDS While there are only 118 or so While there are only 118 or so elementselements listed on the listed on the

periodic table, there are obviously more substances in periodic table, there are obviously more substances in nature than 118 pure elements.  This is because nature than 118 pure elements.  This is because atomsatoms can can reactreact with one another to form new substances called with one another to form new substances called compoundscompounds.  When two or more atoms chemically bond .  When two or more atoms chemically bond together, the resulting compound is unique both chemically together, the resulting compound is unique both chemically and physically from its parent atoms. and physically from its parent atoms. 

Let's look at an example.  Let's look at an example. 

The element sodium is a silver-colored metal that reacts so The element sodium is a silver-colored metal that reacts so violently with water that flames are produced when sodium violently with water that flames are produced when sodium gets wet.  The element chlorine is a greenish-colored gas gets wet.  The element chlorine is a greenish-colored gas that is so poisonous that it was used as a weapon in World that is so poisonous that it was used as a weapon in World War I.  When chemically bonded together, these two War I.  When chemically bonded together, these two dangerous substances form the compound sodium chloride, dangerous substances form the compound sodium chloride, a compound so safe that we eat it every day - common a compound so safe that we eat it every day - common table salt!table salt!

NaClNaCl

+ 

Bonding Forces and EnergiesBonding Forces and Energies Consider two isolated atoms:Consider two isolated atoms:

• When the atoms are at large inter-atomicWhen the atoms are at large inter-atomicseparation distance, the atoms do not exert any force on each separation distance, the atoms do not exert any force on each other.other.

• When the distance is decreased, an attractive force FWhen the distance is decreased, an attractive force FAA starts to starts to actact pulling atoms closer. pulling atoms closer.

• FFAA increases as the atoms gets closer. increases as the atoms gets closer.• But as the atoms get closer a repulsive force FBut as the atoms get closer a repulsive force FRR begin to act. begin to act.• The net force FThe net force FNN between the two atoms is given by: between the two atoms is given by:

FFN N = F= FAA + F + FRR

• At some inter-atomic distance rAt some inter-atomic distance ro, o, FFRR exactly equals F exactly equals FA A and Fand FNN becomes Zerobecomes Zero FFN N = 0 = F= 0 = FAA + F + FRR

• rroo is called the is called the equilibrium inter-atomicequilibrium inter-atomic separation distance at separation distance at which atoms enter into bondingwhich atoms enter into bonding

rroo ≈ 0.3 nm ≈ 0.3 nm

Force vs. Separation DistanceForce vs. Separation Distance

Energy vs. Separation DistanceEnergy vs. Separation Distance

2.5 Bonding Forces and Energies2.5 Bonding Forces and Energies (Contd.) (Contd.)

Sometimes it is more convenient to work with the Sometimes it is more convenient to work with the potential energiespotential energies between two atoms instead of forces.Mathematically, energy (E) and force between two atoms instead of forces.Mathematically, energy (E) and force (F) are related as(F) are related as

E = E = F dr F dr

For atomic systems,For atomic systems,

EENN, E, EAA, and E, and ER : R : the net, attractive, and repulsive the net, attractive, and repulsive

energies for two isolated and adjacent atoms.energies for two isolated and adjacent atoms.

RA

r

R

r

A

r

NN

EE

drFdrF

drFE

2.5 Bonding Forces and Energies2.5 Bonding Forces and Energies (Contd.) (Contd.) Figure 2.8b plots potential energies.Figure 2.8b plots potential energies.

The net curve, which is sum of attractive and repulsive The net curve, which is sum of attractive and repulsive energies, has a potential energy well around its minimum.energies, has a potential energy well around its minimum.

EE00 Bonding energyBonding energy at minimum point, energy required at minimum point, energy required to separate these two atoms to an infinite separation.to separate these two atoms to an infinite separation.

A similar yet complex condition exists for solid materials A similar yet complex condition exists for solid materials because force and energy interactions among many because force and energy interactions among many atoms must be considered. atoms must be considered.

Nevertheless, a bonding energy, analogous to ENevertheless, a bonding energy, analogous to E00 above, above, may be associated with each atom. may be associated with each atom.

2.5 Bonding Forces and Energies2.5 Bonding Forces and Energies (Contd.) (Contd.) A number of material properties depend on EA number of material properties depend on E00, the curve , the curve

shape, and bonding type. For example,shape, and bonding type. For example,

• Materials having large bonding energies typically also Materials having large bonding energies typically also have high melting temperature.have high melting temperature.

• At room temperature, solids formed for large bonding At room temperature, solids formed for large bonding energies, whereas for small energies the gaseous state is energies, whereas for small energies the gaseous state is favored, liquids prevail when energies are of favored, liquids prevail when energies are of intermediate magnitude.intermediate magnitude.

• The mechanical stiffness (modulus of elasticity) is The mechanical stiffness (modulus of elasticity) is dependent on the shape of the force-versus-interatomic dependent on the shape of the force-versus-interatomic separation curve.separation curve.

• The coefficient of thermal expansion (how much expands The coefficient of thermal expansion (how much expands or contracts per degree change in temperature) is or contracts per degree change in temperature) is related to the shape of its Erelated to the shape of its E00-versus-r-versus-r00 curve. curve.

Types of Atomic & Molecular BondsTypes of Atomic & Molecular Bonds

Primary Atomic BondsPrimary Atomic Bonds• Ionic BondsIonic Bonds• Covalent BondsCovalent Bonds• Metallic BondsMetallic Bonds

Secondary Atomic & Molecular BondsSecondary Atomic & Molecular Bonds• Permanent Dipole BondsPermanent Dipole Bonds• Fluctuating Dipole BondsFluctuating Dipole Bonds

Large inter-atomic forces are created Large inter-atomic forces are created by the “by the “coulombiccoulombic” effect produced ” effect produced by positively and negatively charged by positively and negatively charged ions. ions.

Ionic bonds are “Ionic bonds are “non-directionalnon-directional”.”. The “The “cationcation” has a ” has a ++ charge & the charge & the

““anionanion” has the ” has the -- charge. charge. The The cationcation is much smaller than the is much smaller than the anionanion..

Ionic BondingIonic Bonding

IONIC BONDING IN NaClIONIC BONDING IN NaCl

NaClNaCl

IONIC BONDINGIONIC BONDING The attractive energy EThe attractive energy EAA is a function of inter-atomic is a function of inter-atomic

separation distance.separation distance.

EEAA = -A/r = -A/r

Where Where

A = (zA = (z11e)(ze)(z22e)/4e)/400

zz11 and z and z2 2 are the valance of the two ions. are the valance of the two ions.

e is the electron charge (1.6 x 10e is the electron charge (1.6 x 10-19-19 C) C)

oo is the permittivity of vacuum (8.85 x 10 is the permittivity of vacuum (8.85 x 10-12 -12 F/m)F/m)

Repulsive energy for two isolated ions:Repulsive energy for two isolated ions: EERR = B / = B / (r(rnn))

A, B, and n are constants and depends on ionic A, B, and n are constants and depends on ionic system, n is approximately 8.system, n is approximately 8.

Energy and Force relationshipEnergy and Force relationship

Energy is related to force as:Energy is related to force as:

EE = = ∫∫F drF drand thusand thus

F = dE/drF = dE/dr

Useful EquationsUseful Equations

EEAA = -A/r = -A/r

FFAA = dE/dr = -A [d/dr(1/r)] = dE/dr = -A [d/dr(1/r)]

FFAA = A/r = A/r22

Since A = (zSince A = (z11e)(ze)(z22e)/4e)/4oo

FFA A = (z= (z11e)(ze)(z22e)/4e)/4oorr22

oror

r = r = √ √ (z(z11e)(ze)(z22e)/e)/44ooFFAA

Large inter-atomic forces are created Large inter-atomic forces are created by the sharing of electrons to form by the sharing of electrons to form directional bonds.directional bonds.

The atoms have small differences in The atoms have small differences in electronegativity & close to each electronegativity & close to each other in the periodic table.other in the periodic table.

The atoms share their outer The atoms share their outer ss and and pp electrons so that each atom attains electrons so that each atom attains the noble-gas electron configuration. the noble-gas electron configuration.

Covalent BondingCovalent Bonding

HH22 Molecule Molecule

Primary Interatomic Bonding (Contd.)Primary Interatomic Bonding (Contd.) ExamplesExamples of covalent bonding: of covalent bonding:

Nonmetallic elemental molecules (HNonmetallic elemental molecules (H22, Cl, Cl22, F, F22, etc.), etc.)

Dissimilar atoms ( CHDissimilar atoms ( CH44, H, H22O, HNOO, HNO33, and HF ), and HF )

Elemental solids ( Diamond (Carbon), silicon, Elemental solids ( Diamond (Carbon), silicon, germanium )germanium )

Number of covalent bondNumber of covalent bond = { 8 – (No. of valence = { 8 – (No. of valence electrons) }electrons) }

Chlorine :Chlorine :8-7=18-7=1 can bond to only can bond to only oneone other atom. other atom.

Carbon:Carbon: 8-4=48-4=4 can bond to can bond to fourfour other atoms. other atoms.

Covalent bonds may be very strong (diamond) or Covalent bonds may be very strong (diamond) or very weak (bismuth). See Table 2.3.very weak (bismuth). See Table 2.3.

Primary Interatomic Bonding (Contd.)Primary Interatomic Bonding (Contd.) It is possible to have interatomic bonds that are It is possible to have interatomic bonds that are partially partially

ionic and partially covalentionic and partially covalent, and, in fact, very few , and, in fact, very few compounds exhibit pure ionic or covalent bonding.compounds exhibit pure ionic or covalent bonding.

For a compound, the degree of either bond type depends For a compound, the degree of either bond type depends on the relative positions of the constituent atoms in the on the relative positions of the constituent atoms in the periodic table (Figure 2.6) or the difference in their periodic table (Figure 2.6) or the difference in their electronegativitieselectronegativities (Figure 2.7). (Figure 2.7).

The The percent ionic characterpercent ionic character of a bond between elements of a bond between elements A and B (A being the most electronegative) may be A and B (A being the most electronegative) may be approximated by the expression:approximated by the expression:

% ionic character = {1 – exp[-(0.25)(X% ionic character = {1 – exp[-(0.25)(XAA – X – XBB)2]} x 100)2]} x 100

Where XWhere XAA and X and XBB are the electronegativities for the respective are the electronegativities for the respective elements.elements.

Large inter-atomic forces are created Large inter-atomic forces are created by the sharing of electrons in a by the sharing of electrons in a delocalized manner to form strong delocalized manner to form strong non-directionalnon-directional bonding. bonding.

Metallic BondingMetallic Bonding

Cloud of electrons surrounding the positively charged Cloud of electrons surrounding the positively charged corescores

Secondary Atomic & Molecular Bonds [Van Secondary Atomic & Molecular Bonds [Van der Waals Bonds]der Waals Bonds]

Physical Bonds (no electron involvement).Physical Bonds (no electron involvement).

Weak electrostatic bonds.Weak electrostatic bonds.

Bonding occurs between atomic or molecular Bonding occurs between atomic or molecular electric dipoles.electric dipoles.

Bonding Energies are low. The bondsBonding Energies are low. The bonds are are relatively weak.relatively weak.

Fluctuating Induced Dipole BondsFluctuating Induced Dipole Bonds

• Weak electric dipole bonding can take place among Weak electric dipole bonding can take place among atoms due to an instantaneous asymmetrical atoms due to an instantaneous asymmetrical distribution of electron densities around their nuclei.distribution of electron densities around their nuclei.

• This type of bonding is termed fluctuation since the This type of bonding is termed fluctuation since the electron density is continuously changing.electron density is continuously changing.

• Example: Bonding between Argon atomsExample: Bonding between Argon atoms

Polar Molecule-Induced Dipole Bonds Polar Molecule-Induced Dipole Bonds

• Weak intermolecular bonds are formed between atoms Weak intermolecular bonds are formed between atoms or molecules which possess permanent dipoles.or molecules which possess permanent dipoles.

• A dipole exists in a molecule if there is asymmetry in its A dipole exists in a molecule if there is asymmetry in its electron density distribution.electron density distribution.

Secondary Atomic & Molecular Bonds Secondary Atomic & Molecular Bonds [Van der Waals Bonds][Van der Waals Bonds]

Fluctuating Dipole BondsFluctuating Dipole Bonds

Bonding Between Argon electric dipolesBonding Between Argon electric dipoles

Argon atom Argon Electric Diploe

- +

≈≈

Van der waals bond

Polar Molecule-Induced Dipole BondsPolar Molecule-Induced Dipole Bonds Permanent dipole exist in some molecules.Permanent dipole exist in some molecules. Such molecules are called Polar Molecules.Such molecules are called Polar Molecules. Polar Molecules can induce dipoles in Polar Molecules can induce dipoles in

adjacent non-polar molecules and bonding adjacent non-polar molecules and bonding can take place.can take place.

Example: HCl moleculeExample: HCl molecule

ClH

+ -

Permanent Dipole BondsPermanent Dipole Bonds Van der waal bonds also occur between Van der waal bonds also occur between

permanent polar molecules.permanent polar molecules. The bonding energies are higher than the The bonding energies are higher than the

fluctuating induced or polar molecule fluctuating induced or polar molecule induced bonds.induced bonds.

The strongest Secondary Bonding is The strongest Secondary Bonding is Hydrogen Bond.Hydrogen Bond.

Examples of Hydrogen Bonding:Examples of Hydrogen Bonding: HF, HHF, H22O, NHO, NH33