ch 02 - atomic structure & bonding

8/10/2019 Ch 02 - Atomic Structure & Bonding

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CHAPTER

Atomic Structure

Bonding

2-1


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Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Structure of Atoms

ATOMBasic Unit of an Element

10

Neutrally Charged

Diameter : 10 14 m

Accounts for almost all mass

Positive Charge

Mass : 9.109 x 10 28 g

Charge : -1.602 x10 9

CAccounts for all volume

ProtonMass : 1.673 x 10 24 g

NeutronMass : 1.675 x 10 24 g

2-2

arge : . x eu ra arge


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Atomic Number and Atomic Mass

Atomic Number = Number of Protons in the nucleus

Unique to an element Example :- Hydrogen = 1, Uranium = 92

Relative atomic mass = Mass in grams of 6.203 x 1023

Example :- Carbon has 6 Protons and 6 Neutrons. Atomic Mass

= 12.

atom.

One gram mole = Gram atomic mass of an element. Example :-

One gram

12 Grams

6.023 x 1023

Mole of

Carbon

Of CarbonCar on

Atoms2-3


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Periodic Table

Source: Davis, M. and Davis, R., Fundamentals of Chemical Reaction Engineering, McGraw-Hill, 2003.

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Electron Structure of Atoms

Electron rotates at definite energy levels.

Ener is absorbed to move to hi her ener level.

Energy is emitted during transition to lower level.

Energy change due to transition = E =hc

h=Planks Constant

= 6.63 x 10-34 J.s

EmitAbsorb

c= Speed of light

= Wavelength of light

Energy

(Photon)

nergy

(Photon)

Energy levels

2-6

Photon = Electromagnetic radiation


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Quantum Numbers of Electrons of Atoms

Principal Quantum

Number (n)

Subsidiary Quantum

Number l

Represents main energylevels.

Represents sub energylevels (orbital).

Range 1 to 7.

Larger the n higher

Range 0n-1.

Represented by letters, , .

n=2

s orbital

(l = 0)

n=1

=n=1

p Orbital

(l =1)n=3

2-8


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Electron Structure of Multielectron Atom

Maximum number of electrons in each atomic shell is

given by 2n2.

Atomic size (radius) increases with addition of shells.

Electron Configuration lists the arrangement of electronsin orbitals.

Example :-Orbital letters

Number of Electrons

1s2 2s2 2p6 3s2

=

Principal Quantum Numbers

1s2 2s2 sp6 3s2 3p6 3d6 4s2

2-10

Z = atomic no. = no. of proton


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Electron Structure and Chemical Activity

Except Helium, mostnoble gasses (Ne, Ar, Kr, Xe, Rn)are chemically very stable

All have s

2

p6

configuration for outermost shell. Helium has 1s2 configuration

Electropositive elements give electrons during

Cations are indicated by positive oxidation numbers

Example:-

e : s s sp s p sFe2+ : 1s2 2s2 sp6 3s2 3p6 3d6

Fe3+ : 1s2 2s2 sp6 3s2 3p6 3d5

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Electron Structure and Chemical Activity (Cont..)

Electronegative elements accept electrons duringchemical reaction.

Some elements behave as both electronegative andelectropositive.

Electronegat v ty s the degree to wh ch the atom

attracts electrons to itself

Measured on a scale of 0 to 4.1

Example :- Electronegativity of Fluorine is 4.1

Electronegativity of Sodium is 1.

0 1 2 3 4K

Na N O Fl

W

Te

SeH

Electro-

positive

Electro-

negative

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Atomic and Molecular Bonds

Ionic bonds :- Strong atomic bonds due to transfer of

- oms n on e s a e are n a more s a e energy con on compare o un on e

condition. Net decrease of potential energy after bonding.

electrons non-directional

Covalent bonds :- Large interactive force due to

-

Metallic bonds :- Non-directional bonds formed by

sharing of electrons

Permanent Dipole bonds :- Weak intermolecular

bonds due to attraction between the ends of permanent

. Fluctuating Dipole bonds :- Very weak electric dipole

bonds due to asymmetric distribution of electron

densities.

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Ionic Bonding

Ionic bonding is due to electrostatic force of attraction

between cations and anions.

elements.

Electrons are transferred from electropositive to

electronegative atoms

ElectropositiveElement ElectronegativeAtomElectronrans er

Electrostatic (coulombic)

Cation

+ve charge

Anion

-ve charge

Attraction

IONIC BOND

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Ionic Bonding - Example

Ionic bonding in NaCl

1

Atomic no= 17

3p7

Sodium

Atom

ChlorineAtom

=

Na

Chlorine Ion

O

N

I

Na+ -

C

B

N

D2-15

Figure 2.10


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Ionic Force for Ion Pair

Nucleus of one ion attracts electron of another ion.

The electron clouds of ion repulse each other when

(Proton + neutron)

they are sufficiently close.

Force versus separation

oppositely charged ions

Figure 2.11

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Ion Force for Ion Pair (Cont..)

( )( )

( ) ( )a

eZZ

a

ZZF

ee

attractive2

2

21

2

21

44

==

Z1,Z2 = Number of electrons removed oradded during ion formation

=

a = Interionic seperation distance

= Permeability of free space (8.85 x 10-12c2/Nm2)

(n and b are constants)Fn

repulsive

nb

1+

=

eZZF

nnet

nb

12

2

21

+

=

aa0

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Interionic Force - Example

Force of attraction between Na+ and Cl- ions

Z1 = +1 for Na+

, Z2 = -1 for Cl-

e = 1.60 x 10-19 C , 0 = 8.85 x 10-12 C2/Nm2

a0 = Sum of Radii of Na+ and Cl- ions

= 0.095 nm + 0.181 nm = 2.76 x 10-10

m

Na+ Cl-

CeZZ 92192

21)1060.1)(1)(1(

+

a0

( )aattraction

10-212-2

0

.

m)10x/Nm2)(2.76C10x8.85(44

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Ion Arrangements in Ionic Solids

Ionic bonds areNon Directional

Geometric arrangements are present in solids to

maintain electric neutrality.

Example:- in NaCl, six Cl- ions pack around central Na+ Ions

Ionic packing

In NaCl

and CsCl

Figure 2.13

As the ratio of cation to anion radius decreases, fewer

CsCl NaCl

anion surround central cation.

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Bonding Energies

Lattice energies and melting points of ionically

bonded solids are high.

(Measure of bonding strength)

increases (because bonding electrons in larger ions are farther away fromthe attractive influence of the +ve nucleus).

.

Example :-NaCl Lattice energy = 766 KJ/mol

Melting point = 801oC

CsCl Lattice energy = 649 KJ/mol

Melting Point = 646oCBaO Lattice energy = 3127 KJ/mol

Melting point = 1923oC

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Covalent Bonding

In Covalent bonding, outer s and p electrons are

shared between two atoms to obtain noble gas

con gura on.

Takes place between elements

electronegativity and close by

in eriodic table.

In Hydrogen, a bond is formed between 2 atoms by

sharing their 1s1 electronsElectron Overlapping Electron Clouds

H + H H HPair

H H

1s1

Electrons

Hydrogen

Molecule2-22


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Covalent Bonding - Examples

In case of F2, O2 and N2, covalent bonding is formed bysharing p electrons

uor ne gas u er or a s p s are one p e ec ron o

attain noble gas configuration.

F + F F FH

F FBond Energy=160KJ/mol

Oxygen (Outer orbital - 2s2 2p4) atoms share two p electrons

O + O O O O = O

Nitrogen (Outer orbital - 2s2 2p3) atoms share three p electrons

Bond Energy=28KJ/mol

H H N + N Bond Energy=54KJ/molN N N N

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Covalent Bonding in Carbon

Carbon has electronic configuration 1s2 2s2 2p2

roun tate arrangement Ind cates

carbon

Forms two1s 2s 2p

Two filed 2p orbitals

Covalent

bonds

Hybridization causes one of the 2s orbitals promoted to

2p orbital. Result four sp3 orbitals.

Indicatesfour covalent

bonds ares 2p

Four filled sp3 orbitals

formed

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Covalent Bonding in Benzene

Chemical composition of Benzene is C6H6.

The Carbon atoms are arranged in hexagonal ring.

Single and double bonds alternate between the atoms.H

CH H

C C

CH

H

Structure of Benzene Simplified Notations

2-27

.


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Metallic Bonding

Atoms in metals are closely packed in crystalstructure.

towards nucleus of other atoms. Electrons spread out among atoms forming electron

clouds.

These free electrons arePositive Ion

conductivity and ductility*

Since outer electrons are

shared by many atoms,metallic bonds are

-*a mechanical property used to describe the extent to which materials can be deformed plastically without

fracture Valence electron charge cloud2-28


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Metallic Bonds (Cont..)

Overall energy of individual atoms are lowered by

metallic bonds (i.e. from unstable atoms to stable bonding).

Minimum energy between atoms exist at equilibrium

distance a0

,

metallic the bond is (i.e. valence electrons are freer to move).

Example:- Na Bonding energy 108KJ/mol,Melting temperature 97.7oC

Higher the number of valence electrons involved,

.

Example:- Ca Bonding energy 177KJ/mol,

Melting temperature 851oC

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Secondary Bonding

Secondary bonds are due to attractions of electricdipoles in atoms or molecules.

centers exist.

-

Dipole moment= =q.d

nucleus

q= ec r c c arge

d = separation distance

There two types of bonds permanent and.

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Fluctuating Dipoles

Weak secondary bonds in noble gasses.

Dipoles are created due to asymmetrical distribution of

They have complete outer-

valence-electron shells

electron charges.

Electron cloud charge changes with time.

nucleus

+_

Symmetrical

AsymmetricalFigure 2.27

of electron charge (Changes with time)

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Permanent Dipoles

Dipoles that do not fluctuate with time are

called Permanent dipoles (weak bonding forces among covalently.

Examples:-

Arrangement

Of 4 C-H bondsCH4

moment

(Methane)

Asymmetrical

TetrahedralCreates

arrangementpo e

(Chloromethane)

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Hydrogen Bonds

Hydrogen bonds areDipole-Dipole interaction

between polar bonds containing hydrogen

spec a case o permanen po e- po e n erac on e w. po ar mo ecu es

atom. Example :-

In water, dipole is created due to asymmetrical

arrangement of hydrogen atoms.

Attraction between positive oxygen pole andnegative hydrogen pole.

H

105 0O

H dro enFigure 2.28

H Bond

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ch 02 - atomic structure & bonding

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