lecture2 mz

8/10/2019 Lecture2 MZ

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Fundamental conceptsElectrons in atomsPeriodic tableBonding forces and energies

Chapter 2: Atomic structure and interatomic bonding

Fundamental concepts Proton and electron, charged

1.60 x10 -19 C Mass of electron 9.11x10 -31 kg Mass of protons and neutrons

1.67 x 10 -27 kg

Atomic number: the number of protons

Atomic mass =protons+neutrons Isotope Atomic mass unit(amu): 1amu=1/12 C One mole = 6.023x10 23 atoms(Avogadros)

Quarks Gluons Neutron Nucleus

ElectronProtons

Chapter 2: Atomic structure and interatomic bonding


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Electrons in atoms

Atomic models Bohr atomic electrons revolve around the atomic

nucleus in discrete orbital and theenergies of electrons are quantized

Wave-mechanical Electron exhibits both wavelike and

particle-like characteristics, its positionis considered to be a probabilitydistribution

Electrons in atoms ( continue )

Comparison of the (a) Bohrand (b) wave-mechanical atommodels

In terms of electron distribution


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Electron energy states

Bohr hydrogen model Wave-mechanical hydrogen

Principle quantum number n=1, 2 ..; K, L, M, N, OOrbital quantum number l=0, ..n-1; subshell, s, p, d, orf; the shape of the electron subshellSpin moment ms 1/2 or -1/2

Quantum numbers


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Quantum numbers

The smaller n, thelower energyThe smaller l, the lowerenergyThere are someoverlaps in energy,especially for d and fstates

Relative energies of the electronsfor various shells and subshells

Electron configurations

Filled energy states for a sodiumatom

Energy minimum rulePauli exclusionHunds rule: as manyunpaired electrons aspossibleGround state

Valence electrons


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Electronic configurations

ex: Fe - atomic # = 26

valenceelectrons

Adapted from Fig. 2.4,Callister 7e.

1s

2s

2p

K-shell n = 1

L-shell n = 2

3s3p M-shell n = 3

3d

4s

4p4d

Energy

N-shell n = 4

1s2

2s2

2p6

3s2

3p6

3d6

4s2

The periodic table

Columns: Similar Valence Structure

Adapted fromFig. 2.6,Callister 7e.

Electropositive elements:Readily give up electronsto become + ions.

Electronegative elements:Readily acquire electronsto become - ions.

g i v

e u

p 1

e

g i v

e u

p 2

e

g i v

e u

p 3

e i n

e r

t g a s e s

a c c e p

t 1

e

a c c e p

t 2

e

O

Se

Te

Po At

I

Br

He

Ne

Ar

Kr

Xe

Rn

F

ClS

Li Be

H

Na Mg

BaCs

RaFr

CaK Sc

Sr Rb Y


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The periodic table

Ranges from 0.7 to 4.0 ,

Smaller electronegativity Larger electronegativity

Large values: tendency to acquire electrons.

The periodic table( continue )

Period: horizontal rowsGroup and column Same group, same valence electrons, similar properties Group 0, inert gas Group IA, IIA, 1 or 2 excess electrons from stable

structure Transition metals (IVB and IIB).

III A, IVA and VA, semiconductor

Electropositive and electronegativeElectronegativity


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Atomic bonding in solids

Bonding forces andenergies Fn=F A +F R E0 -- bonding energy large bonding E, high

melting point stiffness -- shape of

f-r curve thermal expansion --

E-r curve

Primary interatomic bonding: Ionic Bonding

Predominant bonding in Ceramics

Give up electrons Acquire electrons

NaClMgOCaF 2CsCl


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Ironic bonding

An ionic bond is formed whenan atom loses or gains one ormore electrons from its outershell

Between metallic andnonmetallic

Attractive bonding force--coulombic

Non-directional E A =-A/r, E R =B/r n , n~8

Primary interatomic bonding ( continue )

Primary interatomic bonds ( continue )

Covalent bonding

stable electron configurationsare assumed by sharingelectrons between adjacentatoms

directional many non-metallic elements % ionic character={1-exp[-

(0.25)(X A -XB)2]}x100

shared electronsfrom carbon atom

shared electrons

from hydrogenatoms

H

H

H

H

C

CH 4


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Primary interatomic bonds ( continue )

Metallic bonding sea of valence

electrons floating onion cores

non-directional metal and metal

alloys electrons are

balanced with ioncores in charge

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Bond length , r

Bond energy , E o

Melting Temperature , Tm

Tm is larger if E o is larger.

Properties From Bonding: Tm

r o r

Energyr

larger Tm

smaller Tm

E o =bond energy

Energy

r o r unstretched length


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Summary: Bonding

TypeIonic

Covalent

Metallic

Secondary

Bond EnergyLarge!

Variablelarge-Diamondsmall-Bismuth

Variablelarge-Tungstensmall-Mercury

smallest

CommentsNondirectional ( ceramics )

Directional(semiconductors , ceramicspolymer chains)

Nondirectional ( metals )

Directionalinter-chain ( polymer )inter-molecular

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Ceramics(Ionic & covalent bonding):

Metals(Metallic bonding):

Polymers(Covalent & Secondary):

Large bond energylarge Tmlarge Esmall

Variable bond energymoderate Tmmoderate Emoderate

Directional PropertiesSecondary bonding dominates

small Tmsmall Elarge

Summary: Primary Bonds

s e c o n d a r y b o n d i n g

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