ChE 42D Chapter 1 Ed_11!09!07_structure of Matls_basics

7/31/2019 ChE 42D Chapter 1 Ed_11!09!07_structure of Matls_basics

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Materials Science andEngineering

CHAPTER 1The Structure of Materials

Basic Concepts

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Materials Science

Involves theinvestigation of the

relation between

structure and

properties of

materials

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Materials Engineering

Involves designing or

engineering the structure of

materials on the basis of

material structure-propertyrelationship to produce a

predetermined set of

properties

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Structure

Relates to the arrangement of its internal

components

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Subatomic

Atomic

Microscopic

Macroscopic


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Property

A material trait in terms of the kind and

magnitude of response to a specificimposed stimulus

Definitions are made independent ofmaterial shape and size

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chemical mechanical

electrical magnetic

dimensional


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Four components of materials

science and engineering

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DesignProduction

Utilization


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Elements as building blocks for all

materialsMatter

Elements

Organic substances Inorganic substances

Liquids Solids Gases Liquids Solids Gases

Fuels

ChemicalsPaints

Oils

Beverages

Living organisms

PolymersNatural resins

Food

Soils

Composites

Fuels

Chemicals

Acids

WaterBases

Chemicals

Metals

CeramicsGlasses

Clays

Cements

Composites

Inert gases

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Bohrs Quantum Model of the

AtomOrbital electron

Nucleus

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Suggested in 1913

Electrons exist in stablecircular orbits of fixed

energy (quantized)

ENERGYSTATES/LEVEL

Electrons can only emit

or absorb energy whenmaking a transition

from one possible orbit

to another QUANTUMJUMP

Electron is a particle moving in a discreteorbital.


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Simple atomic structure model

KL

7e-2e-

K L

8e-2e-

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Wave-mechanical model

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Nucleus

Electron probability density

electrons position is

described by a probabilitydistribution (electron

cloud)

Bohr energy levelsseparate into electron

subshells

Defines the exact location of electrons in orbitals (energy level)

Electrons exhibit both wave-likeand particle-like characteristics


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Bohr and Wave-mechanical

models Comparison of

the (a) Bohr and(b) Wave-

mechanical atom

models in terms

of electronic

distribution

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(a) (b)


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Quantum Numbers

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K

LM

N

O

s s s s s 0p p p p 1

d d d 2

f f 3

g 4

Subshells

s: sphericalp: dumbbell-shaped

d: clover in four cases or dumbbell-shaped in one

Principal quantum

number (n): general energy

levels of the electrons (in

terms of distance from the

nucleus); n is + integer;

max. value: 2n2

Secondary quantum

numbers (l): (angular

momentum); defines theshape of the electron

subshell; values 0 to (n-1)


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Quantum Numbers

Third quantum number (ml):

(magnetic moment); the

orientation of an orbital; number

of energy states in a subshell;

values l to l including 0

Fourth quantum number (l):(spin momentum); motion (-

1/2down), (+1/2 up)

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s p d f1 3 5 7

A moving electron will produce a

magnetic field which is affected byexternal magnetic field

No. of energy states

In the absence of an external magnetic field, the states within each subshell

are identical.


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Number of available electron states

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1

3

57

s

p

df

N4

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s

pd

M3

1

3

s

p

L2

1sK1

Number of

electrons

Number of

states

SubshellsShell

designation

Principal

quantum

number, n

?


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Electron configuration

Pauli Exclusion Principle

No two electrons can have the same fourquantum numbers!!!!

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Each electron state can hold no more than two electrons,

which must have opposite signs.


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1 2 3 4 5 6 7

ps

ps

ps

d

psd

psdf

p

s

df

df

s

Principal quantum number, n

Energy

NOTES:

1) The smaller the principal quantum number, the lower the energy level.

2) Within each shell, the energy of a subshell level increases with the value of the (l) quantum number.3) There may be overlap in energy of a state in one shell with states in an adjacent shell.


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The manner by which states are

occupied by electrons Aluminum (13): 1s22s2p63s2p1

Chromium (24): 1s2

2s2

p6

3s2

p6

3d5

4s1

Krypton (36): 1s22s2p63s2p63d54s2p6

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Electrons fill up the lowest possible energy level first in the electron shells and

subshells, two electrons per state.


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GROUND STATE: all electrons occupy the

lowest possible energies in accordanceto Pauli exclusion principle.VALENCE ELECTRONS: electrons in the

outermost filled shell; participate ininteratomic bonding physical/chemicalproperties

STABLE ELECTRON CONFIGURATION:outermost shells are fully filled (s & pstates normally)

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The Periodic Table

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ElectronegativeElectropositive


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Electronegativity Values for Elements

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ELECTRONEGATIVITY the relative tendency of an atom to attract electrons


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Bonding

Secondary bonds

Primary bonds

Ionic Covalent Metallic

Permanent dipole

(also as H bond)London

forces Hydrogen

= attractive bond X O = shared electrons

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Types Bonding

Primary bonds strong atom-to-atom

attractions produced by changes in electronposition of outer (valence) electrons

Secondary bonds weaker than primary

bonds and are formed when atoms ormolecules are attracted by overall electric

fields (dipoles), which often result from the

transfer of electrons in the primary bonding

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Bond Strengths

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0.1-10

10-40

50-1000

200-1000

50-1000

Intermolecular forcesvan der Waals (London,

dipole-dipole) forces

Hydrogen bondingChemical bonding

Ionic

Covalent

Metallic bonding

Energy (kJ/mol)Type of bond


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Ionic bond

M

2e-

L

2e-

LKK

8e- 8e- (ionized)

0e- (ionized)

= attractive bondU


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electrons are transferred from one metal to the nonmetal,

creating ions that attract each other throughout the mass


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Covalent bond

2e-

L

K

8e-

2e-

LK

8e-

FF22U


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electrons are shared between atoms to produce a stable

group of eight


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Metallic bond

++ + +

+ + +

Mg2+ ion cores

Electron cloud from valence electrons

L

8e-

K

2e-

2e-

M

Magnesium atom

Sea of valence electrons

Ion cores

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the electrons are delocalized, or given up to form a

common sea of electrons surrounding the positive ions


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Metallic bond

Electron cloud valence electrons that are not

bound to any particular atom in the solid and aremore or less free to drift throughout the entire

metal; shield the ion cores from mutually

repulsive electrostatic forces and act as a gluethat hold ion cores together

Ion cores remaining non valence electrons

and atomic nuclei form which has a net positivecharge equal to that in magnitude to the total

valence electron charges per atomU


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Dipoles

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+ - + -

Atomic or molecular dipoles

Van der Waals bonding between two dipoles

Atomic nucleus

Electron cloud

Electrically

symmetric

atom

Induceddipole


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Dipole-dipole Forces

the arrangement of electrons and positive

nuclei results in a positively charged field atone end and a negatively charged field at the

other end; e.g., formation of a polar molecule

of HCl Also called polar molecule-induced dipole

bonds

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ClH

+ -


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London forces

attractions are formed when temporary

dipoles are developed due to the motionof electrons; e.g., attraction between

nonpolar molecules and single atoms of

inert gases

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Ne Ne

Attraction between + and -


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Hydrogen bonds (Permanent dipole)

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FH FH

Hydrogen bond

a special case of intermolecular attraction producedbetween certain covalently bonded hydrogen atoms andlone pairs of electrons of another atom; e.g., attractionbetween H atoms in water molecules, H is covalentlybonded to F (HF) & to nitrogen (as in NH3)


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Hydrogen bonding in H2O

molecules

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Seatwork

1. Write the un-ionized and ionized shorthandelectron notation for the following elements:

a. Cu Cu+ and Cu2+ At. No. = 29

b. Fe Fe2+ and Fe3+At. No. = 26

2. Silica (SiO2) can be treated as either acovalently or ionically bonded material.Sketch the valence-electron configuration forsilica both ways. Si = 14, O = 8

3. Despite the strength of the ionic bond, manymaterials with ionic bonds are not considered

good engineering materials. Why not?UniversityofSanCarlos-DepartmentofCh

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ChE 42D Chapter 1 Ed_11!09!07_structure of Matls_basics