Post on 06-Mar-2018
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LECTURE #2: ATOMIC STRUCTURE AND
ATOMIC BONDING
ENGR 151: Materials of Engineering
CHAPTER 1: INTRO
Four components of MS field
Processing, Structure, Properties, Performance
Example: Aluminum Oxide – different processing,
different properties.
CHAPTER 1: INTRODUCTION
What is Materials Science?
Investigating properties and relationships that exist
between structures
“Structure” at the subatomic level
“Property” as a material trait: mechanical,
electrical, thermal, magnetic, optical, deteriorative
“Processing” and “Performance”
CHAPTER 1
Why study Materials Science & Engineering?
Use of materials in design problems
Lockheed F-22/F-35 characteristics
Selecting the right material:
Strength, ductility, deterioration (temperature), cost
Trade-offs are necessary
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CHAPTER 1 - INTRODUCTION
Metals used in structures & machinery
Plastics used in packaging, medical devices, consumer goods, clothing
Ceramics used in electronics (insulative)
Composites are novel materials for all applications listed above
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CHAPTER 2 OBJECTIVES
Understand the elements used to make engineering materials
Review basic chemistry and physics principles
Overview of the materials classes:
Metals: good conductors, strong, lustrous
Polymers: organic, low densities, flexible
Ceramics: clay, cement, glass; insulators
Composites: fiberglass; strength and flexibility
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ENGINEERING MATERIALS’ ORIGIN
Materials engineering has its foundation in
chemistry and physics
Organization of materials
Organic – C containing (H too)
Inorganic – non-living things
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ATOMIC STRUCTURE
Elements
Atomic number (Z, number of protons in nucleus)
Protons, neutrons, electrons (masses)
mp = mn = 1.67 x 10-27 kg, me = 9.11 x 10-31 kg
Atomic Mass (A) = sum of proton and neutron masses in nucleus
Isotope = same element, differing atomic masses E.g. Hydrogen (P = 1, N = 0), Deuterium (P = 1, N = 1), Tritium (P
= 1, N = 2).
Atomic Weight = Average atomic mass of all naturally-occurring isotopes.
amu (atomic mass unit) = 1/12 of atomic mass of carbon 12
One mole = 6.023 x 1023 (Avogadro’s number) atoms
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ELECTRON MODELS
Rutherford’s alpha particle experiment
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Image Courtesy: http://hyperphysics.phy-astr.gsu.edu
ELECTRON MODELS CONTD.
Bohr Model (electrons revolve around nucleus in orbitals)
Nucleus comprised of protons and neutrons
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ELECTRON MODELS CONTD.
Quantum mechanics (electron phenomena)
Used to explain the dual-nature (particle and wave) of the electron.
Electron positions now measured in terms of probabilities rather than being expressed in definitive terms.
Electron Cloud.
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ELECTRON MODELS CONTD.
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Comparison of
the (a) Bohr and (b) wave-mechanical
atom models in
terms of electron distribution.
ELECTRON CONFIGURATIONS
Rules of electron configuration (Table 2.1, pg.
23)
Electrons are quantized (have specific energies –
discrete energy levels)
Quantum numbers (4)
Principal: Position (n, distance of an electron from nucleus)
Azimuthal: Subshell (l)
Determines orbital angular momentum
s, p, d, or f (shape of electron subshell)
Magnetic: Number of energy states per subshell (ml)
s-1, p-3, d-5, f-7
Spin: Spin moment (ms)
+1/2, -1/2
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ELECTRON CONFIGURATIONS – CONTD.
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ELECTRON CONFIGURATIONS – CONTD.
Pauli Exclusion Principle:
No more than two electrons per electron state
Number of electron states per shell determined by magnetic quantum
number
Examples:
3p shell has 3 states (-1, 0, +1), therefore can
accommodate up to 6 electrons (2 electrons per state).
3d shell has 5 states (-2, -1, 0, +1, +2), therefore can
accommodate up to 10 electrons (2 electrons per state).
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ELECTRON CONFIGURATIONS – CONTD.
Valence electrons occupy the outermost filled
shell
Stable electron configurations have the
outermost shell completely filled
Noble gases – He, Ne, Ar
Inert elements, do not enter into chemical reactions
Chemical reactivity is a function of outer shell
electron configuration
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QUICK REVIEW (TABLE 2.2, PG. 22)
How many valence electrons do they have?
Hydrogen, 1s1
Aluminum, 1s22s22p63s23p1
Chlorine, 1s22s22p63s23p5
Answer: 1, 3, 7
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THE PERIODIC TABLE
Significance?
Dictionary of Information
Assists in materials selection process
THE PERIODIC TABLE – CONTD.
Significance?
Elements in the same column have similar
characteristics, similar chemical properties
E.g. noble gases, halogens, alkali metals
ELECTRONEGATIVITY
Measures the tendency of an element to give up or accept
valence electrons
Electropositive elements (e.g. alkali metals)
Capable of giving up few valence electrons to become positively charged
(e-, negative charge)
Electronegative elements (e.g. halogens)
Readily accept electrons to form negatively charged ions.
Also share electrons (covalent bonding)
Electronegativity increases left to right, bottom to top
Atoms accept electrons if shells are closer to nucleus
Example: Na gives up one electron, Cl accepts the electron to
form NaCl
ELECTRONEGATIVITY – CONTD.
MATERIALS FROM ELEMENTS
Elements used in:
Elemental state – W, Cr, Ni, etc.
Alloys – combination of metals
Solutions – chemical bonding
Compounds – combination in definite proportions
Mixtures – physical blend
Molecule – smallest part of a compound
ATOMIC BONDING
To understand the physical properties behind
materials, we must have an understanding of
interatomic forces that bind atoms together.
At large distances, the interactions between
two atoms are negligible…BUT…as they come
closer to each other they start to exert a force
on each other.
ATOMIC BONDING
There are two types of forces that are both
functions of the distance between two atoms:
1) Attractive Force (FA) – Depends on
bonding between atoms
2) Repulsive Force (FR) – Originates due
to repulsion between atoms’ individual
(negatively-charged) electron clouds
ATOMIC BONDING
Magnitude of an attractive force varies with
distance.
The Net Force (FN) is the sum of the attractive
and repulsive forces:
ATOMIC BONDING – CONTD.
When FA = FR the net force is zero:
(State of equilibrium)
In a state of equilibrium, the two atoms will remain separated by the distance, ro. Attractive force is the same as repulsive force at ro.
For many atoms, ro is approximately .3 nm or 3 angstroms (Å)
ATOMIC BONDING – CONTD.
Another way to represent this relationship in attractive and repulsive forces is to look at potential energy relationships.
Force-energy relationships: Both force and energy are functions of distance r
Measure of amount of work done to move an atom from infinity (zero force) to a distance r.
Alternatively:
ATOMIC BONDING – CONTD.
Energy relationships:
EN = net energy
EA = attractive energy
ER = repulsive energy
ATOMIC BONDING – CONTD.
Energy relationships:
Why does zero force correspond to minimum energy?
ATOMIC BONDING – CONTD.
The net potential energy curve has a trough
around its minimum. The potential energy
minimum is ro away from the origin.
Force is the derivative of energy.
ATOMIC BONDING – CONTD.
The Bonding Energy, Eo, refers to the vertical
distance between the minimum potential
energy and the x-axis. This is the energy that
would be required to separate the atoms to an
infinite separation.
Force and Energy plots become more complex
in actual materials. Why?
ATOMIC BONDING ENERGY
Magnitude of bonding energy and shape of
energy-versus-interatomic separation curve
vary from material to material AND depend on
the type of bonding that is taking place
between atoms.
HOMEWORK (DUE WED, 2/15/17)
Read Chapter 2 (pgs. 18-40)
Complete problems 2.2, 2.7, 2.9, 2.21, 2.23
Complete all work in pencil
Show all work (if applicable)
Circle calculated answers
Quiz next Wednesday (2/15/17)