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)