ENGR 151: Materials of Engineering LECTURE #2: ATOMIC STRUCTURE AND ATOMIC BONDING

Transcription

1 ENGR 151: Materials of Engineering LECTURE #2: ATOMIC STRUCTURE AND ATOMIC BONDING

2 CHAPTER 1: INTRO Four components of MS field Processing, Structure, Properties, Performance Example: Aluminum Oxide different processing, different properties.

3 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

4 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 4

5 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 5

6 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 6

7 ENGINEERING MATERIALS ORIGIN Materials engineering has its foundation in chemistry and physics Organization of materials Organic C containing (H too) Inorganic non-living things 7

8 ATOMIC STRUCTURE Elements Atomic number (Z, number of protons in nucleus) Protons, neutrons, electrons (masses) m p = m n = 1.67 x kg, m e = 9.11 x 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 naturallyoccurring isotopes. amu (atomic mass unit) = 1/12 of atomic mass of carbon 12 One mole = x (Avogadro s number) atoms 8

9 ELECTRON MODELS Rutherford s alpha particle experiment Image Courtesy: 9

10 ELECTRON MODELS CONTD. Bohr Model (electrons revolve around nucleus in orbitals) Nucleus comprised of protons and neutrons 10

11 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. 11

12 ELECTRON MODELS CONTD. Comparison of the (a) Bohr and (b) wave-mechanical atom models in terms of electron distribution. 12

13 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 (m l ) s-1, p-3, d-5, f-7 Spin: Spin moment (m s ) +1/2, -1/2 13

14 ELECTRON CONFIGURATIONS CONTD. 14

15 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). 15

16 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 16

17 QUICK REVIEW (TABLE 2.2, PG. 22) How many valence electrons do they have? Hydrogen, 1s 1 Aluminum, 1s 2 2s 2 2p 6 3s 2 3p 1 Chlorine, 1s 2 2s 2 2p 6 3s 2 3p 5 Answer: 1, 3, 7 17

18 THE PERIODIC TABLE Significance? Dictionary of Information Assists in materials selection process

19 THE PERIODIC TABLE CONTD. Significance? Elements in the same column have similar characteristics, similar chemical properties E.g. noble gases, halogens, alkali metals

20 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

21 ELECTRONEGATIVITY CONTD.

22 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

23 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.

24 ATOMIC BONDING There are two types of forces that are both functions of the distance between two atoms: 1) Attractive Force (F A ) Depends on bonding between atoms 2) Repulsive Force (F R ) Originates due to repulsion between atoms individual (negatively-charged) electron clouds

25 ATOMIC BONDING Magnitude of an attractive force varies with distance. The Net Force (F N ) is the sum of the attractive and repulsive forces:

26 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)

27 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:

28 ATOMIC BONDING CONTD. Energy relationships: EN = net energy EA = attractive energy ER = repulsive energy

29 ATOMIC BONDING CONTD. Energy relationships:

30 Why does zero force correspond to minimum energy?

31 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.

32 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?

33 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.

34 HOMEWORK (DUE WED, 2/15/17) Read Chapter 2 (pgs ) 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)