ch02-revised-D2L
Chapter Overview
Introduction
Title: Atomic Structure and Bonding
Source: Foundations of Materials Science and Engineering, 5th Ed., by Smith and Hashemi
Objectives
Describe the nature and structure of atoms, including electronic structure.
Explain primary bond types: ionic, covalent, and metallic.
Discuss covalent bonding in carbon.
Understand various types of secondary bonds.
Analyze the effects of bond types and strengths on mechanical and electrical performance.
Explore mixed bonding in materials.
History of Atomic Theory
17th Century: Robert Boyle - Elements made of simple bodies.
19th Century: John Dalton - Introduces atoms as particles of matter.
Concept of radioactivity by Henri Becquerel and Marie & Pierre Curie.
Discovery of electrons by J.J. Thomson.
Protons found by Ernest Rutherford in 1910.
Neutrons discovered by James Chadwick in 1932.
Atom Structure
Basic Unit of Element
Diameter of atom: 10^{-10} m.
Nucleus diameter: 10^{-14} m - Neutrally charged.
Accounts for the majority of the atom's mass.
Key Particles
Electrons
Mass: 9.109 x 10^{-28} g, Charge: -1.602 x 10^{-19} C
Protons
Mass: 1.673 x 10^{-24} g, Charge: +1.602 x 10^{-19} C
Neutrons
Mass: 1.675 x 10^{-24} g, Neutral charge.
Atomic Number and Atomic Mass
Atomic Number = Number of protons in the nucleus.
Unique to each element (e.g., Hydrogen = 1, Uranium = 92).
Relative Atomic Mass: Mass of 1 mole of atoms (6.022 x 10^{23}).
Mass Number (A): Sum of protons and neutrons (e.g., Carbon: 6 protons + 6 neutrons = 12).
Isotopes: Variants of an element with the same atomic number but different mass numbers.
Example Problems
Problem 1: Alloy Composition
Given a 100 g alloy (75 wt% Cu and 25 wt% Ni)
Calculate the atomic percentages of Cu and Ni.
Problem 2: Intermetallic Compound
Calculation for the simplest formula given 42.04 wt% Ni and 57.96 wt% Al.
Quantum Theory and Electron Structure
Planck's Quantum Theory
Energy emission in discrete quanta.
Equation: E = hν = hc/λ
Bohr’s Theory
Electrons occupy specific energy levels.
Energy change upon transitions between levels quantified (ΔE = h).
Energy in Hydrogen Atom
Energy levels determined by principal quantum numbers (n).
Ionization energy defined.
Emission Spectrum
Graphical representation of emitted photons from energy transitions in hydrogen.
Quantum Mechanical Model
Uncertainty Principle by Heisenberg
Impossible to know both position and momentum of electrons simultaneously.
Probability distributions of electron density.
Electron Density and Wave Functions
Solutions to wave equation yield electron density distributions (orbitals).
Quantum Numbers
Principal Quantum Number (n)
Indicates main energy levels (n = 1 to 7).
Subsidiary Quantum Number (l)
Indicates sub-levels (s, p, d, f).
Covalent Bonding
Basics of Covalent Bonds
Sharing of electrons between atoms.
Examples: H2, F2, O2, N2 - prescribed bond energies.
Ionic Bonding
Ionic Bond Formation
Electron transfer from metallic to non-metallic elements.
Strong bonds due to electrostatic attraction between cations and anions.
Ionic Properties
High melting points, high lattice energies.
Metallic Bonding
Characteristics of Metallic Bonds
Non-directional bonds characterized by electron cloud sharing.
Properties include conductivity and malleability.
Secondary Bonding
Types of Secondary Bonds
Permanent Dipole Bonds: Due to fixed charge distributions.
Fluctuating Dipole Bonds: Occur due to asymmetrical charge distributions.
Definitions
Atomic Number: Protons count in an atom's nucleus.
Isotopes: Same atomic number but different atomic masses.
Electronegativity: Atom's tendency to attract electrons.
Bonding Energy: Energy to separate two atoms to infinity.