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.