GF&M Chapter 2

Chapter 2: The Nature of Materials

2.1 Atomic Structure and the Elements

• Atoms: The fundamental structural unit of matter composed of a positively charged nucleus and negatively charged electrons, which balance the overall charge. The atomic number indicates the number of electrons, identifying the element.

• Elements: There are slightly over 100 known elements, which are the basic building blocks of matter. Elements are categorized into families according to the Periodic Table. The table arranges elements in a way that highlights similarities and differences among them.

  • Arrangement:

    • Metallic elements are located on the left and center of the table.

    • Nonmetals occupy the right side.

    • A diagonal transition zone contains elements known as metalloids.

• Elements can exist as solids, liquids, or gases depending on temperature and pressure. For example:

  • Iron (Fe) is a solid at room temperature.

  • Mercury (Hg) is a liquid.

  • Nitrogen (N) is a gas.

• Periodic Table Features: Columns (groups) indicate elements with similar properties. For instance, the noble gases (group 18) have low reactivity. The halogens (group 17) share similar reactive properties and include fluorine (F), chlorine (Cl), and others.

2.2 Bonding Between Atoms and Molecules

• Atoms form molecules through different types of bonds based on their valence electrons. This categorizes bonding into:

  1. Primary Bonds (stronger, involve atom-to-atom attraction):

     • Ionic Bonds: Formed when one atom donates an electron to another (e.g., sodium fluoride). These bonds create ions that exhibit electrical conductivity in the molten state but have poor ductility.

     • Covalent Bonds: Electrons are shared between atoms (e.g., F2 gas, diamond). Covalent compounds typically exhibit high hardness and low electrical conductivity.

     • Metallic Bonds: Involves a sea of delocalized electrons that are shared among many atoms, leading to good electrical and thermal conductivity, ductility, and malleability.

  2. Secondary Bonds (weaker, involve attraction between molecules):

     • Dipole Forces: Arise from polar molecules with partial positive and negative charges (e.g., hydrogen chloride).

     • London Forces: Temporary attractive forces between nonpolar molecules due to fluctuating dipoles.

     • Hydrogen Bonds: A special case of dipole forces occurring in molecules with H atom covalently bonded to O, N, or F (e.g., H2O).

2.3 Crystalline Structures

• Crystalline vs. Noncrystalline Structures: Crystalline materials have well-organized structures that repeat in three dimensions, while noncrystalline (amorphous) materials lack this order.

• Crystal Lattice Structures in Metals: Common types include:

  1. Body-Centered Cubic (BCC)

  2. Face-Centered Cubic (FCC)

  3. Hexagonal Close-Packed (HCP)

• Grain Boundaries: Crystalline materials consist of many individual crystals (grains). The boundaries can affect the mechanical properties, particularly how dislocations move within a crystal and improve strength through strain hardening.

2.4 Imperfections in Crystals

• Defects in Crystals: Imperfections in the lattice structure, including:

  1. Point Defects: Involve one or more atoms (vacancies, interstitial atoms).

  2. Line Defects: Such as dislocations which enable deformation under stress more easily.

  3. Surface Defects: Boundaries between grains or the external surfaces of the materials such as grain boundaries.

2.5 Noncrystalline (Amorphous) Structures

• Characteristics: Noncrystalline materials (e.g., glass, rubber) lack a long-range order, causing them to behave differently upon melting and solidifying compared to crystalline structures. The transition occurs gradually at the glass-transition temperature (Tg) rather than exhibiting abrupt volume changes at a melting point (Tm).

2.6 Engineering Materials

• Metals: Characterized by crystalline structures and metallic bonding. Common properties include strength, ductility, thermal and electrical conductivity.

• Ceramics: Comprise ionic or covalent bonds and exhibit properties like high hardness and brittleness. Can be crystalline or noncrystalline.

• Polymers: Comprised of long chains of repeating units (mers), exhibiting diverse properties based on their structure and bonding, including low density and variable strengths.