Chemistry: Solids and Unit Cells
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Questions and Answers

What is the coordination number for the Body-Centered Cubic (BCC) crystal structure?

  • 4
  • 8 (correct)
  • 12
  • 6

    • How many total atoms are there in a Body-Centered Cubic (BCC) unit cell?

  • 1
  • 4
  • 8
  • 2 (correct)

    • What is the atomic packing factor (APF) for the Body-Centered Cubic (BCC) structure?

  • 0.52
  • 0.74
  • 0.68 (correct)
  • 0.78

    • How many corner atoms contribute to the unit cell in the Hexagonal Close-Packed (HCP) structure?

    <p>6</p> Signup and view all the answers

    What additional feature distinguishes the Hexagonal Close-Packed (HCP) structure from cubic structures?

    <p>Plane arrangement with three additional atoms in midplane</p> Signup and view all the answers

    What is the basic structural unit or building block of the crystal structure called?

    <p>Unit cell</p> Signup and view all the answers

    Which of the following statements about amorphous solids is true?

    <p>They are arranged in an irregular manner</p> Signup and view all the answers

    Which unit cell type includes atoms at all eight corners and one atom in the center?

    <p>Body-Centered Cubic (BCC)</p> Signup and view all the answers

    What is the atomic packing factor characteristic of Face-Centered Cubic (FCC) structures?

    <p>0.74</p> Signup and view all the answers

    Which type of crystal structure has a coordination number of 12?

    <p>Face-Centered Cubic (FCC)</p> Signup and view all the answers

    What type of solid is Polonium classified as based on its crystal structure?

    <p>Simple cubic element</p> Signup and view all the answers

    How are atoms arranged in a Simple Cubic (SC) crystal structure?

    <p>At corners only</p> Signup and view all the answers

    Which of the following is NOT a type of cubic unit cell?

    <p>Tetragonal</p> Signup and view all the answers

    What is the formula to compute the theoretical density of a metallic solid?

    <p>$\rho = \frac{n \cdot A}{V_C \cdot N_A}$</p> Signup and view all the answers

    Which of the following types of crystals is characterized by composed charged species?

    <p>Ionic Crystals</p> Signup and view all the answers

    What is an important consideration when classifying ionic crystals?

    <p>The radii of the ions must be known.</p> Signup and view all the answers

    How is the edge length of the unit cell in NaCl determined using ionic radii?

    <p>It is twice the sum of the ionic radii of Na+ and Cl-.</p> Signup and view all the answers

    What is true about the radii of anions and cations in ionic crystals?

    <p>Anions are generally larger than cations.</p> Signup and view all the answers

    What is the edge length of the unit cell of NaCl as determined experimentally?

    <p>564 pm</p> Signup and view all the answers

    Which of the following properties of crystals are influenced by the types of forces holding particles together?

    <p>Melting point, density, and hardness</p> Signup and view all the answers

    What is the value of Avogadro's number?

    <p>6.022 x $10^{23}$ atoms/mol</p> Signup and view all the answers

    Study Notes

    Solids

    • A solid has a definite shape and volume because its particles are tightly packed.
    • Solids can exist as crystals with regular lattices or as amorphous solids with irregular arrangements.

    Unit Cells

    • Unit cells are the fundamental building blocks of crystal structures.
    • There are three main types of unit cells: simple cubic (SC), body-centered cubic (BCC), and face-centered cubic (FCC).

    Simple Cubic (SC)

    • The SC unit cell has atoms only at the corners of a cube.
    • Polonium is the only element with a simple cubic structure.

    Face-Centered Cubic (FCC)

    • FCC unit cells have atoms at each corner and in the center of each face of the cube.
    • Important characteristics of an FCC structure include a coordination number of 12 and an atomic packing factor of 0.74.
    • The volume of an FCC unit cell can be calculated using the formula: VC = (4√2)R³
    • Examples of FCC structures include copper, silver, and gold.

    Atomic Packing Factor

    • The atomic packing factor represents the proportion of space occupied by atoms in a unit cell.
    • It is calculated by dividing the volume of atoms in the unit cell by the volume of the unit cell.
    • A higher atomic packing factor indicates a more efficient packing of atoms.

    Body-Centered Cubic (BCC)

    • BCC unit cells have atoms at each corner and one atom at the center of the cube.
    • Important characteristics of a BCC structure include a coordination number of 8 and an atomic packing factor of 0.68.
    • The edge length (a) of a BCC unit cell is related to the atomic radius (R) by the equation: a = 4R/√3.
    • Examples of BCC structures include iron, chromium, and tungsten.

    Hexagonal Close-Packed (HCP)

    • HCP unit cells have atoms at the corners of a hexagon, a single atom in the center of the hexagon, and three atoms between the top and bottom planes.
    • Important characteristics of an HCP structure include a coordination number of 12 and an atomic packing factor of 0.74.
    • The number of atoms per unit cell in an HCP structure can be calculated using the equation: N = (12 * 1/6) + 2 + 3 = 6.
    • Examples of HCP structures include magnesium, zinc, and titanium.

    Density Computations

    • The theoretical density of a solid can be calculated using the equation: ρ = (n * A) / (VC * NA).
    • Where:
      • n = number of atoms per unit cell
      • A = atomic weight
      • VC = volume of the unit cell
      • NA = Avogadro’s number (6.022 x 10^23 atoms/mol)

    Types of Crystals

    • Crystals are classified into four types based on the forces holding the particles together: ionic, covalent, molecular, and metallic.

    Ionic Crystals

    • Ionic crystals are composed of charged species (anions and cations).
    • Anions and cations in ionic crystals are generally quite different in size, and their radii need to be considered for understanding the structure and stability of the compound.
    • The edge length of an ionic crystal unit cell is often related to the sum of the ionic radii.
    • Examples of ionic crystals include NaCl (sodium chloride), CaCl2 (calcium chloride), and MgO (magnesium oxide).

    Conclusion

    • Understanding the basic concepts of crystal structures, unit cells, density computations, and different crystal types is crucial in the study of engineering materials.
    • These concepts provide insights into the properties and behaviors of materials used in various engineering applications.

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    Crystal Structures - Basic Concepts PDF

    Description

    Explore the unique properties of solids and their structures with this quiz. Learn about different types of unit cells, including simple cubic and face-centered cubic, along with their characteristics and examples. Test your knowledge of atomic packing factors and crystal arrangements.

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