11.3 Structures of Solids

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    Solid Structures

    amorphous solids – a solid with no ordered structure

    Cubic Unit Cells

    Simple Cubic

    Atoms occupy the corners

    1 atom per unit cell

    Coordination Number = 6

    a = 2r          (a = edge length; r = radius)

    Packing Efficiency = 52%

    simple cubic

    Body-Centered Cubic (BCC)

    Atoms occupy the corners and body center

    2 atoms per unit cell

    Coordination Number = 8

    √3a = 4r          (a = edge length; r = radius)

    Packing Efficiency = 68%

    body centered cubic

    Face-Centered Cubic (BCC)

    Atoms occupy the corners & face centers

    4 atoms per unit cell

    Coordination Number = 12

    √2a = 4r          (a = edge length; r = radius)

    Packing Efficiency = 74%

    face centered cubic

    Close-Packing of Spheres

    Hexagonal Close-Packed

    2 alternating layers of spheres (ABAB)

    hexagonal close-packed

    Cubic Close-Packed

    3 alternating layers of spheres (ABCABC)

    hexagonal close-packed

    Bonding in Solids

    Molecular Solids

    Held together by intermolecular forces

    Relatively low melting points

    H2O: m.p. = 0ºC

    C3H8: m.p. = -188ºC

    Network Covalent Solids

    Held together by covalent bonds

    High Melting Points

    Examples: Cdiamond and SiO2 (quartz)

    SiO2: m.p. = 1700ºC

    Ionic Solids

    Held together by ionic bonds

    High Melting Points

    Higher charges & smaller ions lead to stronger ionic bonds & higher melting points

    LiF: m.p. = 845ºC

    CsF: m.p. = 682ºC

    Li2O: m.p. = 845ºC

    Metallic Solids

    Held together by metallic bonds

    melting points of metals