prelims mod 3

• General characteristics of solids

  • They have definite mass, volume, and shape
  • Intermolecular distances are short
  • Intermolecular forces are strong
  • Their constituent particles (atoms, molecules, or ions) have fixed positions and can only oscillate about their mean positions
  • They are incompressible and rigid
• Crystalline Solid


  Ordered and Repeated Arrangement
• Amorphous Solid


  Random Arrangement
• Types of Solids (On the basis of arrangement of particle)

  • Crystalline Solids
  • Amorphous Solids
• Crystalline Solids


  • Regular arrangement of particles
  • Long range order
  • Have sharp melting points
  • Anisotropy (show different properties in different directions)
  • When cut, new surfaces are plain and smooth
• Crystalline Solids


  • CuSO4, diamond, graphite, NaCl, sugar, etc
• Amorphous Solids


  • Random arrangement of particles
  • Only short range order
  • Broad melting points
  • Isotropic behavior (show same properties in different directions)
  • When cut, new surfaces have irregular surface
• Amorphous Solids


  • Coal, coke, glass, plastic, rubber, etc
• Marble (hard)


  Crystalline solid
• Chalk (soft)


  Amorphous solid
• Quartz (Crystalline SiO2)


  • Anisotropic (polarizes light)
  • Higher density
  • Higher melting temperature
  • Presence of smooth surface when cut
• Glass (Amorphous SiO2)


  • Isotropic (does not polarize light)
  • Lower density
  • Lower melting temperature
  • Presence of unnatural streaky patches
• Types of Solids (On the basis of nature of interparticle forces)

  • Molecular solids
  • Covalent solids
  • Ionic solids
  • Metallic solids
• Molecular solids


  • Most organics, and inert gases (O2, N2, H2, I2, H2O)
• Covalent solids


  • 3D collection of atoms bound by shared valence electrons
  • Difficult to deform because bonds are directional
  • High melting point (difficult to deform)
  • No free electrons → poor electrical conductor
  • Most solids absorb photons → opaque
• Covalent solids


  • C (diamond), SiO2, B
• Ionic solids


  • Individual atoms act like closed-shell, spherical structures leading to non directional binding
  • Commonly salts that are held together by the strong force of attraction between ions of opposite charge
  • Tight packed arrangement → poor thermal conductors
  • No free electrons → poor electrical conductors
  • Strong forces → hard, and high melting points
• Ionic solids


  • NaCl, CaF2
• Metallic solids


  • Constructed of atoms which have very weakly bounded outer electrons
  • Large number of vacancies in orbitals (not enough energy available to form covalent bonds)
  • Electrons aren't tightly bound to individual atoms, and are free to migrate through the metal. As a result, metals are good conductors of electricity and heat.
• Metallic solids


  • Hg, Na, Au, W
• Summarized characteristics of types of crystalline solids

  • Molecular solids: Poor conductors of heat and electricity, Low melting point, Soft, Low density, Dull surface
• Crystalline structures

  Regular repeating pattern (unit cell) called the crystalline lattice
• Three simple cubic crystalline structures

  • Simple Cubic (SC)
  • Body-Centered Cubic (BCC)
  • Face-Centered Cubic (FCC)
• Cubic crystalline structures

  • Coordination Number - the number of atoms touching a particular atom, or the number of nearest neighbors
  • Number of atoms in a unit cell - based on the total contribution of the atoms composing the unit cell
  • Relationship of atomic radius (r) and cube edge length (a)
  • Atomic Packing Factor (APF) - the fraction of space occupied by atoms assuming that atoms are hard spheres
• Atomic Packing Factor (APF)


  APF % = (volume of atoms in a unit cell / volume of unit cell) * 100%
• Simple Cubic (SC)


  • Very inefficient and rarely seen in nature due to low packing density
  • Atoms touch each other along the cube edge
  • Coordination Number = 6
  • Contains 8 x 1/8 = 1 atom/unit cell
  • Relationship of r and a: r = 1/2 a
  • APF % = 52%
• Body-Centered Cubic (BCC)


  • Atoms touch each other along cube diagonals
  • Contains 1 center atom in contact with 8 corner atoms
  • Coordination Number = 8
  • Contains 8 x 1/8 + 1 = 2 atoms per unit cell
  • Relationship of r and a: r = 3/4 a
  • APF % = 68%
• Face-Centered Cubic (FCC)


  • Atoms touch each other along face diagonals
  • Contains 6 face atoms and 8 corner atoms
  • Coordination Number = 12
  • Contains 6 face(1/2) + 8 corners (1/8) = 4 atoms/unit cell
  • Relationship of r and a: r = 2/4 a
  • APF % = 74%
• Calculating theoretical density of cubic crystalline structures

  ρ = (n*A) / (Vc*NA)

  Where:

  n = no. of atoms / unit cell

  A = atomic weight (g/mol)

  Vc = volume / unit cell (cm3/unit cell) = a3

  NA = Avogadro's number = 6.022x1023 atoms/mol
• Important conversion factors:

  1 cm = 107 nm

  1 cm = 1010 pm
• Polymer


  Large molecule composed of many repeating sub-units
• Monomer


  The repeating sub-unit of a polymer
• Polymers can have high molecular weight, reaching more than 1,000,000 g/mol
• Types of polymers

  • Homopolymer
  • Copolymer
• Homopolymer


  A polymer containing only one monomer
• Copolymer


  A polymer containing two or more different monomers
• Examples of natural polymers


  • Proteins
  • Carbohydrates
  • Nucleic acids
  • Natural rubber
• Proteins


  • Polymers of amino acids
  • Play a key role in nearly all biological processes
  • Compose 15% of our body
• Carbohydrates


  • Polymers of simple sugars (monosaccharides)
  • Empirical formula is CH2O
  • Functions: food storage/source, structural material
• Nucleic acids


  • Store and transfer genetic information
  • Direct the synthesis of new protein
  • Types: DNA, RNA