Chemical bonds aren't just about atoms sticking together. Intermolecular forces and crystal structures shape how molecules interact and form larger structures. These concepts are key to understanding why materials behave the way they do.

From weak van der Waals forces to strong ionic bonds, the way atoms and molecules arrange themselves determines a material's properties. We'll explore how these forces create different crystal types and structures, influencing everything from melting points to conductivity.

Intermolecular Forces

Van der Waals and Hydrogen Bonding

• Van der Waals forces represent weak attractions between molecules
  • Arise from temporary fluctuations in electron distribution
  • Include dipole-dipole interactions and London dispersion forces
  • Strength increases with molecular size and polarizability
• Hydrogen bonding occurs between a hydrogen atom bonded to a highly electronegative atom and another electronegative atom
  • Stronger than other intermolecular forces but weaker than covalent bonds
  • Crucial in determining properties of water, DNA, and proteins
  • Affects boiling points, solubility, and viscosity of substances

Dipole-Dipole and London Dispersion Forces

• Dipole-dipole interactions result from the attraction between polar molecules
  • Molecules with permanent dipoles align to minimize potential energy
  • Strength depends on the magnitude of molecular dipoles
  • Influences physical properties like boiling points (acetone)
• London dispersion forces exist between all molecules, even non-polar ones
  • Caused by instantaneous dipoles formed by electron movement
  • Strength increases with molecular size and surface area
  • Responsible for condensation of noble gases (liquid helium)
  • Play a significant role in the behavior of large molecules (proteins)

Types of Crystals

Ionic and Covalent Crystals

• Ionic crystals form from electrostatic attractions between oppositely charged ions
  • Characterized by high melting points and brittleness
  • Conduct electricity when molten or dissolved in water
  • Common in salts (sodium chloride)
• Covalent crystals consist of atoms held together by covalent bonds in three-dimensional networks
  • Exhibit extreme hardness and high melting points
  • Generally poor conductors of electricity
  • Include diamond (carbon atoms) and quartz (silicon and oxygen atoms)

Metallic and Molecular Crystals

• Metallic crystals comprise positively charged metal ions in a sea of delocalized electrons
  • Display high electrical and thermal conductivity
  • Possess malleability and ductility
  • Found in pure metals and alloys (copper, steel)
• Molecular crystals form from molecules held together by intermolecular forces
  • Generally have low melting points and are soft
  • Poor conductors of electricity
  • Include ice (water molecules) and dry ice (carbon dioxide molecules)

Crystal Structure

Lattice and Unit Cell

• Crystal lattice represents the three-dimensional, periodic arrangement of atoms or molecules
  • Determines the overall structure and symmetry of the crystal
  • Can be described by translation vectors
• Unit cell serves as the smallest repeating unit of the crystal structure
  • Contains all the structural information of the entire crystal
  • Defined by lattice parameters: edge lengths and angles
  • Comes in seven crystal systems (cubic, tetragonal, orthorhombic)

Coordination and Packing

• Coordination number indicates the number of nearest neighbors for each atom or ion
  • Affects physical properties like density and hardness
  • Varies depending on the crystal structure (6 for sodium chloride, 4 for diamond)
• Packing efficiency measures the fraction of space occupied by atoms or ions in a crystal
  • Calculated as the ratio of atomic volume to unit cell volume
  • Higher packing efficiency generally leads to increased density
  • Face-centered cubic (FCC) structures have the highest packing efficiency (74%)

Polymorphism and Structure Analysis

• Polymorphism occurs when a substance can crystallize in multiple crystal structures
  • Different polymorphs exhibit distinct physical properties
  • Affects pharmaceutical industry (different drug absorption rates)
  • Examples include carbon (graphite and diamond) and calcium carbonate (calcite and aragonite)
• X-ray crystallography determines the atomic and molecular structure of crystals
  • Uses X-ray diffraction patterns to deduce crystal structure
  • Provides information on bond lengths, angles, and molecular packing
  • Crucial in fields like structural biology and materials science