Lecture Notes on Intermolecular Forces, Surface Tension, and Crystal Structures

Intermolecular Forces and Surface Tension

• Intermolecular Forces (IMFs): Forces that mediate interaction between molecules, including hydrogen bonding, dipole-dipole interactions, and dispersion forces.
• Hydrogen Bonding: Special type of dipole-dipole interaction where hydrogen is bonded to a highly electronegative atom (like N, O, F).
  • Example: Water (H2O) can form up to four hydrogen bonds due to its two hydrogen atoms and two lone pairs.
  • Ethylene glycol (C2H6O2) can form up to six hydrogen bonds.
• Surface Tension: Energy required to stretch a surface. Related to the strength of IMFs.
  • Water has a higher surface tension than ethylene glycol despite forming fewer hydrogen bonds due to stronger cohesion among water molecules.
  • Cohesion refers to IMFs between like molecules, while adhesion is between unlike molecules (e.g., water adhering to glass).

Viscosity

• Viscosity: Measure of a fluid's resistance to flow. High viscosity indicates strong intermolecular forces.
  • Water has lower viscosity compared to glycerol, which is similar to ethylene glycol.

Properties of Water

• Unique Properties: Small size, high polarity, expands when it freezes (ice floats).
• Important temperature: 4°C is when water reaches its maximum density before expanding.

Crystal Structures

• Types of Solids:
  • Crystalline solids possess an ordered structure with unit cells.
  • Amorphous solids lack a defined order.
• Unit Cells: Smallest repeating unit that shows the entire pattern of a crystal structure.
  • Simple Cubic, Body-Centered Cubic (BCC), Face-Centered Cubic (FCC): Different types of unit cells.

Structure and Composition

• Lattice Points: Positions in a crystal structure where atoms, ions, or molecules reside.
• Unit Cell Parameters: Defined by the angles (alpha, beta, gamma) and edge lengths (a, b, c).

X-ray Crystallography

• X-ray Crystallography: Technique used to determine the atomic and molecular structure by diffracting X-rays through a crystal.
• Bragg's Equation: Used to calculate the distances between layers in a crystal based on the angle of incidence and wavelength of X-rays.

Types of Crystals

1. Ionic Crystals: Composed of cations and anions, high melting point, poor conductor in solid state but conducts when molten or dissolved.
2. Covalent Network Solids: Atoms held by covalent bonds, high melting point, poor conductor of electricity (e.g., diamond).
3. Molecular Crystals: Molecules held by IMFs, low melting point, poor conductors (e.g., ice, benzene).
4. Metallic Crystals: Metal atoms in a sea of electrons, good conductors, variable hardness and melting points.

Coordination Numbers and Closest Packing

• Coordination Number: Number of nearest neighbors surrounding an atom or ion in a crystal structure.
  • Simple cubic: 6
  • Body-centered cubic: 8
  • Face-centered cubic: 12
• Closest Packing: Arrangement to maximize space use; types include hexagonal close packing and cubic close packing.

Calculating Densities and Volumes

• Using unit cell dimensions and atomic/molecular weights to determine densities.
• Importance of converting units correctly (e.g., picometers to centimeters).

Conclusion

Understanding these concepts provides foundational knowledge for analyzing material properties and behaviors at the molecular and atomic levels in chemistry.