London Dispersion Forces Overview

Overview

• Topic: London dispersion forces (LDF), a type of intermolecular force.
• Present in all atoms and molecules; they dominate in nonpolar molecules.
• Arise from temporary dipoles and induced dipoles due to electron motion.
• Strength of LDF depends on number of electrons, polarizability, and molecular surface area.
• LDF strength strongly affects boiling point and physical state (gas, liquid, solid).

Nature and Origin of London Dispersion Forces

• All atoms and molecules have electrons moving randomly around the nucleus.
• In a perfectly nonpolar atom, electrons are usually evenly distributed.
• At any instant, electrons may be more on one side than the other.
• This uneven electron distribution creates a temporary dipole.
• Temporary dipole:
  • One side becomes electron rich → partial negative charge (δ−).
  • Opposite side becomes electron poor → partial positive charge (δ+).
• This temporary dipole is short-lived, constantly changing with electron motion.

Temporary Dipole–Induced Dipole Interaction

• A temporary dipole in one atom can affect a neighboring neutral atom.
• The partial positive side of the first atom attracts electrons of the second atom.
• This attraction distorts the electron cloud of the second atom.
• Distorted electron cloud in the second atom creates an induced dipole.
• Now both atoms have temporary dipoles aligned with opposite charges facing.
• The attraction between these dipoles is the London dispersion force.
• LDF is therefore a temporary dipole–induced dipole attraction.
• These interactions are:
  • Weak compared to many other intermolecular forces.
  • Very short-lived and constantly forming and breaking.

Polarizability and Number of Electrons

• Polarizability: how easily an electron cloud can be distorted to form a dipole.
• Larger atoms with more electrons are more polarizable.
• More electrons → higher chance of uneven electron distribution at any moment.
• Greater polarizability → stronger temporary and induced dipoles.
• Therefore, London dispersion forces increase with increasing polarizability.
• Polarizability depends mainly on:
  • Total number of electrons in the atom or molecule.
  • Overall size of the electron cloud.

London Dispersion Forces and Noble Gases

• Noble gases are monatomic, nonpolar, and interact mainly via LDF.
• More electrons in the noble gas atom → stronger LDF → higher boiling point.

• Trend observed:
  • As number of electrons increases from He to Kr, LDF strength increases.
  • Boiling point becomes less negative (increases) as LDF increases.
• Conclusion:
  • Higher LDF → molecules stick together more strongly → higher boiling point.

London Dispersion Forces and Halogen Molecules

• Homonuclear halogen molecules (F₂, Cl₂, Br₂, I₂) are nonpolar.
• They differ mainly in number of electrons and molecular size.
• More electrons in the molecule → stronger LDF → higher boiling point.

• I₂ has the most electrons and is the most polarizable.
• I₂ therefore has the strongest London dispersion forces and highest boiling point.
• Physical state correlation:
  • Gases (F₂, Cl₂) → weakest LDF and lowest boiling points.
  • Liquid (Br₂) → intermediate LDF, moderate boiling point.
  • Solid (I₂) → strongest LDF, highest boiling point.

Relationship between LDF and Boiling Point

• Boiling requires separating molecules from each other.
• Stronger intermolecular forces → more energy needed to separate molecules.
• Therefore:
  • More London dispersion forces → higher boiling point.
• For nonpolar substances:
  • As number of electrons increases, LDF increase.
  • Increased LDF cause higher boiling points.
• LDF are one type of intermolecular force; others include:
  • Dipole–dipole interactions (between permanent dipoles).
  • Hydrogen bonding (a special strong dipole interaction).

Molecular Shape, Surface Area, and London Dispersion Forces

• For molecules with the same molecular formula and same number of electrons:
  • LDF strength can still differ due to molecular shape and surface area.
• Larger contact surface area between molecules → stronger London dispersion forces.
• Straight-chain molecules usually have more surface area than highly branched ones.
• More surface area allows more effective attractions between electron clouds.

Example: Pentane vs Neopentane

• Both molecules: C₅H₁₂ (same number of electrons).
• Difference lies in structure and shape.

• Pentane:

  • Straight chain, more elongated shape.
  • Larger surface area in contact with neighboring molecules.
  • Stronger London dispersion forces.
  • Higher boiling point.

• Neopentane:

  • Highly branched, more compact, roughly spherical shape.
  • Smaller effective surface area for contact.
  • Weaker London dispersion forces.
  • Lower boiling point.

• General rule:

  • For isomers with equal electrons, straight-chain > branched in LDF strength.
  • Higher surface area → stronger LDF → higher boiling point.

Key Terms and Definitions

• London dispersion forces (LDF):

  • Weak intermolecular forces arising from temporary dipole–induced dipole interactions.
  • Present in all atoms and molecules; main force in nonpolar substances.

• Temporary dipole:

  • Short-lived separation of charge in an atom or molecule.
  • Caused by random uneven electron distribution at a moment.

• Induced dipole:

  • Dipole created in an atom or molecule by the electric field of a nearby dipole.
  • Electron cloud becomes distorted due to attraction or repulsion.

• Polarizability:

  • Measure of how easily an electron cloud can be distorted to form a dipole.
  • Increases with number of electrons and size of the atom or molecule.

• Intermolecular forces:

  • Forces of attraction between separate molecules (or atoms in noble gases).
  • Include London dispersion forces, dipole–dipole interactions, and hydrogen bonding.

Summary of Main Relationships

• More electrons in a nonpolar atom or molecule → higher polarizability → stronger LDF.
• Stronger LDF → higher boiling point and often more condensed physical state.
• For similar molecules:
  • Larger atoms (e.g., I vs F) show stronger LDF.
  • Greater molecular surface area (straight chain) increases LDF compared to branched structures.
• In comparing nonpolar substances:
  • First compare number of electrons.
  • If equal, compare molecular shape and surface area.