Intermolecular forces

• Lewis structure


  A diagram that shows the valence electrons in a molecule, either as pairs of bonding electrons represented by a line or non-bonding electrons represented by dots
• Exceptions to the octet rule: Incomplete octet for B, Be; Expanded octet for P, S, and Si
• Draw the Lewis structure for CH2O

  Step by step process
• VSEPR Theory


  Valence shell electron pair repulsion theory can be used to deduce the shapes of covalent molecules based on the theory that electrons repel one another to be as far apart as possible in space
• Non-bonding electrons (also called lone pair electrons) are electrons that are not involved in a covalent bond
• Draw the Lewis structure for NH3

  Step by step process
• Lewis Structures (Method 1)

  1. Show all the valence electrons in a molecule, the bonding and non-bonding (lone pairs) of electrons

  2. Count the total number of electrons

  3. Determine the number of electrons needed for each atom to achieve an octet

  4. Subtract 1 from 2 to get the number of bonding electrons in the molecule

  5. Add electrons to each atom until octet is achieved

  6. Finally, the final number of electrons should equal step 1
• Learning Goals

  • Apply the valence shell electron pair repulsion (VSEPR) theory to predict, draw and explain the shapes of molecules
  • Use molecular shape, understanding of symmetry, and comparison of the electronegativity of elements to explain and predict the polarity of molecules
  • Explain the relationship between observable properties, including vapour pressure, melting point, boiling point and solubility, and the nature and strength of intermolecular forces, including dispersion forces, dipole–dipole attractions and hydrogen bonding within molecular covalent substances
• Bonding electrons are those that are shared between two atoms in a covalent bond
• VSEPR Theory

  • Used to predict the geometry (shape) of molecules
  • Electron pairs (NB or lone pairs) repel each other and spread as far as possible
  • Each pair of electrons is described as occupying an electron domain, a field of electron density
  • Single, double and triple bonds and lone pairs of electrons count as one electron domain
• Lewis structures are 2D and tell us nothing about the shape
• Lewis structure revisions
• Objectives of VSEPER Theory
• Lewis Dot Method -2


  Draw the Lewis structure for molecules
• VSEPR Theory

  • Molecular geometries based on 2, 3, and 4 electron domains
  • Distinguish between electron domain geometry and molecular geometry
• Electron domain


  A field of electron density where single, double, and triple bonds and lone pairs of electrons count as one electron domain
• Method 1: Drawing Molecular geometries


  Identify valence electrons, sigma bonds, total valence electrons, electron domains, and refer to the table of molecular geometry
• Determining electron domain and Geometry

  Double or triple bonds count as one electron domain, 1 non-bonding pair of electron = 1 electron domain, 1 bonded pair of electron = 1 electron domain
• VSEPR Theory

  Valence Shell Electron Pair Repulsion Theory predicts the shape of molecules based on the repulsion of electron pairs in the outer shell of an atom
• Coordinate Covalent bonds (Dative Bonds)
• VSEPR theory is based on the repulsion of negatively charged electron pairs in the outer shell of an atom
• VSEPR Theory Summary

  Subtract 2.5 from main bond angles for every additional lone pair of electrons
• Resonance Structures
• Understanding the shape of molecules helps to understand physical properties like boiling point and solubility
• Checking for understanding


  • What is the requirement for hydrogen bonding to occur?
  • Which of the following substances form hydrogen bonds: NH3, H2S, CF4, CHF3, Ethanol?
• Dipole-dipole forces

  • Methanal (polar): boiling point: -19 °C
  • Ethane (non-polar): boiling point: -88.5 °C
• Hydrogen bonds

  • Ethane (non-polar, dispersion forces): boiling point: -88.5 °C
  • Methanal (polar, dipole-dipole forces): boiling point: -19 °C
  • Methanol (polar, hydrogen bonding): boiling point: 64.7 °C
• Checking for understanding

  • Which substance has the strongest dipole-dipole forces: HCl or HI?
  • Which of these substances has dipole-dipole interactions: CH4 or CH3Cl?
• Dispersion forces

  • The weakest of intermolecular forces
  • Result from the constant movement of electrons creating temporary dipoles
• Hydrogen bonds hold the two strands of the DNA molecule together
• Hydrogen bonding


  • A particularly strong form of dipole-dipole force
  • Only occurs between highly polar molecules in which a hydrogen atom is covalently bonded to an oxygen, a nitrogen or a fluorine atom
  • Occurs when O, F, and N are small and highly electronegative, creating a strong dipole, attracting the hydrogen atom to the lone electron pairs of a neighbouring molecule
  • Can occur between molecules of the same substance or between molecules of different substances
  • Greatly affects the melting and boiling points of a substance
• Intermolecular forces are the electrostatic attractive forces that exist between molecules. They are 10-100 times weaker than strong intramolecular bonds such as ionic, metallic and covalent bonds. Intermolecular forces affect many physical properties of substances
• Types of intermolecular forces

  • Dipole-dipole forces
  • Hydrogen bonding
  • Dispersion forces
• Learning Goals

  • Apply the valence shell electron pair repulsion (VSEPR) theory to predict, draw and explain the shapes of molecules
  • Use molecular shape, understanding of symmetry, and comparison of the electronegativity of elements to explain and predict the polarity of molecules
  • Explain the relationship between observable properties, including vapour pressure, melting point, boiling point and solubility, and the nature and strength of intermolecular forces, including dispersion forces, dipole–dipole attractions and hydrogen bonding within molecular covalent substances
• Hydrogen bonds are responsible for the unique properties of water such as its high boiling point, surface tension, and the lower density of ice compared to liquid water
• Dipole-dipole forces

  • Only occur between permanent polar molecules
  • Result from the attraction between the positive end of a polar molecule and the negative end of another polar molecule
  • Are relatively weak since the partial charges δ+ and δ- on the molecules are small
  • The higher the polarity of the molecules, the higher the dipole-dipole forces will be
  • Directly affect the melting and boiling points: a substance that contains stronger dipole-dipole forces has a higher melting and boiling point than a substance that has weaker dipole-dipole forces or no dipole-dipole forces at all
• Arranging a central atom linked to 2 other atoms

  Bonding electrons are arranged as far apart as possible in a linear shape
• Reason for smaller bond angle in trigonal pyramidal shape compared to tetrahedral shape

  Lone pair of electrons occupies more space than bonding electrons, pushing the bonds further apart resulting in a smaller angle
• Arranging a central atom linked to 3 other atoms

  Bonding electrons are arranged as far apart as possible in a trigonal planar shape with a 120° angle between the bonds
• Trigonal Pyramidal shape


  Formed when the central atom forms 3 covalent bonds and has one lone pair of electrons, with a bond angle of 107°