Intramolecular vs Intermolecular

  • Intramolecular bonding: what holds atoms together within the molecule

           ○ Covalent or ionic

  • Intermolecular forces: forces between (rather than within) molecules

Intermolecular Forces

  • Weaker interactions that occur between molecules (3 types)
  1. Hydrogen bonding (strongest)
  2. Dipole-dipole forces (intermediate)
  3. London-dispersion forces (weakest)
  • Intramolecular forces are a lot stronger than intermolecular forces
  • The weaker the forces, the more likely the substance is to exist as a gas → bcuz particles are able to move far apart since they are not held together very strongly
  • Molecules orient themselves to maximize the ⊕,⊝ interactions and to minimize ⊕,⊕ and ⊝,⊝ interactions

Dipole-Dipole Forces

  • Dipole-Dipole Attraction: Molecules with dipole moments (polar molecules) can attract each other electrostatically by lining up so that their positive and negative ends are close to each other

            ○ Can be attractive or repulsive

  • The strength of the interactions is directly related to the magnitude of the dipole

           ○ The more polar the molecule is (bigger electronegativity difference) → stronger the D-D attraction

                   ■ Force becomes weaker as the distance between the dipoles increases (larger atoms)

  • Molecules with D-D have atoms held more tightly together → higher MP and BP → more likely to be a liquid at room temp

Hydrogen “Bonding”

  • Hydrogen bonds: A very strong type of dipole-dipole attraction that occurs when hydrogen is covalently bonded (intramolecular) to N, O, or F

           ○ → not HB bcuz H is covalently bonded to Carbon

  • It is very strong due to the large difference in electronegativity and hydrogen’s small size that allows dipoles to be close → need more energy to overcome bonds → higher MP and BP 

London Dispersion Forces

  • ALL covalent compounds experience LDF (aka van der waal forces)

           ○ Do not use the word “has”

  • Is the only force present in noble gas atoms and nonpolar molecules

           ○ Forces also exist between polar molecules but usually not as strong as D-D

  • In non-polar molecules the electron charge is usually evenly distributed but it is possible that at a particular moment in time it is not (e- are always moving in their orbitals) → distorts electron cloud → separation of charge creates instantaneous dipoles in molecule → induces another instantaneous dipole in another molecule → short/weak attraction between molecules
  • Polarizability: how easy the electron cloud of an atom can be induced into a dipole

         ○ Elements with more electrons (greater molar mass) = more polarizable molecule = electron cloud can be more easily induced into a dipole = stronger LDF = higher MP/BP

  • Force is usually weak but can be significant in large atoms/molecules (Ex: F2, Cl2, Br2) → is the only force affected by mass

Ion-Dipole Forces

  • Attractive force between an ion and the oppositely charged end of a polar molecule → strongest force ○ Ex: NaCl and water Forces Between Polar and Nonpolar Molecules
  • Dipole-induced dipole interaction: a permanent dipole on a polar molecule can induce a dipole on a neighboring nonpolar molecule

      ○ Thus polar molecules and nonpolar molecules exhibit an attraction for one another ■ Explains why oxygen and CO2 can be dissolved (slightly) in water

  • Factors that affect strength → nature of both molecules

     ○ The larger the magnitude of the dipole in a polar molecule → the better able it is to induce a dipole in a neighboring molecule.

     ○ Nonpolar molecules with a greater number of electrons have an increased polarizability (LDF) → dipole more easily induced

Factors Affected by IMF’s

Properties that increase with stronger InterMF’s → bcuz molecules experience stronger attractions for each other and it takes more energy to overcome that attraction

  • Melting and boiling points
  • Surface tension
  • Heat of vaporization
  • Viscosity

Properties that decrease with stronger InterMF’s

  • Vapor pressure
  • Volatility: how easily a liquid evaporates

Comparing IMF’s

  • If the size of molecules is similar → H-bonding > D-D > LDF ○ If sizes are comparable → LDFs are similar
  • If the molecules are of different size → more complex → very imp. for AP exam

         ○ LDF’s can become more significant than D-D or H-B when the size of molecules is bigger

             ■ Occurs when the difference in magnitude of LDFs in A exceed the magnitude of all the other interactions in B, giving A more IMFs total

  • More linear compounds (less branched) = greater surface area for contact between molecules = stronger


         ○ Ex: for questions involving isomers

  • Remember: when asked to compare properties, make sure to list all the InterMFs are present in both molecules and their relative strengths