Intermolecular forces

• Intermolecular forces of attraction are attractive forces present in between molecules
• The four main types of intermolecular forces are:

  • London dispersion forces
  • Dipole-dipole forces
  • Ion-dipole forces
  • Hydrogen bonding forces
• Dipole-dipole interactions are attractive forces that are a moderately strong type of IMFA and are present in between polar molecules
• Hydrogen bonding is a special kind of dipole-dipole force that exists when hydrogen is bonded to the most electronegative atoms, namely F, O, or N
• London dispersion forces (LDFs) are the weakest type of IMFA and are present in between all electrically neutral molecules—polar and nonpolar molecules
• Induced dipoles occur when a nonpolar atom becomes polar due to the presence of an ion or a dipole
• The strength of LDFs depends on the polarizability of the molecule
• Polarizability refers to the ease at which the electron cloud can be distorted
• The only intermolecular force present in these compounds is the London dispersion forces
• Nonpolar covalent compounds are molecules with zero dipole moment due to equally shared electrons between the atoms
• To determine the intermolecular force present for a molecule:

  1. Check for polar and nonpolar bonds

  2. Check for polar and nonpolar molecule

  3. Check geometric structure

  4. Overall molecular size
• Intermolecular forces are the attractive or repulsive forces between molecules of a compound
• Intermolecular forces exist between molecules, not between atoms
• All intermolecular forces are electrostatic and natural
• Three types of intermolecular forces studied at the college level are:

  • Hydrogen bonding: exists between molecules like water, hydrogen fluoride, and ammonia
  • Dipole-dipole forces: exist between two polar molecules like HCl, sulfur dioxide, etc.
  • London dispersion forces: exist between two non-polar molecules like hydrogen gas, fluorine gas, chlorine gas, oxygen gas
• Intermolecular forces only exist in non-metals, not in metals
• Metals contain intramolecular forces like metallic bonds
• Boiling point is directly proportional to intermolecular forces
• Strong intermolecular forces require more thermal energy to separate molecules, leading to a higher boiling point

  • Weak intermolecular forces require less thermal energy to separate molecules, leading to a lower boiling point
• For example, water has a high boiling point of 100 degrees Celsius due to strong intermolecular forces, while acetone has a low boiling point of 50 degrees Celsius due to weak intermolecular forces
• The strongest intermolecular force is hydrogen bonding
• Hydrogen bonding occurs when hydrogen is bonded to a strongly electronegative element like in hydrogen fluoride, water, and NH3
• The weakest intermolecular force is London dispersion forces
• London dispersion forces are considered weak and increase as the number of electrons in a molecule increases
• For example, the boiling point of water is 100 degrees Celsius, while the boiling point of methane is -162 degrees Celsius
• The boiling point of decane is 174 degrees Celsius, showing an increase in London dispersion forces with more electrons