Intermolecular Forces, Electronegativity and Bond Properties

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. (a) Describe hydrogen bonding, limited to molecules containing N–H and O–H groups, including ammonia and water as simple examples
    (b) Use the concept of hydrogen bonding to explain the anomalous properties of H₂O (ice and water): its relatively high melting and boiling points; its relatively high surface tension; the density of the solid ice compared with the liquid water

  2. Use the concept of electronegativity to explain bond polarity and dipole moments of molecules

  3. (a) Describe van der Waals' forces as the intermolecular forces between molecular entities other than those due to bond formation, and use the term van der Waals' forces as a generic term to describe all intermolecular forces
    (b) Describe the types of van der Waals' forces: instantaneous dipole–induced dipole (id-id) forces, also called London dispersion forces; permanent dipole–permanent dipole (pd-pd) forces, including hydrogen bonding
    (c) Describe hydrogen bonding and understand that hydrogen bonding is a special case of permanent dipole–permanent dipole forces between molecules where hydrogen is bonded to a highly electronegative atom

  4. State that, in general, ionic, covalent and metallic bonding are stronger than intermolecular forces


1. Introduction to Intermolecular Forces

Intermolecular forces are forces of attraction that exist between molecules. These are different from the bonds within molecules (like covalent bonds, which hold atoms together inside a molecule).

Think of it this way:

  • Intramolecular forces = forces inside a molecule (e.g., the covalent bond between H and Cl in HCl)
  • Intermolecular forces = forces between molecules (e.g., the attraction between one HCl molecule and another HCl molecule)

Intermolecular forces are much weaker than the chemical bonds that hold atoms together within molecules, but they are very important because they determine:

  • Whether a substance is a solid, liquid, or gas at room temperature
  • The melting point and boiling point of a substance
  • How soluble a substance is in water

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