Intermolecular Forces

EXAM QUESTION: Predict 2 possible shapes which molecules having the general formula AB2. (2011, HL, 7 marks) SEE ABOVE FOR EXPLANTION

Intermolecular Forces

  1. This refers to the forces which occur between molecules.

i.e.

Intramolecular forces

    • Includes ionic + covalent bond

molecule

    • Refers to bond which keeps the atoms together in a molecule.

molecule

Intramolecular forces: refers to the forces which occur within atoms of a molecule

  1. These forces determine the melting and boiling point of a substance.

Three types intermolecular forces:

  • Dipole – Dipole [permanent bond]
  • Hydrogen Bonding [special type of permanent dipole – dipole]
  • Van der Waals’ Forces [temporary dipole]
  1. Van der Waals’ Forces: Occurs in atoms or molecules where the difference in electronegativity is 0 and there should be no polarity. They are caused by temporary dipoles produced by random movement of electrons . E.g. H-H or He
  2. Dipole – Dipole Forces: Forces of attraction between a positive are of one atom and a negative area in another atom e.g. between HCl > Hδ+ – Clδ-
  3. Hydrogen bond: This is the attraction between, a slightly positive atom in one molecule and a slightly negative N, F, or O in an adjacent molecule.

Note:

  • Hydrogen bonds form when a hydrogen atom is bonded to a small electronegative atom.
  • Hydrogen bonds are stronger than Van Der Waals or dipole dipole.

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