Polarity and Intermolecular Forces Diatomic molecules Nonpolar Polar Polyatomic molecules Nonpolar Polar Bond moments and resultant dipole moments in NH3 and NF3. Practice Which of the following bonds are polar covalent, which are non polar covalent, and which are ionic? a. The bond in CsCl Ionic b. The bond in H S 2 Polar covalent c. The NN bond in H2NNH2 Non Polar covalent Intramolecular forces vs Intermolecular forces Intramolecular forces- the forces which mediate interaction between molecules, including forces of attraction or repulsion which act between molecules and other types of neighboring particles, e.g., atoms or ions. – Metallic bond – Covalent bond, Network Covalent – Ionic bond Intermolecular forces-any force that holds together the atoms making up a molecule or compound. This includes all types of chemical bonds. Inter are usually weaker than intramolecular forces. – Dispersion, aka London – Dipole-Dipole – Hydrogen bonding-strongest 1. London dispersion force (aka dispersion force) • Attractive forces that arise as a result of temporary dipoles induced in atoms or molecules by the movement of the electrons for a short moment in time • Usually increase with molar mass • weakest of the three 2. Dipole-dipole • Attractive forces between two polar molecules • Medium in strength of the three This is where the force of two polar molecules are attracted to each other with electrostatic force denoted with a red dashed line 3. Hydrogen Bond • Forms between the hydrogen end of one dipole and the fluorine, oxygen, or nitrogen (that have at least one lone pair) end of another dipole • strongest of the three forces The hydrogen bond attractive force is denoted by the black dotted lines. IMF’s • ALL molecules have dispersion forces • All polar molecules have dipole-dipole forces and dispersion forces • Molecules that H-bond have all 3 Solubility of polar molecules • Properties are due to intermolecular forces • Like dissolves like • Polar substances will dissolve polar molecules • Non polar substances will dissolve non- polar molecules ❖Ionic substances will dissolve in polar molecules Intermolecular forces affect properties such as: • melting point and boiling point • Surface tension • Capillary action • Vapor pressure • Miscibility (mixability) .