Aim: What holds molecules to each other? DO NOW: EXPLAIN WHY A MOLECULE CONTAINING POLAR BONDS IS NOT NECESSARILY A POLAR MOLECULE. GIVE AN EXAMPLE OF SUCH MOLECULE.

Intermolecular Forces  The attraction between molecules  They are what make solid and liquid molecular compounds possible.

Attractions Between Molecules Intermolecular attractions are weaker than ionic, covalent, and metallic bonds There are 2 main types of attractions between molecules: Van der Waals and Hydrogen

Van der Waals Forces 1. dipole interactions – polar molecules attracted to one another 2. dispersion forces – caused by the motion of electrons (weakest of all forces)

Dipole interactions  Occur when polar molecules are attracted to each other.  Slightly stronger than dispersion forces.  Opposites attract but not completely hooked like in ionic solids. H F  

Dipole Interactions    

Dispersion Forces  Occur between nonpolar molecules. When the electrons happen to momentarily move to one end of the molecule there is a temporary dipole.  Depend on the number of electrons  Bigger molecules and decrease in distance between molecules  More electrons  stronger forces  Fluorine is a gas  Bromine is a liquid  Iodine is a solid

Dispersion Forces  Dispersion forces even occur between monatomic molecules - molecules of noble gases, like helium, which consist of a single atom

Hydrogen Bond Hydrogen Bonds are forces where a hydrogen atom is weakly attracted to an unshared electron pair of another atom

Hydrogen bonding  Are the attractive force caused by hydrogen bonded to F, O, or N.  F, O, and N are very electronegative so it is a very strong dipole. (very electronegative and have small atomic radii)  The strongest of the intermolecular forces.

Hydrogen Bonding H H O ++ -- ++ H H O ++ -- ++

Hydrogen bonding H H O H H O H H O H H O H H O H H O H H O

Molecule-Ion Attraction  The attraction between the ion of an ionic compound, such as salt, and molecules of water (or other polar liquids).  When you put salt in water, the positive ion Na + is attracted to the negative end of the water molecule, and the negative ion Cl - is attracted to the positive end of water.  ectfolder/flashfiles/thermochem/solutionSalt.html ectfolder/flashfiles/thermochem/solutionSalt.html

Network Solids A Network Solid contains atoms that are all covalently bonded to each other. Consist of molecules that do not melt until the temperature reaches 1000ºC or higher (Example: diamond, silicon carbide) Melting a network solid would require breaking bonds throughout the solid (which is difficult to do)

Practice Question 1 Which type of bonding accounts for the unusually high boiling points of water? (1) Ionic bonding (2) Covalent bonding (3) Hydrogen bonding (4) Network bonding

Practice Question 2 The attractions that allow molecules of krypton to exist in the solid phase are due to (1) Ionic bonds (2) Covalent bonds (3) Molecule- ion forces (4) Van der Waals forces

Practice Question 3 The dispersion forces of attraction between molecules always become stronger as molecular size (1) increases, and the distance between the molecules increases (2) Increases, and the distance between the molecules decreases (3) Decreases, and the distance between the molecules increases (4) Decreases, and the distance between the molecules decreases