CHEMISTRY 20 CHEMICAL BONDING Types of Molecular Bonding

Morning Assignment What determines a molecules polarity? What are two ways that bonds occur?

2 Types of Bonds Intermolecular Bonds – attractions (bonds) between molecules Intramolecular Bonds – attractions within molecules (covalent bonds)

Intermolecular Bonding Until now we have considered intramolecular bonding Intermolecular Bonding – � Electrostatic attractive forces b/w molecules � Can only occur in substances that form covalent bonds � Weaker that ionic, covalent, or metallic bonds

Types of Intermolecular Bonding Van der Waals Forces – there are 2 kinds 1) � � Dipole – dipole Forces if the substance contains polar molecules, the pos. end of one is attracted to the neg. end of another Draw Picture form notes

Types of Intermolecular Bonding 2) � � � � London Dispersion Forces Exist between all molecules Due to electrostatic attractions between temp. dipoles At any instant e- may be found closer to one end of a nuclei Creates temp. dipole - one end slightly neg. Attracted to neighboring atom which has been induced Not permanent, no shift of e. More strength as molecules get bigger. More e-

Hydrogen Bonding Strongest of the intermolecular bonds Usually Hydrogen and a strong electronegative element (F or N, or O but could be other Group 17) � The Hydrogen bonds covalently to the strong electronegative element which has strong pull on e� This leaves the molecule very polar so it then bonds to other polar molecules. Usually another of the same molecule � Hydrogen appears to be sharing e- pairs from 2 molecules � There must be at least one lone pair for it to work

Example Hydrogen Bonding Hydrogen & Flourine

Effects of Intermolecular Bonds on Physical Properties Trend: As molar mass increases, so do melting point and boiling point Give students copy of SNAP Pg 72 Other Properties � Surface Tension – molecules in liquid are attracted to others in all directions, but surface molecules only downward + sideways � Capillary Action – water climbs narrow tube because of polar attraction to glass � Vaporization – water vapourizes slower than gas because their molecules are attracted to each other

Assignment Reading � Text Pages 105 – 118 Questions � SNAP Page 76 #1 � SNAP Page 78 #’s 2, 3 � SNAP Page 79 #4 � SNAP Page 79 Practice test #1 Extra Practice Text Pages 117 – 118 Section 3. 4 questions