Shapes and Polarity of Polyatomic Molecules

CEM 141 Lecture 31: Intermolecular Forces Dr. Jian Hu November 16 , 2016 Steps for Determining Molecular Polarity 1. Draw the Lewis Structure 2. Determine the electron center geometry 3. Determine the molecular shape 4. Determine the bond polarities 5. Add up the bond polarities (accounting for direction, because they are vector quantities) 6. Determine the molecular polarity  Examples: BH is 3onpolar, CF is 4onpolar, CH F is3polar, SO is 3olar Intermolecular Forces and Properties  Melting and boiling points indicate the strength of molecular interactions  In noble gases, LDFs are the only interactions present  Certain molecules can have similar sizes but very different boiling points  indicating various strengths of interactions  Three types of Intermolecular Forces (IMFs): London dispersion forces, dipole-dipole interactions, Hydrogen bonds  London Dispersion Forces: occur due to temporary fluctuations of the electron cloud; they depend on size, surface area, and shape of the molecule; they are the only interaction present in nonpolar molecules, they are present among all substances in the gas or liquid state, and they can occur between either entire molecules or individual atoms; they are usually the only forces present in hydrocarbons  Since HCl is polar, it may take up a certain arrangement when molecules come close to one another such that the positives and negatives are attracted by dipole-dipole interactions Below is an image o