Polarity and Intermolecular Forces Molecular Geometry
Polar Bonds Polar bonds have a positive and negative end. The charge difference is due to the difference in electronegativity of the two bonded atoms. –Electronegativity is a measure of how much an atom wants to gain electrons. F is the most electronegative atom on the Periodic Table.
Polar Bonds The direction of greatest electronegativity is shown with an arrow called a dipole. C F and C H The dipole point toward the atom with the greatest electronegativity (closest to F)
Polar Molecules are molecules which have an uneven distribution of charge. One side of the molecule is negative while one side of the molecule is slightly positive. Non-polar molecules are molecules in which there is no net separation of charge. The electrons are evenly distributed. There is no net separation of charge. Polar Molecules
Polarity of Molecules Non-polar molecules – the dipoles are all equal and opposite. They cancel out and the molecule is non- polar. Cl Be Cl Polar molecules – the dipoles are not equal and opposite so the molecule has a net dipole and is polar. H C N Net Dipole No Net Dipole
There are two steps in determining the polarity of a molecule: Step 1: Use electronegativities to determine the direction of the dipoles for each bond making up the molecule. (F is the most electronegative) Step 2: Determine if there is a net dipole by looking at the shape of the molecule. Polarity of Molecules
Step 2: Determine the shape of the molecule To determine the molecular shape of a molecule we must first determine its Lewis dot diagram. According to VSEPR theory, since the carbon atom is surrounded by two electron clouds (remember multiple bonds only count as one cloud), the shape of this molecule must be linear. Polarity of Molecules
Step 2: Determine the shape of the molecule (continued) Once we know the shape of the molecule, we must analysize how the electrons are distributed to determine if there is an even distribution (non-polar) or uneven distribution (polar). In this case we know that the oxygen is more electronegative (closer to F) than the carbon and therefore should pull the electrons out away from the carbon. Polarity of Molecules
If we look at the charge distribution in each bond, we get the following: Since the oxygen is more electronegative than the carbon, the electrons will be pulled toward the oxygen atoms and away from the carbon atoms. But, the dipoles are equal and opposite so the molecule is non-polar. No Net Dipole
Example #2 – SO 2 To determine the molecular shape of a molecule we must first determine its Lewis dot diagram. According to VSEPR theory, since the sulfur atom is surrounded by three electron clouds (remember multiple bonds only count as one cloud), the shape of this molecule must be bent.
If we look at the charge distribution in each bond, we get the following: Since the polarity of the bonds and the shape of the molecule result in an uneven distribution of charge – SO 2 is a polar molecule. Net dipole
Now that you have seen how to apply the two steps to determine the polarity of molecules, see if you can predict the polarity of the following: 1. H 2 O 6. PH 3 2. CCl 4 3. Ammonia (NH 3 ) 4. SO 3 5. CH 3 Cl