Polarity Guided Inquiry (Textbook 10.4) Essential Questions How do the bonds between atoms, as well as electron location, determine interactions between molecules? How do electronegativity and molecular shape determine the polarity of a molecule? How does Bond Polarity affect Polarity? Molecule Polarity is a measure of how electrons are distributed in the outer regions of the molecule. It is similar to bond polarity in that it is a measure of how unequally electrons are shared. It is different than bond polarity because bond polarity is a measure of how unequally the electrons are shared in a bond, while molecular polarity is a measure of how unequally the electrons are distributed throughout the outer region of the entire molecule, not just a particular bond. polarity is the sum of all the bond polarities. A highly polar molecule will have an uneven distribution of electrons around the outer regions of the molecule, which will result in areas of positive and negative charges. The image on the left shows water, H2O, with two highly polar O- H bonds. The negative ends of dipoles are pointing toward the oxygen. This coupled with the asymmetrical (bent) shape of the water molecule causes an uneven distribution of electrons throughout the outer regions of the molecule. The image on the right shows the electron density for water. Notice that the density is much higher on the oxygen side of the molecule than at either of the hydrogen sides. This results in a partial negative charge at the oxygen side and partial negative charges at the hydrogen sides. A non- polar molecule has the electrons uniformly distributed around the outer edges. A non- polar molecule can have polar bonds if the bonds are arranged symmetrically so the electrons are distributed uniformly throughout the outer regions of the molecule. Carbon tetrafluoride, CF4, is an example of a non- polar molecule with 4 highly polar bonds. The image on the left shows CF4 with four highly polar C- F bonds. The image on the right shows the electron density of the CF4 molecule. The electron density is uniform throughout the outer regions of the molecule. That s because the 4 strong C- F bond dipoles are arranged symmetrically in opposite directions. So they cancel each other out. The net result is that there are no areas of positive and negative partial charges in the outer region of the molecule.
Molecule Polarity Simulation w/ 3 Atoms Run the Molecule Polarity simulation from the PhET web site: http://phet.colorado.edu/en/simulation/molecule- polarity. Select the Three Atom tab at the top of the screen. Set the View parameters to show the bond dipole, molecular dipole and partial charges. Turn the electric field off. Run the simulation with different combinations of electronegativities for atoms A, B and C as specified in the data table below. Watch what happens to the size and direction of the bond dipoles, molecular dipoles and partial charges. Record your observations in the data table. Use your results to answer the questions. Use the key below to help you run the simulation. Electronegativity Settings: less, middle and more Bond Dipole & Dipole Strength: zero, medium and strong Direction: north, south, east, west, northeast, northwest, southeast and southwest Electronegativity Settings A B Bond Dipole C B Bond Dipole Dipole Atom A Atom B Atom C Strength Direction Strength Direction Strength Direction Less Less Less Less Less Middle Less Less More Less Middle Less Less Middle Middle Less Middle More Middle Less Less Middle Less Middle Middle Less More Middle Middle Less Middle Middle Middle Middle Middle More Middle More Less Middle More Middle Middle More More More Middle Less More Middle Middle More Middle More
1. What combination of electronegativities created a non- polar molecule? 2. What combinations of electronegativities created a moderately polar molecule? 3. What combinations of electronegativities created a highly polar molecule? 4. How do changes in the strength of the individual bond dipoles affect the strength and direction of the molecular dipole? 5. How do changes in the direction of the individual bond dipoles affect the strength and direction of the molecular dipole? 6. What is the relationship between the molecular dipole and the individual bond dipoles?
Molecule Polarity Simulation w/ Real Molecules http://phet.colorado.edu/en/simulation/molecule- polarity In the previous simulation with 3 atoms the molecular symmetry was always the same, bent. In the real molecule simulation we will be able to see how molecular shape affects molecular dipole. Select the Real Molecules tab at the top of the screen. Pick 5 molecules from the list that have the following shapes: linear, bent, trigonal planar, trigonal pyramidal and tetrahedral. Complete the information below for each of the molecules. Dipoles and Dipoles Linear Bent Trigonal planar Trigonal pyramidal
Dipoles and Dipoles tetrahedral Pick 5 other molecules from the simulation list that represent the 5 molecular shapes, but do not run their simulations. Draw their electron- dot notation, predict their shape, use the electronegativity periodic table to determine their bond polarity and then sketch the molecule showing the correct shape, bond dipoles and molecular dipole. Then run the simulation to check your answers. Dipoles and Dipoles
Dipoles and Dipoles 7. Is it possible to have a polar molecule that is made up of non- polar bonds? Explain why. 8. Is it possible to have a non- polar molecule that is made up of polar bonds? Explain why.