VSEPR and Polarity

3.091: Introduction to Solid State Chemistry, Fall 2018 Recitation 9 1 VSEPR Lewis dot structures are a great tool to visualize how electrons can be arranged in molecules. Further, drawing resonance structures and determining the formal charge can help us determine which structures are most stable. However, neither of these tools provide much insight into the physical configuration of a molecule in 3D space. For this, we turn to Valence Shell Electron Pair Repulsion theory, or VSEPR. Once we draw a viable Lewis structure, we can use the following chart to translate the 2D representation to a 3D geometry: Chart courtesy of Boundless.com. License: CC BY-SA. This content is excluded from our Creative Commons license. For more information, see https://ocw.mit.edu/fairuse. Example: Draw Lewis dot diagrams and determine the 3D VSEPR geometry of the following molecules: CH4 , NH3 , H2 O, SO3 , SO2 , CO2 1 3.091: Introduction to Solid State Chemistry, Fall 2018 Recitation 9 Lewis dot diagram Electrons around central atom VSEPR description CH4 -4 groups of electrons in bonds -0 lone pairs tetrahedral NH3 -3 groups of electrons in bonds -1 lone pair trigonal pyramidal H2 O -4 groups of electrons in bonds -0 lone pairs bent SO3 -4 groups of electrons in bonds -0 lone pairs trigonal planar SO2 -4 groups of electrons in bonds -0 lone pairs bent CO2 -4 groups of electrons -0 lone pairs linear 2 Sketch of 3D model 3.091: Introduction to Solid State Chemistry, Fall 2018 Recitation 9 2 Polarity The difference in electronegativity across a molecule can generate electric dipole moments. Dipole moments are vector quantities, and by convention point from a more positive region of charge to a more negative region. If individual dipoles within a molecule cancel, there is no net dipole. Example: Determine whether CO2 and H2 O have a net dipole moment. In CO2 , the two electronic dipoles are exactly opposite and cancel each other, so there isn’t a net dipole. Carbon dioxide is not a polar molecule. In H2 O, the electronic dipoles don’t fully cancel, so there is a net dipole moment. Water is a polar molecule! 3