6.9: VSEPR and Polarity
- Page ID
- 408819
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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:
\(\mathrm{CH}_4, \mathrm{NH}_3, \mathrm{H}_2 \mathrm{O}, \mathrm{SO}_3, \mathrm{SO}_2, \mathrm{CO}_2\)
- Answer
-
Lewis dot diagram Electrons around central atom VESPR description Sketch of 3D model \(\mathrm{CH}_4\) 
- 4 groups of electrons in bonds
- 0 lone pairs
tetrahedral 
\(\mathrm{NH}_3\) 
-3 groups of electrons in bonds
-1 lone pair
trigonal pyramidal 
\(\mathrm{H}_2\mathrm{O}\) 
- 4 groups of electrons in bonds
- 0 lone pairs
bent 
\(\mathrm{SO}_3\) 
- 4 groups of electrons in bonds
- 0 lone pairs
trigonal planar 
\(\mathrm{SO}_2\) 
- 4 groups of electrons in bonds
- 0 lone pairs
bent 
\(\mathrm{CO}_2\) 
- 4 groups of electrons
- 0 lone pairs
linear 
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 \(\mathrm{CO}_2\) and \(\mathrm{H}_2\mathrm{O}\) have a net dipole moment.
- Answer
-
In \(\mathrm{CO}_2\), 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 \(\mathrm{H}_2\mathrm{O}\), the electronic dipoles don’t fully cancel, so there is a net dipole moment. Water is a polar molecule!