(Polar molecules, Non-polar molecules, etc.)
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Hi! What does Dr. Lavelle mean by sigma bonds having an electron density with a cylindrical symmetry around the internuclear axis and pi bonds having an electron density on each side of the internuclear axis? Where would the internuclear axis be present?
I believe that when Dr. Lavelle mentions cylindrical symmetry he is just simply demonstrating to us the fact that if you were to rotate an atom with a sigma bond, the bond will not be broken because they are connected to each other end-to-end and are in constant alignment. I like to think of it as if you were placing two of the exact same fans face to face with each other. The axis where the center of the fans meets is the internuclear axis where the sigma bond is formed, and the blades connecting to them are the orbitals that would form the pi bonds. No matter how much you spin the fan blades, the centers of the fans do not move and are representative of the fact that rotations do not break a sigma bond because it is the axis in which the fan blades are being spun, the internuclear axis. However, only when the fan blades are precisely aligned with one another can pi bonds be formed. If the blades are spun out of alignment, those pi bonds would be broken. This results from the fact that fan blades lie outside of the cylindrical symmetry that comes from bonding at the internuclear axis.
Also, since both orbital electron densities in a pi bond are in alignment, this creates an overlap beyond the internuclear axis that is not within these densities. In the sigma bond, the overlap of the orbitals occurs within the axial plane in which the nuclei sit on.
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