Why is a pi bond weaker than sigma bond?

1 Answer
Truong-Son N.
Aug 31, 2015

A #pi# bond is indeed weaker than a #sigma# bond of the same #n#.

A #sigma# bond is made with colinear atomic orbital orientations (in the same axis), in any of the following ways:

  • An #ns# orbital with another #ns# orbital.
  • An #ns# orbital with an #np_z# orbital.
  • An #np_z# with an #np_z# orbital.

This head-on overlap allows for a nice increase in the electron density for the generated #sigma# molecular orbitals.

In contrast, a #pi# bond is made using parallel atomic orbital orientations:

  • An #np_x# orbital with another #np_x# orbital.
  • An #np_y# orbital with another #np_y# orbital.

This parallel overlap does not allow for as much of an increase in electron density as in the colinear overlaps that generate #sigma# molecular orbitals. Greater orbital overlap makes for stronger bonds.

You can see these overlaps here:

http://images.tutorvista.com/

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