A single bond is almost always a sigma bond, and a double bond is almost always made up of a sigma bond and a pi bond. There are very few exceptions to this rule. Which of the following species have violated this generalization?

A. N2

B. O2

C. F2

D. C2

E. B2.

The exception here is

C2 and B2

both the bonds are pi bond in the above cases

Explanation:

The fault here is the unforeseen energy order of molecular orbitals. Since if filled, sigma 2px will be firmly pushed away by the electron density of orbitals already filled with sigma 2s (both electron densities lie on the inter-nuclear axis). Because of this, sigma 2px's energy is found shockingly greater than pi 2py and pi 2pz.

The electrons therefore occupy pi 2py and pi 2pz orbitals as opposed to normal sigma 2px (which remains vacant) while filling.

The existence of these 4 electrons in orbitals pi 2py and pi 2pz results in two pi bonds being formed.