Exchange of electrons in f orbital If an f orbital has 6 unpaired electrons then how many electrons can be exchanged. 
According to me if 2 electrons are added then a half filled f orbital would be formed which is stable (not as much as a completely filled orbital but still quite stable) however, the answer is supposedly 15.

If one knows the reason please do tell.
There are six unpaired electrons, and we can swap any two of them, so we have:
$${6\choose2} = 15$$