Structure of AsF3Cl2 Since the molecule has 5 bonding pairs and zero lone pairs, the shape should be trigonal bipyramidal. But one thing i am confused about is which atom(s) will occupy the axial and the planar positions, respectively, and why? 

Thanks
The axial positions see 3 substituents at 90 degrees, while the equatorial positions only see 2.

To first order, it is generally favorable to place the smaller substituents in the axial positions. Unfortunately, when you have fluorine, higher order effects (like minimizing dipole moment) may overcome steric considerations, but I would still prefer to place the fluorines axially as my first guess.