# Which element should be placed in axial position bulky one or the smaller one?

In one question for finding $\ce{P(CH3)2H3}$ structure? Which one should be placed in axial position? I think $\ce{-CH3}$ should be axial position to minimise repulsion.

According to Inorganic chemistry (Meissler and Tarr), electronegativity and symmetry actually play a huge role. More electronegative groups tend to occupy axial positions, even if they are smaller, and symmetry is always preferred. For an example of the role of symmetry consider, $\ce{PF3(CF3)2}$ ($\ce{-CF3}$ are axial) vs $\ce{PF2(CF3)3}$ ($\ce{F}$ are axial). For an example of electronegativity being more important than size look at $\ce{PCl3F2}$ which has the $\ce{F}$ at the axial positions.
In this case, $\ce{-CH3}$ groups are both bigger, more electronegative, and putting them at axial positions will be symmetric (and cause the net dipole to cancel); with all these considerations, the $\ce{-CH3}$ groups will be axial.