Question

Would the `PF_2^+` molecule be trigonal planar or bent?

Answer 1

Well, how many valence electrons has we got? And I think you are right.

Explanation:

There are `5(P)+2xx7(F)-1=19-1=18` `"electrons"`, which we distribute around 3 centres; i.e. 9 electron pairs........

And I make this: `F-ddotP^(+)-F`.

There are 3 regions of electron density around the central phosphorus atom: the two `P-F` bonds; and the lone pair. Electronic geometry is predicted to be trigonal planar; `/_F-P-F>120^@`, due to the disproportionate influence of the `P` lone pair.

Either your teacher got it wrong, or we have misinterpreted the problem. Both scenarios are possible.