Question

Which of the following species do not exist?

• A. $F_3^{-}$
• B. $I_3^{-}$
• C. ${PH}_5$
• D. ${PF}_5$

Answer 1

Answer: (A), (C)

The correct options are
A $F_3^{-}$
C ${PH}_5$

$F_3^{-}$ and $P{H}_5$ species does not exist.
In $F_3^{-}$ does not form a Lewis structure because fluorine has 7 valance electrons.$F_3^{-}$ totals 22 valance electrons. To bond 3 $F$ atoms together in a Lewis structure there is only space for 20 valance electrons. Remaining 2 electrons have to be fit in the near by $d$ orbital. Fluorine being a period 2 element does not have a $d$ orbital and can not form a $F_3^{-}$.
In case of $P{H}_5$, hydrogen is only very slightly more electronegative than Phosphorus so does not pull significantly on the surrounding electronic charge. Phosphorus pentahydride is a very highly unstable compound and $P{H}_5$ does not form.