Question

The shape of \[XeF_5^ + \]ion is:
A. Pentagonal
B. Octahedral
C. Square pyramidal
D. Trigonal bipyramidal

Answer 1

Hint: To find out the hybridisation of any molecule, one needs to find out the steric number of the central atom using its formula. Steric number is the number of sigma bonds on the central atom plus the number of lone pairs on an atom.

Complete step-by-step answer:
Steric number is equal to ½ (number of valence electrons + number of singly bonded species with the atom whose hybridisation to be determined - cationic charge on molecule + anionic charge on molecule).
In \[XeF_5^ + \], number of valence electrons of xenon atom are 8 as its electronic configuration is \[[Kr]4{d^{10}}5{s^2}5{p^6}\]. To the central atom xenon, five fluoride atoms are singly bonded by sigma bonds. So, the number of singly bonded species with the xenon atom whose hybridisation to be determined is five. The molecule has cationic charge of +1.
Putting all these values in the formula of steric number, we get
Steric number = \[\dfrac{1}{2}(8 + 5 - 1)\]= 6
For molecules having steric number six, have the hybridisation of \[s{p^3}{d^2}\]. It can either be an octahedral geometry or square bipyramidal geometry. Since the molecule \[XeF_5^ + \]has only five sigma bonds, its geometry is square pyramidal.

Hence, the correct option is (C).

Note: We can have confusion as to why the molecular geometry of \[XeF_5^ + \] is not trigonal bipyramidal even if it has five bond pairs. This is because the central atom xenon has a lone pair too along with those bonded pairs. The lone pair causes repulsion and results in square pyramidal.