Question

The ${{\text{I}}_{\text{3}}}^ -$ ion has-
A.Five equatorial lone pairs on the central I atom and two axial bonding pairs in a trigonal bi-pyramidal arrangement.
B.Five equatorial lone pairs on the central I atom and two axial bonding pairs in a pentagonal bi-pyramidal arrangement
C.Three equatorial lone pairs on the central I atom and two axial bonding pairs in a trigonal bi-pyramidal arrangement.
D.Two equatorial lone pairs on the central I atom and three axial bonding pairs in a trigonal bi-pyramidal arrangement

Answer 1

Notice that in the trigonal bi-pyramidal structure only $3$ equatorial lone pairs along with $2$ axial bonding pairs are present. Iodine electronic configuration is ${\text{n}}{{\text{s}}^{\text{2}}}{\text{4}}{{\text{p}}^{\text{5}}}$.

Complete step by step answer:
Let’s start with discussing the options that are provided to us, the first option that is five equatorial lone pairs on the central I atom and two axial bonding pairs in a trigonal bi-pyramidal arrangement. It is to be noticed that in trigonal bi-pyramidal structure only $3$ equatorial lone pair along with $2$ axial bonding pairs are present, so the option A is incorrect.
Similarly in the fourth option the information provided to us is not right. So, option D. is also incorrect.
Now the second and third option is left with us. Now the molecule given to us is ${{\text{I}}_{\text{3}}}^ -$, and the outer electronic configuration of if I is ${\text{4}}{{\text{s}}^{\text{2}}}{\text{4}}{{\text{p}}^{\text{5}}}$ and central atom is having $2$ bonds with one electron acceptance. So its electronic configuration will become ${\text{4}}{{\text{s}}^{\text{2}}}{\text{4}}{{\text{p}}^{{\text{3 + 1}}}}$ and $2$ bonds. So clearly $3$ lone pairs along with $2$ bond pairs are present. Hence the answer will be three equatorial lone pairs on the central I atom and two axial bonding pairs in a trigonal bi-pyramidal arrangement.

$\therefore$ Option C is correct .

Note:
Since we know that, the trigonal bi-pyramidal molecular geometry is the one with surrounding atoms and no lone pair. Three of the angles are at ${120^ \circ }$ and the other two are in between them in the atom's axis. So it’s like one pyramid up and one pyramid below, hence the name is trigonal bi-pyramidal.