Question

Among the following species which two have a trigonal pyramidal shape?
A. $${\text{N}}{{\text{I}}_{\text{3}}}$$
B. $${{\text{I}}_{\text{3}}}^{\text{ - }}$$
C. $${\text{S}}{{\text{O}}_{\text{3}}}^{{\text{2 - }}}$$
D. $${\text{N}}{{\text{O}}_{\text{3}}}^{\text{ - }}$$

Answer 1

Hint: The Lewis concept was not able to explain the shape of the molecules. We can identify the shape of the molecule by the Valence shell electron pair repulsion theory. This theory of valence shell has been based on the electron pair repulsion which is in the valence shell of the central atom.

Complete step by step answer:
The postulates for the VSEPR theory is the following:
The unpaired electrons which are present in the valence shell of the central atom have the tendency to form the bond with that of unpaired electrons of the surrounding atoms and those that are paired electrons remain as the lone pairs present in the valence shell of the central atom.
The electrons should be arranged in a way that the hybrid orbitals should lie far from each other so that they achieve maximum stability. The bond pair and lone pair repel each other.
The geometry and shape of the molecule is decided by the number of bond pairs and the lone pairs which surround the central atom.
The electron pairs should be arranged in a way that the repulsive forces must be minimum.
The trigonal pyramidal is a molecular shape in which three bond pairs and lone pairs are present around a central atom in a molecule. so the three bond pair and one lone pair is present in $${\text{N}}{{\text{I}}_{\text{3}}}$$ and $${\text{S}}{{\text{O}}_{\text{3}}}^{{\text{2 - }}}$$so the correct answer is option (I) and (III). So the shape of $${\text{S}}{{\text{O}}_{\text{3}}}^{{\text{2 - }}}$$ and $${\text{N}}{{\text{I}}_{\text{3}}}$$ will be

So, the correct answer is Option A,C.

Note: The shape of the molecule determines the boiling point and the melting point of the molecule. There are three different types of repulsion which are present and they are lone pair and lone pair repulsion, lone pair and bond pair repulsion and bond pair and bond pair repulsion. The bond pair bond pair is least repulsive.