Animated Solution for Chemistry - Chemical Bonding and Molecular Structure: Consider the following species : ${\text{SOCl}}_2$, ${\text{XeOF}}_4$, ${\text{ClF}}_3$, ${\text{ClF}}_5$, ${\text{XeF}}_5^{+}$, ${\text{SO}}_3^{2-}$, ${\text{XeF}}_3^{+}$, ${\text{SF}}_4$ List-I contains different molecular shapes and List-II contains total number of species with the same molecular shapes from the given species. Match each entry in List-I with the appropriate entry in List-II.

Imagine you are presented with a fascinating lineup of eight different molecules and ions: ${\text{SOCl}}_2$, ${\text{XeOF}}_4$, ${\text{ClF}}_3$, ${\text{ClF}}_5$, ${\text{XeF}}_5^{+}$, ${\text{SO}}_3^{2-}$, ${\text{XeF}}_3^{+}$, and ${\text{SF}}_4$. Your mission is to act as a molecular architect, determining their exact three-dimensional shapes and grouping them into specific geometric categories. This is a classic test of your chemical intuition and mastery over molecular bonding.

VSEPR Theory

To crack this puzzle, we must rely on our master tool: the Valence Shell Electron Pair Repulsion (VSEPR) theory. The core principle is beautifully simple: electron pairs around a central atom repel each other and will arrange themselves in space to be as far apart as possible.

The shape is dictated by the Steric Number (SN), which is simply the sum of the sigma bond pairs and the lone pairs on the central atom.

Let's systematically break down each species.

Let's start with the sulfur compounds. In thionyl chloride (${\text{SOCl}}_2$), sulfur is the central atom. It sits in Group 16, possessing 6 valence electrons. It forms one double bond with oxygen and two single bonds with chlorine atoms. This uses up 4 of its valence electrons, leaving 2 electrons that pair up to form exactly 1 lone pair.

With 3 bonding domains and 1 lone pair, the Steric Number is $3+1=4$. The foundational electron geometry is tetrahedral, but because we cannot "see" the lone pair when looking at the molecular shape, the remaining atoms form a Pyramidal structure.

Similarly, the sulfite ion (${\text{SO}}_3^{2-}$) features a central sulfur atom. The $2-$ charge effectively gives sulfur 8 valence electrons to work with. It forms 3 bonds with oxygen atoms, leaving 1 lone pair. Again, $\text{SN}=4$, resulting in another Pyramidal species. That gives us a total of two pyramidal molecules.

Now, let's look at the heavier, more exotic atoms: ${\text{XeOF}}_4$, ${\text{ClF}}_5$, and ${\text{XeF}}_5^{+}$. What do they have in common?

Take Xenon oxytetrafluoride (${\text{XeOF}}_4$). Xenon, a noble gas, brings 8 valence electrons. It forms a double bond with oxygen and four single bonds with fluorine. This uses 6 electrons, leaving exactly 1 lone pair.

Chlorine pentafluoride (${\text{ClF}}_5$) has chlorine (7 valence electrons) forming 5 single bonds, leaving 1 lone pair.

The xenon pentafluoride cation (${\text{XeF}}_5^{+}$) starts with Xenon (8 electrons), loses one due to the positive charge (leaving 7), and forms 5 bonds, again leaving 1 lone pair.

All three species have 5 bonding domains and 1 lone pair, giving a Steric Number of $5+1=6$. The base geometry is an octahedron. When one of the six positions is occupied by a bulky lone pair, the remaining five atoms form a striking Square Pyramidal shape. We have found three species for this category.

Moving on to chlorine trifluoride (${\text{ClF}}_3$) and the xenon trifluoride cation (${\text{XeF}}_3^{+}$). In ${\text{ClF}}_3$, chlorine uses 3 of its 7 valence electrons to bond, leaving 4 electrons, which form 2 lone pairs.

With 3 bonds and 2 lone pairs, the Steric Number is 5, corresponding to a trigonal bipyramidal electron geometry. Here is where the physics gets fascinating: the equatorial positions are roomier ($120^{\circ }$ apart) compared to the cramped axial positions ($90^{\circ }$). Because lone pairs demand more space, they exclusively occupy the equatorial positions. Hiding these two lone pairs reveals a distinct T-shaped geometry. ${\text{XeF}}_3^{+}$ follows the exact same logic. Thus, we have two T-shaped species.

Finally, let's examine sulfur tetrafluoride (${\text{SF}}_4$). Sulfur uses 4 of its 6 valence electrons to bond with fluorine, leaving 1 lone pair. With a Steric Number of 5 and only one equatorial lone pair, the resulting molecular shape is a See-saw. This is the only species in our list with this unique shape.

Let's bring it all together and match our findings to the lists provided in the problem:

(P) See-saw: We found exactly 1 species (${\text{SF}}_4$). So, P matches with (1). (Q) T-Shaped: We found 2 species (${\text{ClF}}_3,{\text{XeF}}_3^{+}$). So, Q matches with (2). (R) Trigonal Planar: Did we find any? No, zero. So, R matches with (5). (S) Square Pyramidal: We found 3 species (${\text{XeOF}}_4,{\text{ClF}}_5,{\text{XeF}}_5^{+}$). So, S matches with (3).

The puzzle is perfectly solved. Always remember that while VSEPR gives us these beautiful idealized shapes, the strong repulsive force of lone pairs will slightly compress the actual bond angles in reality!