Question

Which of the following species are hypervalent?
1. \[Cl{O_4}^ - \] 2. \[B{F_3}\] 3. \[S{O_4}^{2 - }\] 4. \[C{O_3}^{2 - }\]
A. 1, 2, 3
B. 1, 3
C. 3, 4
D. 1, 2

Answer 1

Hint: A hypervalent molecule or a hypervalent species is a compound in which the given molecule contains one or more main group elements, and have more than eight electrons in their valence shell. This phenomenon is also commonly known as ‘expanded octet’.

Complete Step-by-Step answer:
To determine which of the given molecules are hypervalent, let us first understand the molecular structures of these compounds:
 \[Cl{O_4}^ - \]

As we can observe in this structure, chlorine is forming 6 double bonds and 1 single bond. This brings the total number of electrons present in the valence shell of chlorine to be:
 \[ = {\text{ }}6{\text{ }}\left( 4 \right){\text{ }} + {\text{ }}1{\text{ }}\left( 2 \right){\text{ }} = {\text{ }}12\] electrons
Hence, the given species is hypervalent
 \[B{F_3}\]

As we can observe in this structure, boron is forming 3 single bonds. This brings the total number of atoms present in the valence shell of boron to be:
 \[ = {\text{ }}3\left( 2 \right){\text{ }} = {\text{ }}6\] electrons
Hence, the given species is not hypervalent
 \[Cl{O_4}^ - \]

As we can observe in this structure, sulphur is forming 2 double bonds and 2 single bonds. This brings the total number of electrons present in the valence shell of sulphur to be:
 \[ = {\text{ }}2{\text{ }}\left( 4 \right){\text{ }} + {\text{ }}2{\text{ }}\left( 2 \right){\text{ }} = {\text{ }}10\] electrons
Hence, the given species is hypervalent
 \[C{O_3}^{2 - }\]

As we can observe in this structure, carbon is forming 2 single bonds and 1 double bond. This brings the total number of atoms present in the valence shell of carbon to be:
 \[ = {\text{ }}2\left( 2 \right){\text{ }} + {\text{ }}1\left( 4 \right){\text{ }} = {\text{ }}8\] electrons
Hence, the given species is not hypervalent
Hence, we can conclude that, the species that are hypervalent are \[Cl{O_4}^ - \] and \[Cl{O_4}^ - \]

Hence, Option B is the correct option.

Note: Early considerations of the geometry of hypervalent molecules returned familiar arrangements that were well explained by the VSEPR model for atomic bonding. In order to account for the observed bond angles, bond lengths and apparent violation of the Lewis octet rule, several alternative models have been proposed.