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Question

Explain the following:


A) CO2 is a gas whereas SiO2 is a solid.

B)Silicon forms [SiF6]2 ion whereas the corresponding fluoro compound of carbon is not known.


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Solution

a)
CO2 is a linear and nonpolar molecule.

• It has a very weak intermolecular forces of attraction (i.e. Van Der Waals forces) and therefore exists in gaseous form.

SiO2 is a covalent three-dimensional network solid is whether each Silicon atom is covalently bonded in a tetrahedral manner to 4 oxygen atoms.

• Each O Atom in turn is covalently bonded to another Si as shown in the diagram:


• Like C, Si cannot form pπpπ bonding with other atoms and therefore forms 4 covalent bonds with O atoms

• Thus, the entire structure may be considered as a giant molecule forming a solid structure.


b)

•Valence shell of C atom does not have any vacant d-orbitals and therefore cannot expand its octet by increasing its covalency beyond4 covalent bonds.

• Si has an empty 3d orbital and can accept lone pairs of electrons to expand its covalency beyond 4 bonds.

• Thus, Si can accept electrons from F ion to form 6 bond and exist as[SiF6]2. Valence shell of C atom does not have any vacant d-orbitals and therefore cannot expand its octet by increasing its covalency beyond 4 covalent bonds.

Si has an empty 3d orbital and can accept lone pairs of electrons to expand its covalency beyond 4 bonds.

•Valence shell of C atom does not have any vacant d-orbitals and therefore cannot expand its octet by increasing its covalency beyond 4 covalent bonds. • Si has an empty 3d orbital and can accept lone pairs of electrons to expand its covalency beyond 4 bonds. • Thus, Si can accept electrons from F ion to form 6 bond and exist as [SiF6]2. Thus, Si can accept electrons from F ion to form 6 bond and exist as [SiF6]2.

• Also, the smaller size of C cannot accommodate the six F atoms around it.

• Due to all these factors, C is unable to form [CF6]2 anion.




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