Ncert Exercises & Solutions

Explain the following

1. CO₂ is a gas whereas SiO₂ is a solid.

2. Silicon forms ${SiF}_{6}^{2 -}$ ion whereas corresponding fluoro compound of carbon is not known.

(a) CO₂ has a linear structure. Its dipole moment is zero. It is believed that CO₂ molecule is a resonance hybrid has the following structures.

O = C = O \leftrightarrow O^{-} - C \equiv O^{+} \leftrightarrow O^{+} \equiv C - O^{-}

The CO₂ molecules are held together by weak van der Waals' forces and thus, it exists as gas. In SiO₂, due to large electronegative difference between Si and O, the Si-O bonds have considerable ionic nature.

Therefore, silica has three dimensional network like structure in which Si-atom is tetrahedrally bonded to four oxygen atoms and each oxygen atom is bonded to two silicon atoms by covalent bonds.

There is not discrete SiO₂ molecule. It is a network solid with octahedral coordination.

(b) In silicon, vacant 3d orbitals are available due to which it can accommodate electrons from 6 fluorine atoms, thereby forming

{SiF}_{6}^{2 -}

ion. However, in case of C only 2p² filled orbitals are available thus, it cannot expand their covalency more than 4. Thus,

{CCl}_{6}^{2}

is not known.

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