Ncert Exercises & Solutions

Carbon and silicon both belong to the group 14, but inspite of the stoichiometric similarity, the dioxides, (i.e., carbon dioxide and silicon dioxide), differ in their structures. Comment.

All members of group 14 form dioxides of formula MO₂ i.e., for carbon CO₂ and silica SiO₂. CO₂ has a linear structure since, its dipole moment is zero. Both the oxygen atoms are linked by double bonds. The C-atom is sp hybridised.

{O = C = O}_{sp - p (σ) sp - p (σ)}^{p - p (π) p - p (π)}

However, C-O bond length is 1.15 Å, which is less than calculated value of C=O bond. Hence, it shows resonance hybrid of following structures.

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

But, the structure of silica is entirely different form that of CO₂.Si-O bonds have a considerable ionic character due to large electronegative difference between Si and O. As a result, silica has a three dimensional structure in which silica atom is tetrahedrally bonded to four oxygen atoms and each oxygen atom is bonded to two silicon atoms by covalent bonds.

There is no discrete SiO₂ molecule but entire crystal as a gian molecule. Si-O bonds are strongly making it solid having high melting point.

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