Explain the following
(a) `CO_(2)` is a gas whereas `SiO_(2)` is a solid
(b) Silicon forms `SiF_(6)^(2-)` ion whereas corresponding fluoro compound of carbon is not known.

Explanation

(i) F or answer, consult section 11.11.
(ii) Silicon atoms has vacant 3d orbtials in its valence shell. It is in a position to accept electron pairs from six `F^(-)` ions and expand its co-ordination number upto 6. Therefore, the compound `SiF_(6)^(2-)` can exist. on the other hand, carbon has no vacant 3d orbitals and cannot expand its co-ordination number upto six. tehrefore, the compound `CF_(6)^(2-)` does not exist.