question_answer 1)

  Explain the following : (a) \[C{{O}_{2}}\] is a gas whereas \[Si{{O}_{2}}\] is a solid. (b) Silicon forms \[SiF_{6}^{2-}\] ion whereas corresponding fluoro compound of carbon is not known.

Answer:

  (a) Because of its small size and good \[\pi \]-overlap with other small atoms, carbon forms strong double bonds with two oxygen atoms to give discrete \[C{{O}_{2}}\] molecules. Silicon atom, on account of large size, does not have good\[\pi \]-overlap with other atoms. It uses its four valence electrons to form four single bonds directed towards the four apices of a tetrahedron (\[s{{p}^{3}}\]-hybridization). Each oxygen is linked with two silicon atoms, i.e., a giant three dimensional structure comes into existence which is very stable. Thus, \[C{{O}_{2}}\] is a gas and \[Si{{O}_{2}}\] is a solid. (b) Silicon has 3d-orbitals in the valence shell and thus expands its octet giving \[s{{p}^{3}}{{d}^{2}}\]-hybridization while d-orbitals are not present in the valence shell of carbon. It can undergo sp3 -hybridization only. Thus, carbon is unable to form \[CF_{6}^{2-}\] anion.