NEET Chemistry Chemical Bonding and Molecular Structure / रासायनिक आबंधन एवं आणविक संरचना NEET PYQ-Chemical Bonding Molecular

  • question_answer
    Among the following, the pair in which the two species are not isostructural, is:                                    [AIPMT (S) 2004]

    A)  \[Si{{F}_{4}}\] and \[S{{F}_{4}}\]       

    B)  \[IO_{3}^{-}\] and \[Xe{{O}_{3}}\]

    C)  \[B{{H}_{4}}\] and \[NH_{4}^{+}\]

    D)       \[PF_{6}^{-}\] and \[SF_{6}^{{}}\]

    Correct Answer: A

    Solution :

    \[Si{{F}_{4}}\] and \[S{{F}_{4}}\] are not is structural because \[Si{{F}_{4}}\] is tetrahedral due to \[\text{s}{{\text{p}}^{3}}\]hybridisation of Si.
    \[_{14}Si=1{{s}^{2}},2{{s}^{2}},2{{p}^{6}},3{{s}^{2}}3{{p}^{2}}\] (In ground state)
    \[_{14}Si=1{{s}^{2}},2{{s}^{2}},2{{p}^{6}},3{{s}^{2}}3{{p}^{2}}\] (In excited state)
    \[_{14}Si=1{{s}^{2}},2{{s}^{2}},2{{p}^{6}},3{{s}^{1}}3{{p}^{3}}\] (In excited state)
    Hence, four equivalent\[s{{p}^{3}}\]-hybrid orbitals are obtained and they are overlapped by four p-orbitals of four fluorine atoms on their axes.
    Thus it shows following structure :
    While \[S{{F}_{4}}\] is not tetrahedral but it is distorted tetrahedral because in it S is \[s{{p}^{3}}\] d-hybrid.
    \[_{16}S=1{{s}^{2}},2{{s}^{2}}2{{p}^{6}},3{{s}^{2}}3p_{x}^{2}3p_{y}^{1}3p_{z}^{1}\]
    (In ground state)
    \[=1{{s}^{2}},2{{s}^{2}}2{{p}^{6}},3{{s}^{2}}3p_{x}^{1}3p_{y}^{1}3p_{z}^{1,}3d_{xy}^{1}\]
    \[s{{p}^{3}}\] d-hybridisation
    (In first excitation state)
    Hence, five \[s{{p}^{3}}\] d-hybrid orbitals are obtained.
    One orbital is already paired and rest four are overlapped with four p-orbitals of four fluorine atoms on their axis in trigonal bipyramidal form. This structure is distorted from trigonal bi-pyramidal to tetrahedral due to involvement of repulsion between lone pair and bond pair.


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