11th Class Chemistry Chemical Bonding and Molecular Structure

  • question_answer 109)   Comprehension given below is followed by some multiple choice questions. Each question has one correct option. Choose the correct option. Molecular orbitals are formed by overlap of atomic orbitals. Two atomic orbitals combine to form two molecular orbitals called bonding molecular orbital (BMO) and antibonding molecular orbital (ABMO). Energy of antibonding orbital is raised above the parent atomic orbitals that have combined and the energy of bonding orbital is lowered than the parent atomic orbitals. Energies of various molecular orbitals for elements hydrogen   to   nitrogen   increase   in   the   order \[\sigma 1s<{{\sigma }^{*}}1s<\sigma 2s<{{\sigma }^{*}}2s<\]\[(\pi 2{{p}_{x}}\approx \pi 2{{p}_{y}})\]\[<\sigma 2{{p}_{z}}<({{\pi }^{*}}2{{p}_{x}}\approx {{\pi }^{*}}2{{p}_{y}})<{{\sigma }^{*}}2{{p}_{z}}\] and for oxygen and fluorine order of energy of molecular orbitals is given below : \[\sigma 1s<{{\sigma }^{*}}1s<\sigma 2s<{{\sigma }^{*}}2s<\sigma 2{{p}_{z}}<\] \[(\pi 2{{p}_{x}}\approx \pi 2{{p}_{y}})<({{\pi }^{*}}2{{p}_{x}}\approx {{\pi }^{*}}2{{p}_{y}})<{{\sigma }^{*}}2{{p}_{z}}.\] Different atomic orbitals of one atom combine with those atomic orbitals of the second atom which have comparable energies and proper orientation. Further, if the overlapping is head on, the molecular orbital is called 'sigma' \[(\sigma )\] and if the overlap is lateral, the molecular orbital is called 'pi', (\[\pi \]). The molecular orbitals are filled with electrons according to the same rules as followed for filling of atomic orbitals. However, the order for filling is not the same for all molecules or their ions. Bond order is one of the most important parameters to compare the strength of the bonds. Which of the following statements is correct? (a) In the formation of dioxygen from oxygen atoms 10 molecular orbitals will be formed. (b) All the molecular orbitals in the dioxygen will be completely filled. (c) Total number of bonding molecular orbitals will not be same as total number of antibonding orbitals in dioxygen. (d) Number of filled bonding orbitals will be same as number of filled antibonding orbitals.

    Answer:

      (a) Oxygen atom has configuration \[\frac{1{{s}^{2}},2{{s}^{2}},2p_{x}^{2}2p_{y}^{1}2p_{z}^{1}}{5\,orbitals}\] Two oxygen atoms overlapping five atomic orbitals each and thus, forming ten molecular orbitals.


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