Question

There are five electron pairs in outer shell, then structure and bond angle as predicted by the Sidgwick- Powell theory is:
A.Octahedron, ${90^{\text{o}}}$
B.Trigonal bipyramidal, ${120^{\text{o}}}$ and ${90^{\text{o}}}$
C.Pentagonal bipyramidal, ${72^{\text{o}}}$ and ${90^{\text{o}}}$
D.Tetrahedron, ${109^{\text{o}}}28'$

Answer 1

To answer this question you must recall the Sigdwick- Powell theory. The Sigdwick Powell theory is commonly known as the VSEPR theory. The Valence shell electron pair repulsion theory proposes that the hybridized orbitals in an atom arrange themselves in such a way so as to minimize the repulsion between them, hence determining the geometry of a molecule on the basis of its hybridization.

Complete answer:
The Sigdwick- Powell theory or the Valence shell electron pair repulsion theory proposes that all valence shell electrons surrounding the central atom arrange themselves in such a manner so as to be as far away from each other as possible to avoid repulsion between the electron pairs.
If there are 5 valence electrons in a given compound, assuming in this case that the given atom has vacant $d -$ orbitals in its valence shell and thus, it can excite its electrons to achieve a maximum covalency of 5. So the atom will undergo a hybridization of $s{p^3}d$. The structure of the molecule will be trigonal bipyramidal. Three pairs lie in the equatorial plane at an angle of ${120^{\text{o}}}$ and two bonds lie in axial plane forming an angle of ${90^{\text{o}}}$

Thus, the correct answer is B.

Note:
The concept of mixing of atomic orbitals in order to form new hybrid orbitals that possess different shapes and energies as compared to the original parent atomic orbitals is known as hybridisation. Hybrid orbitals are suitable to form chemical bonds of equal energies. Also hybridization of orbitals leads to the formation of more stable compounds because hybrid orbitals have lower energy than the unhybrid orbitals.