Vsepr Theory And Molecular Geometry

PCl_5 adopts a trigonal bipyramidal geometry due to sp^3d hybridisation of phosphorus. In this structure, three chlorine atoms occupy equatorial positions (in the horizontal plane, separated by 120 degrees from each other) and two chlorine atoms occupy axial positions (above and below the equatorial plane, at 90 degrees to the equatorial bonds). The axial bonds are longer (219 pm) than the equatorial bonds (204 pm). The reason lies in the different repulsion environments: each axial bond pair experiences repulsion from three equatorial bond pairs at 90 degrees (where repulsion is most severe) as well as from the other axial bond at 180 degrees (which is negligible due to the linear arrangement). In contrast, each equatorial bond pair has repulsion only from two axial bond pairs at 90 degrees and two equatorial bond pairs at 120 degrees. Since 90-degree repulsions are stronger than 120-degree repulsions, axial bonds face more total repulsion, forcing electrons outward and lengthening the bonds. Option 'Less steric hindrance from fluorine substituents' is irrelevant because PCl_5 has chlorines, not fluorines. Option 'Greater pi character' is incorrect; sp^3d bonds do not have significant pi character. Option 'Shorter bond due to ionic character' is factually wrong; the axial bonds are longer, not shorter, due to greater repulsion. This is an NCERT Class 11 concept tested regularly in JEE. Plausibility: a longer bond corresponds to a weaker bond, consistent with the reduced stability of axial Cl in PCl_5 compared to equatorial Cl.