Explain why $PCl_5$ is trigonal bipyramidal whereas $IF_5$ is square pyramidal.

(N/A) $PCl_5$: Phosphorus $(Z=15)$ has the ground state electronic configuration $[Ne] 3s^2 3p^3$. In the excited state,one $3s$ electron is promoted to the $3d$ orbital,resulting in five unpaired electrons. Phosphorus undergoes $sp^3d$ hybridisation,leading to a trigonal bipyramidal geometry.
$IF_5$: Iodine $(Z=53)$ has the ground state valence shell configuration $5s^2 5p^5$. In the excited state,two electrons from the $5p$ orbitals are promoted to the $5d$ orbitals,creating five unpaired electrons for bonding and one lone pair. Iodine undergoes $sp^3d^2$ hybridisation. Due to the presence of one lone pair and five bond pairs,the geometry is square pyramidal.