Question

Explain why $PC{l_5}$ is trigonal bipyramidal and $I{F_5}$ is square pyramidal.

Answer 1

The geometry of a molecule can be determined by the shape of a molecule. It is the arrangement of the atoms and the bonds in a molecule. The shapes of the molecules can be bent, Seesaw, trigonal pyramidal, T- Shaped, square pyramid, square planar, etc. according to the arrangement of atoms and bonds in the molecules. Also, the shapes can be determined based on the hybridization of the molecule. Various theories are there to determine the shape of the molecule (For example- VSEPR Theory, Molecular orbital theory, Linear combination of atomic orbitals).

Complete step by step answer:
The phosphorus atom in $PC{l_5}$ has five valence electrons in its outermost shell i.e. $3{s^2}3{p^3}$ and to make five bonds with five chlorine atoms, one of its electrons will get excited from 3s to 3d due to which the hybridization will be $s{p^3}d$. The $s{p^3}d$ hybridization state gives the trigonal bipyramidal geometry. But in $I{F_5}$ molecule the iodine atom has seven valence electrons in its outermost shell i.e. $5{s^2}5{p^5}$ and to make five bonds with the fluorine atoms, two of its electron gets excited from 5p to 5d orbital due to which the hybridization will be $s{p^3}{d^2}$. The $s{p^3}{d^2}$ hybridization state gives the square pyramidal geometry.

Additional Information:
There are three different types of hybridization involved in the case of carbon.
$s{p^3}$(tetrahedral) hybridization- When one 2s-orbital and three 2p-orbitals of excited carbon get intermixed to form four hybrid orbitals then these hybrid orbitals are known as $s{p^3}$hybrid orbitals. This phenomenon is called $s{p^3}$ hybridization.
$s{p^2}$ (Trigonal) hybridization- When 2s-orbital and two p- orbitals $\left( {2{p_{x\,}}\,,\,2{p_y}} \right)$ of excited carbon atom gets intermixed to form three hybrid orbitals, then these orbitals are known as $s{p^2}$ hybrid orbitals. This phenomenon is known as $s{p^2}$ hybridization.
$sp$ (Linear or diagonal) hybridization- When one 2s- orbital and one p-orbital $\left( {2{p_x}} \right)$ of the excited carbon atom get intermixed to give two hybrid orbitals, then these hybrid orbitals are known as $sp$ hybrid orbitals. This hybrid orbital is known as $sp$ hybridization.

Note: The concept of intermixing of the orbitals of an atom which have nearly the same energy to give the same equivalent orbitals with the same energy, identical shapes, and symmetrical orientations in space is known as hybridization.