Question

why PH3 is possible but PH5 is not

Answer 1

In case of PH5, The P atom uses sp3d hybrid orbitals. Since d orbital has higher energy than s and p orbitals. To hybridise d orbital with them, its energy should be equivalent to s and p. The F and Cl has ability to make the s, p and d orbitals equivalent in energy due to their higher electronegativity. But due to lower electronegativity of hydrogen, d orbital could not be hybridised with s and p. So sp3d hybridisation is not possible, Hence PH5 does not exists while PCl5 is possible.

PH3 is possible because only s and three p orbitals are used which can be hybridise easily due to their equal energy.

Answer 2

Answer:

To understand it first we need to understand why PCL₅ Exists but PH₅ does not :

1. Before forming bonds with CL or H, P atom needs to hybridize its orbitals to SP³D by exciting its electrons so that it can easily make bonds  and share 5 electrons
2. But, the excitation of electrons of central atom only happens if the approaching atom is more electronegative than the Central Atom
3. Since CL is more Electronegative than P, P can excite its electrons and make 5 SP³D orbitals to bond with CL. BUT since H is not more electronegative than P, P cannot excite its electrons. Hence it cannot form bonds with 5 H atoms AND IT CANNOT HYBRIDIZE ITS ORBITALS ACCORDING TO DRAGO'S RULE.

Now, when P cant excite its electrons while bonding with H, electronic Config. of P before bonding with H will be 1S²2S²2P⁶3S²3P³ . Hence an unhybridized P atom will need 3 more electrons to become more stable. Hence instead of bonding with 5 H atoms, it only bonds with 3 H atoms and attains stability.

And that is the reason, my fellas PH₃ exists but PH₅ does not.