ph3 exit but ph5 does why?and how we say this form of compond exit and this not how?i.e how we know that how many bound can  be formed 

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.

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