Question

Why are pentahalides more covalent than trihalides?

Answer 1

Hint: We know that higher the oxidation state of the central atom, more will be its polarizing power, which in turn, increases the covalent character of the bond formed between the central atom and the halogen atom.

Complete step by step solution:
We know that trihalides are formed by all elements of group 15 of periodic tables while pentahalides are formed by all the elements of group 15 except nitrogen due to absence of vacant d-orbital in outermost shell.
The oxidation state of +5 in pentahalide is more as compared to +3 oxidation state in trihalides due to higher positive oxidation state of the central atom in case of pentahalide.
These atoms will have larger polarizing power than the halogen atom attached to them. So, we can easily say that the central atom in case of pentahalides will tend to polarize more the halogen atom to which it is attached.
But in case of trihalides we can see that due to +3 oxidation state the central atom will polarize the halogen atom to a lesser extent as compared to pentahalide state.
Therefore, according to Fajan's rule, more the polarization, larger will be the covalent character of the bond.
Hence due to larger polarization of bond in pentahalide state as compared to trihalide state, we can easily say that the pentahalides are more covalent than trihalides.

Note: Nitrogen does not form pentahalide due to non-availability of d-orbital in its valence shell.
Pentahalides are more covalent than trihalides.
All trihalides of elements of group 15 except nitrogen are stable.
All trihalides of group-15 except $ [Bi\mathop F\nolimits_3 ] $ are predominantly covalent in nature.