Why are pentahalides more covalent than trihalides?

Explanation

The elements of group 15 have five electrons (two in the s-and three in the p-orbitals) in their respective valence shells. Since it is difficult ot loss all the three electrons of form `E^(3+)` or even more difficult to lose all the five valence electrons (two s- and three p-) to form `E^(5+)` ions, therefore, higher elements have no tendency to form ionic compounds. Instead they form covalent compounds by sharing of electrons.
Since elements in the +5 oxidation state have less tendency to loss electrons than in the +3 oxidation state, therefore, elements in the +5 oxidation state are more covalent than in the +3 oxidation state. In other words, pentahalides are more covalent than trihalides.