(i) These trihalides are mainly covalent with the exception of BiF3 which is ionic.
(ii) The ionic character of trihalides increases in going down the group.
(iii) Like hydrides, these trihalides have pyramidal structure.
(iv) These trihalides except NX3 can be easily hydrolysed by water.
The inability of trihalides of N to hydrolyse is attributed to the non-availability of vacant d-orbitals in nitrogen.
(v) The trihalides of P, As, Sb (especially fluorides and chlorides) act as Lewis acids and combine with Lewis bases
PF3 + F2 PF5
SbF3 + 2F- [SbF5]2-
(vi) The pentahalides in general, have less thermal stability as compared to trihalides.
(vii) All the pentahalides act as Lewis acids. It is because the central atom can easily accept the halide ions due to presence of vacant d-orbital and can extend their co-ordination number.
(viii) PCl5 exists as molecule in gaseous state but in solid state it exists as [PCl4]+[PCl6]- and is ionic in nature. PBr5, PI5 also exists in the ionic form in solid state.
Reactivity towards oxygen: the elements of this group combine with oxygen directly or indirectly to form a large number of different types of oxides.
Nature of oxides
All the oxides of nitrogen except NO and N2O and phosphorus are strongly acidic: oxides of arsenic are weakly acidic; oxides of antimony are amphoteric and those of bismuth are weakly basic.
Reason: the change in character from acidic to basic can be explained on the basis of the size of atoms. As the size of nitrogen atom is small and it has a strong positive field, it interacts with water more strongly pulling the electron pair between O - H bond and thus release of H+ ions.
However, this tendency diminishes with the increase in size and therefore decreases the acidic character or conversely increases the basic character.
As far as the stability of the oxides is connected it is found that oxides having elements in the higher oxidation state become less stable as we move down to group. This is because of the import pair effect.