The rationalization of the properties of the group 15 elements (nitrogen, phosphorus, arsenic, antimony and bismuth) and their compounds is difficult, despite there being some general similarities in trends of the group 13, 14 and 15 elements, e.g. increase in metallic character and stabilities of lower oxidation states on descending the group. Although the ‘diagonal’ line  can be drawn between As and Sb, formally separating non-metallic and metallic elements, the distinction is not well defined and should be treated with caution.

    Very little of the chemistry of the group 15 elements is that of simple ions. Although metal nitrides and phosphides that react with water are usually considered to contain N^3- and P^3- ions, electrostatic considerations make it doubtful whether these ionic formulations are correct. The only definite case of a simple cation in a chemical environment is that of Bi³⁺‏, and nearly all the chemistry of the group 15 elements involves covalently bonded compounds. The thermochemical basis of the chemistry of such species is much harder to establish than that of ionic compounds. In addition, they are much more likely to be kinetically inert, both to substitution reactions (e.g. NF₃ to hydrolysis, [H₂PO₂]^- to deuteration), and to oxidation or reduction when these processes involve making or breaking covalent bonds, as well as the transfer of electrons. Nitrogen, for example, forms a range of oxoacids and oxoanions, and in aqueous media can exist in all oxidation states from ‏+5 to ^_3, e.g. [NO₃]^-, N₂O₄, [NO₂] ^-, NO, N₂O, N₂, NH₂ OH, N₂H₄, NH₃. Tables of standard reduction potentials (usually calculated from thermodynamic data) or potential diagrams are of limited use in summarizing the relationships between these species. Although they provide information about the thermodynamics of possible reactions, they say nothing about the kinetics. Much the same is true about the chemistry of phosphorus. The chemistry of the first two members of group 15 is far more extensive than that of As, Sb and Bi, and we can mention only a small fraction of the known inorganic compounds of N and P.