The Alternations in Stability of Compounds of the Elements of Group V J. H. HILDEBRAND University of California, Berkeley, California
T
HE teacher of freshman chemistry, in his search for relations illustrating the value of the Periodic Svstem for internolatine chemical and ~hvsicalbehaviori, must acknokedge"to his studen;s,.if he is strictly honest, that some relations are not so simple as we might like to have them in the interest of sparing mental effort. There is one such irregularity that I noticed many years ago, and which was duly recorded; namely, the alternations in stability of the oxides and chlorides' of the elements of Group V. This has surely not escaped the attention of others but it appears, nevertheless, never to have been discussed. It is well known that there are no stable pentachlorides of nitrogen, arsenic, and bismuth, whereas the pentachlorides of phosphorus and antimony are quite stable. A similar difference exists for the pentoxides. Although the pentoxides of nitrogen and arsenic exist, they are stronger oxidizing agents than those of phosphorus and antimony. The elements in the neighboring groups show evidences of a similar alternation. For example, the oxides and oxyacids of chlofine and iodine are more numerous than those of fluorine and bromine. The absence of eka-iodine of course prevents the series from being so complete. Again, in Group VI, although we can hardly include oxygen in the series of oxides, the remaining dioxides show a somewhat zigzag character if their heats of formation are plotted against atomic number. In Curve A of Figure 1 the heats of formation of the liquid trichlorides for the Group V elements are plotted against their atomic numbers. In the case of bismuth trichloride we know directly only the heat of formation of the solid. The heat of fusion has,been estimated by assuming that the entropy of fusion vould be at least as great as that for antimony trichloride. The salt-like character of bismuth trichloride would indicate that its entropy of fusion should be greater than the lower limit estimated in the above manner so that its point on the curve should be lowered, if anything. The instability of nitrogen trichloride is due mainly to the high energy of the nitrogen molecule so that it is better to eliminate the differences due to the differentenergies required to as has been yield the gaseous atoms of the done in Curve B. using the values given by Bichowsky and R o s s i ~ i . ~This raises the point for nitrogen trichloride but preserves the zigzag character. Curve C represents similarly the heat of formation of half a mol.
of each trioxide from oxygen gas and from the gaseous atom of the Group V elements. There are. of course. a number of factors enterinz into the stability, but the'one that appears mainly responsible for the facts noted above is the zigzag character of the higher ionizing potentials of the gaseous atoms.
Atomic Number
FIGURE~.-COP.RELATIUNS BETWEEN IONlZWG POTENTIAL AND COMPOUND STABILITY FOR GROUP V ELEME~S
Curve A , heats of formation of liquid trichlorides from elements; curve B, same, from elements in state of monatomic pases: curve C. like B. but for one-half mol. of trioxidesj curve D; energy for removing three electrons from atom.
The figures for these are more complete for the Group V ~ elements than for those in G~~~~~vI or vII, ~ gives the values for the first three potentials, TABLE 1
-
L A T I ~ AND R HILDEBRAND, "Reference book of inorganic chemistry," The Macrnillan Company, New York City, 1929, p. 167. and 1940, p. 209. 2 B ~ c n o w sAND ~ ~ ROSSINI."Thermochemistry of chemical substances," Reinhold Publishing Corporation, New York City, 1936.
I O N ~ Z ~ NPOO T B N T I A L P B L B C T R ~ N - Y O L O ~
1 2 3
N
P
A,
Sb
~i
14.47 29.46 47.3
11.0 9.7 30.0
10.5 20.1 28.1
8.36 (18.0) 24.7
7.25 16.6 25.4
60.7
58.7
51.1
-
Sun 91.2
291
-
-
-
-
49.3
b
l
~
taken from the tabulation by Rice.3 Their sum repre- two. The magnitude of the alternation from anything sents the energy of forming the tri-positive gaseous ion. like a smooth curve is more than adequate to account These are plotted against atomic number in Figure 1, for the alternations in the heats of formation. Any atCurve D. The zigzag character of the line is evident. tempt to treat the matter in more quantitative fashion If the potentials of the fourth and fifth electrons are in- would have to take into account several other factors, cluded, a similar zigzag is obtained. not yet easy to evaluate quantitatively, such as the latThe broken character of Curve D is of course related tice energies of the solids and the changing proportions to the fact that the underlying electrons are more radi- of ionic to covalent bond character. It seems reasoncally changed in the first and thirdsteps thanin the other able, however, to assume that alternations in ionizing potential would be reflected in alternations in the *ICE. "Electronic structure and chemical binding," McGraw- strength of covalent bonds parallel to those for ionic bonds. Hill Book Co., Inc., New York City, 1940, pp. 998-9.
