And we find in reality that, for instance, the properties of bromine, whose atomic weight is almost the mean between those of iodine and chlorine, occupy a mean position between those of these two elements. The second measurable property of the elements is their equivalence or their capacity for forming compounds of definite forms . Thus carbon or nitrogen in this respect differs widely from the halogens. Although the form ClO 2 corresponds with NO 2 and CO 2 , yet the last is the highest oxide of carbon, whilst that of nitrogen is N 2 O 5 , and for chlorine, if there were an anhydride of perchloric acid, its composition would be Cl 2 O 7 , which is quite different from that of carbon. In respect to the forms of their compounds the halogens, like all elements of one family or group, are perfectly analogous to each other, as is seen from their hydrogen compounds:
HF, HCl, HBr, HI.
Their oxygen compounds exhibit a similar analogy. Only fluorine does not give any oxygen compounds. The iodine and bromine compounds corresponding with HClO3 and HClO4 are HBrO3 and HBrO4, HIO3 and HIO4.
On comparing the properties of these acids we can even predict that fluorine will not form any oxygen compound. For iodine is easily oxidised—for instance, by nitric acid—whilst chlorine is not directly oxidised. The oxygen acids of iodine are comparatively more stable than those of chlorine; and, generally speaking, the affinity of iodine for oxygen is much greater than that of chlorine.
Here also bromine occupies an intermediate position. In fluorine we may therefore expect a still smaller affinity for oxygen than in chlorine—and up to now it has not been combined with oxygen. If any oxygen compounds of fluorine should be obtained, they will naturally be exceedingly unstable.
The relation of these elements to hydrogen is the reverse of the above. Fluorine has so great an affinity for hydrogen that it decomposes water at the ordinary temperature; whilst iodine has so little affinity for hydrogen that hydriodic acid, HI, is formed with difficulty, is easily decomposed, and acts as a reducing agent in a number of cases.
From the form of their compounds the halogens are univalent elements with respect to hydrogen and septivalent with respect to oxygen, N being trivalent to hydrogen (it gives NH3) and quinqui-valent to oxygen (it gives N2O5), and C being quadrivalent to both H and O as it forms CH4 and CO2.
And as not only their oxygen compounds, but also their hydrogen compounds, have acid properties, the halogens are elements of an exclusively acid character.
Such metals as sodium, potassium, barium only give basic oxides. In the case of nitrogen, although it forms acid oxides, still in ammonia we find that capacity to give an alkali with hydrogen which indicates a less distinctly acid character than in the halogens. In no other elements is the acid-giving property so strongly developed as in the halogens.
In describing certain peculiarities characterising the halogens, we shall at every step encounter a confirmation of the above-mentioned general relations.
As fluorine decomposes water with the evolution of oxygen, F 2 + H 2 O = 2HF + O, for a long time all efforts to obtain it in free state by means of methods similar to those for the preparation of chlorine proved fruitless.