Physical Properties of Nitrogen

Nitrogen is a colourless, odourless, and tasteless gas which is non-poisonous, but does not support life. It neither burns in the ordinary way nor supports the combustion of carbon compounds. It is not inert, however, like argon, and when once in a state of chemical combination is extremely active.

Prior to 1898 the determinations of the weight of 1 litre of the gas under standard conditions gave values varying between 0.9713 and 0.9729, when the weight of 1 litre of air was taken as unity. These values, however, referred to atmospheric nitrogen, which of course contained the inert gases of Group O, and Rayleigh's later determinations resulted in the value 0.97209 (air=1). For nitrogen obtained by chemical processes Rayleigh found the value of 0.96737, the weight of 1 litre being 1.25092 grams at N.T.P., while the corresponding results of Leduc are 0.9671 (air=1) and 1.2507 gram respectively.

More recently, Moles, as the weighted mean of a large number of determinations, calculates the weight of 1 litre of nitrogen at N.T.P. (latitude 45°) as 1.2507±0.0001 gram.

With reference to oxygen (=16.000), the density of nitrogen is 14.008, and to water at 4° C. the density is 0.0012507.

When nitrogen combines with 3 atoms of hydrogen or chlorine, or with 3 univalent radicals, it forms 3 duplet valency bonds, in each of which 1 electron has been contributed by each element. It thus completes the "octet" or stable shell, and may be regarded either as a cube, of which the edges form the bonds, or preferably as a tetrahedron, at the angles of which are situated pairs of electrons. The latter method has the advantage of representing in an intelligible manner a trivalent combination (such as HCN and N₂), and of being uniform with the system so successfully adapted to the representation of carbon compounds. The single bond is thus represented by contact at angles, the double by contact along edges, and the triple by contact over faces, of two adjacent tetrahedra. These types will be found in F₂, O₂, and N₂ respectively; and the superior stability or greater inertness of the N⇔N molecule may be due to this configuration.

The electronic models of chemical combination have supplied a logical and consistent mode of representing the valencies even of compounds, which could not be so represented by any of the older theories. As nitrogen has always held a central position in the field of discussion, this is perhaps a suitable place in which to summarise the conclusions which have been reached at present. Historically, the idea of electro valency as combination with or rejection of the electron was first suggested by Thomson, and more definitely by Ramsay. The explanation of ordinary valency of the organic type as being due to pairs of shared electrons owes much to G. N. Lewis. Finally, the idea of mixed bonds, which is a logical development of electron theories, was developed by Lowry, and some results were stated in the form of postulates by the author as follows, under (3) below. They are particularly helpful in dealing with co-ordination compounds.

The three kinds of valency are:

1. Electrovalency, the electrostatic force which unites the ions of a salt, e.g. N⁺ . . . . Cl^-, either in solution or in the solid crystal. Of this kind are the oxygen and halogen valencies of the metals.
2. Covalency, which unites the atoms of non-polar compounds of an organic type, e.g. CH₄. Of this kind are the hydrogen valencies of the elements, particularly the non-metals. In this case, each atom shares one or more of its electrons with the other.
3. Electro-covalency, which unites some of the atoms of co-ordinated groups, such as K₄[Fe(CN)₆], [Co(NH₃)₆](NO₂)₃. This mixed bond consists of a pair of electrons, but also produces an internal field of force or latent polarity. This kind of valency also probably unites the non- metals with one another, or with oxygen in the oxy-acids. In this case, two adjacent atoms share as before two electrons, but both electrons are supplied by one only of the sharing atoms.

Two compounds which can be represented by similar figures containing the same numbers of octets similarly combined are said to be "isosteric." Such pairs often show a close similarity in physical properties; for example, we have nitrogen and carbon monoxide, nitrous oxide and carbon dioxide. The atoms can be represented by tetrahedra, but it is more convenient to use cubes in cases where this is possible. These are conventionally represented below by formulae, in which the dots between the atoms represent shared or binding electrons:



The cyanide and isocyanide ions are also isosteres of carbon monoxide,

[:C:::N:]^-

It is not, however, to be expected that these isosteres will all exhibit the same reactivity; for an ion, such as the ammonium ion, is obviously in a different state from methane. Even, however, where the physical state is the same as in CO and N₂, a closer examination will show differences in the nature of the bonds. Thus, while all three bonds uniting the atoms in the nitrogen molecule may be normal shared duplets, one of the CO bonds must be a " mixed bond." So also in the CO₂ molecules, all the valencies being hydrogen valencies are normal duplets, whereas in N₂O each nitrogen is united by a normal and by a mixed bond.

Oxides and Oxy-acids of Nitrogen

In all these compounds except N₂O₃ and HNO₂ the mixed bond must be present. Nitric acid itself is probably a co-ordination compound if represented by the strict octet theory, as:



Since, however, the X-ray examination of crystalline nitrates has shown that the three oxygen atoms are symmetrically placed round the nitrogen, it is usual to represent the nitrogen atom with a sextet instead of an octet of electrons thus:

or

in the symbols proposed above.

The Ammines

Since the ordinary valencies of nitrogen are saturated in ammonia, this radical is necessarily attached to the central metal atom by mixed bonds, which are thus found to be identical with the "auxiliary valencies," etc., postulated by Werner and others. Indeed, all co-ordination complexes may be shown to contain mixed bonds. The numbers of electrons thus added to the central atom are in many cases those required to make up the next stable configuration or zero group gas. As has been pointed out by Lowry, a shortage of one or two electrons may be tolerated, but hardly an excess. Cobalt hexammine chloride is an example of the completion of the stable shell by the addition of "lone pairs" on the ammonia radical.

Thus in



the cobalt has gained 12e^- from the NH₃ and lost 3e^- to the halogen atoms, the net gain being 9, which raises its atomic number from 27 to 36, i.e. that of krypton.