Chapter VII: Nitrogen and the Rare Elements: Argon, Helium, Neon, Krypton, Xenon

~Historical.~ Nitrogen was discovered by the English chemist Rutherford in 1772. A little later Scheele showed it to be a constituent of air, and Lavoisier gave it the name _azote_, signifying that it would not support life. The name _nitrogen_ was afterwards given it because of its presence in saltpeter or niter. The term azote and symbol Az are still retained by the French chemists.

~Occurrence.~ Air is composed principally of oxygen and nitrogen in the free state, about 78 parts by volume out of every 100 parts being nitrogen. Nitrogen also occurs in nature in the form of potassium nitrate (KNO_{3})--commonly called saltpeter or niter--as well as in sodium nitrate (NaNO_{3}). Nitrogen is also an essential constituent of all living organisms; for example, the human body contains about 2.4% of nitrogen.

~Preparation from air.~ Nitrogen can be prepared from air by the action of some substance which will combine with the oxygen, leaving the nitrogen free. Such a substance must be chosen, however, as will combine with the oxygen to form a product which is not a gas, and which can be readily separated from the nitrogen. The substances most commonly used for this purpose are phosphorus and copper.

1. _By the action of phosphorus._ The method used for the preparation of nitrogen by the action of phosphorus is as follows:

The phosphorus is placed in a little porcelain dish, supported on a cork and floated on water (Fig. 26). It is then ignited by contact with a hot wire, and immediately a bell jar or bottle is brought over it so as to confine a portion of the air. The phosphorus combines with the oxygen to form an oxide of phosphorus, known as phosphorus pentoxide. This is a white solid which floats about in the bell jar, but in a short time it is all absorbed by the water, leaving the nitrogen. The withdrawal of the oxygen is indicated by the rising of the water in the bell jar.

2. _By the action of copper._ The oxygen present in the air may also be removed by passing air slowly through a heated tube containing copper. The copper combines with the oxygen to form copper oxide, which is a solid. The nitrogen passes on and may be collected over water.

~Nitrogen obtained from air is not pure.~ Inasmuch as air, in
addition to oxygen and nitrogen, contains small amounts of
other gases, and since the phosphorus as well as the copper
removes only the oxygen, it is evident that the nitrogen
obtained by these methods is never quite pure. About 1% of the
product is composed of other gases, from which it is very
difficult to separate the nitrogen. The impure nitrogen so
obtained may, however, be used for a study of most of the
properties of nitrogen, since these are not materially affected
by the presence of the other gases.

~Preparation from compounds of nitrogen.~ Pure nitrogen may be obtained from certain compounds of the element. Thus, if heat is applied to the compound ammonium nitrite (NH_{4}NO_{2}), the change represented in the following equation takes place:

NH_{4}NO_{2} = 2H_{2}O + 2N.

~Physical properties.~ Nitrogen is similar to oxygen and hydrogen in that it is a colorless, odorless, and tasteless gas. One liter of nitrogen weighs 1.2501 g. It is almost insoluble in water. It can be obtained in the form of a colorless liquid having a boiling point of -195° at ordinary pressure. At -214° it solidifies.

~Chemical properties.~ Nitrogen is characterized by its inertness. It is neither combustible nor a supporter of combustion. At ordinary temperatures it will not combine directly with any of the elements except under rare conditions. At higher temperatures it combines with magnesium, lithium, titanium, and a number of other elements. The compounds formed are called _nitrides_, just as compounds of an element with oxygen are called _oxides_. When it is mixed with oxygen and subjected to the action of electric sparks, the two gases slowly combine forming oxides of nitrogen. A mixture of nitrogen and hydrogen when treated similarly forms ammonia, a gaseous compound of nitrogen and hydrogen. Since we are constantly inhaling nitrogen, it is evident that it is not poisonous. Nevertheless life would be impossible in an atmosphere of pure nitrogen on account of the exclusion of the necessary oxygen.

~Argon, helium, neon, krypton, xenon.~ These are all rare
elements occurring in the air in very small quantities. Argon,
discovered in 1894, was the first one obtained. Lord Rayleigh,
an English scientist, while engaged in determining the exact
weights of various gases, observed that the nitrogen obtained
from the air is slightly heavier than pure nitrogen obtained
from its compounds. After repeating his experiments many times,
always with the same results, Rayleigh finally concluded that
the nitrogen which he had obtained from the air was not pure,
but was mixed with a small amount of some unknown gas, the
density of which is greater than that of nitrogen. Acting on
this assumption, Rayleigh, together with the English chemist
Ramsay, attempted to separate the nitrogen from the unknown
gas. Knowing that nitrogen would combine with magnesium, they
passed the nitrogen obtained from the air and freed from all
known substances through tubes containing magnesium heated to
the necessary temperature. After repeating this operation, they
finally succeeded in obtaining from the atmospheric nitrogen a
small volume of gas which would not combine with magnesium and
hence could not be nitrogen. This proved to be a new element,
to which they gave the name _argon_. As predicted, this new
element was found to be heavier than nitrogen, its density as
compared with hydrogen as a standard being approximately 20,
that of nitrogen being only 14. About 1% of the atmospheric
nitrogen proved to be argon. The new element is characterized
by having no affinity for other elements. Even under the most
favorable conditions it has not been made to combine with any
other element. On this account it was given the name argon,
signifying lazy or idle. Like nitrogen, it is colorless,
odorless, and tasteless. It has been liquefied and solidified.
Its boiling point is -187°.

Helium was first found in the gases expelled from certain
minerals by heating. Through the agency of the spectroscope it
had been known to exist in the sun long before its presence on
the earth had been demonstrated,--a fact suggested by the name
helium, signifying the sun. Its existence in traces in the
atmosphere has also been proven. It was first liquefied by
Onnes in July, 1908. Its boiling point, namely -269°, is the
lowest temperature yet reached.

The remaining elements of this group--neon, krypton, and
xenon--have been obtained from liquid air. When liquid air is
allowed to boil, the constituents which are the most difficult
to liquefy, and which therefore have the lowest boiling points,
vaporize first, followed by the others in the order of their
boiling points. It is possible in this way to make at least a
partial separation of the air into its constituents, and Ramsay
thus succeeded in obtaining from liquid air not only the known
constituents, including argon and helium, but also the new
elements, neon, krypton, and xenon. These elements, as well as
helium, all proved to be similar to argon in that they are
without chemical activity, apparently forming no compounds
whatever. The percentages present in the air are very small.
The names, neon, krypton, xenon, signify respectively, new,
hidden, stranger.

EXERCISES

1. How could you distinguish between oxygen, hydrogen, and nitrogen?

2. Calculate the relative weights of nitrogen and oxygen; of nitrogen and hydrogen.

3. In the preparation of nitrogen from the air, how would hydrogen do as a substance for the removal of the oxygen?

4. What weight of nitrogen can be obtained from 10 l. of air measured under the conditions of temperature and pressure which prevail in your laboratory?

5. How many grams of ammonium nitrite are necessary in the preparation of 20 l. of nitrogen measured over water under the conditions of temperature and pressure which prevail in your laboratory?

6. If 10 l. of air, measured under standard conditions, is passed over 100 g. of hot copper, how much will the copper gain in weight?

Has made many studies in the physical properties of substances; discovered helium; together with Lord Rayleigh and others he discovered argon, krypton, xenon, and neon; has contributed largely to the knowledge of radio-active substances, showing that radium gradually gives rise to helium; professor at University College, London]