A nitrogen atom has 7 electrons, with 2 in the K shell and 5 in the L shell. It is an element of group 15 of the periodic table.
The element nitrogen consists of \mathrm{N_2} molecules, in which two atoms are joined by sharing three pairs of electrons. This bond is called a triple bond, and because it is very strong, nitrogen hardly reacts at room temperature.
Nitrogen makes up about 78 % of the air by volume, and it is a colourless, odourless gas.
When nitrogen and hydrogen are reacted at about 500 °C and at a pressure about 100 to 300 times atmospheric pressure, using a catalyst made mainly of iron, ammonia is formed.
\mathrm{N_2} + 3\mathrm{H_2} \longrightarrow 2\mathrm{NH_3}
This reaction also goes in the reverse direction, so the nitrogen and hydrogen do not all turn into ammonia at once. The ammonia formed is cooled, turned into a liquid and removed, and the nitrogen and hydrogen that are left are reacted again.
This method is called the Haber-Bosch process, and it was put into industrial use in Germany in the early 20th century. The ammonia produced is used as a raw material for fertilizers and nitric acid.
In the laboratory, ammonia is made by heating a mixture of ammonium chloride and calcium hydroxide.
2\mathrm{NH_4Cl} + \mathrm{Ca(OH)_2} \longrightarrow \mathrm{CaCl_2} + 2\mathrm{H_2O} + 2\mathrm{NH_3}
Ammonia is a colourless gas with a pungent smell, and it is lighter than air and very soluble in water. For this reason, it is collected by upward delivery: the gas is led to the top of an upside-down container and pushes the air out downwards.
When a little water is squeezed into a flask filled with ammonia, the ammonia dissolves and the pressure inside the flask falls, so the water in the beaker rises up the tube and sprays into the flask as a fountain. If phenolphthalein has been added to the water in the beaker, the fountain turns pink, because a solution of ammonia is basic.
Nitric acid is manufactured from ammonia by the Ostwald process. First, ammonia is reacted with oxygen from the air, using platinum as a catalyst, to form nitrogen monoxide.
4\mathrm{NH_3} + 5\mathrm{O_2} \longrightarrow 4\mathrm{NO} + 6\mathrm{H_2O}
The nitrogen monoxide then reacts further with oxygen to form nitrogen dioxide.
2\mathrm{NO} + \mathrm{O_2} \longrightarrow 2\mathrm{NO_2}
When nitrogen dioxide is reacted with water, nitric acid is formed. The nitrogen monoxide formed at the same time is oxidized again and used once more.
3\mathrm{NO_2} + \mathrm{H_2O} \longrightarrow 2\mathrm{HNO_3} + \mathrm{NO}
Nitric acid is a strong acid, and it is also a strong oxidizing agent. For this reason, it dissolves copper and silver, which do not dissolve in hydrochloric acid or dilute sulfuric acid.
When copper is put into dilute nitric acid, nitrogen monoxide is given off, and when it is put into concentrated nitric acid, nitrogen dioxide is given off.
3\mathrm{Cu} + 8\mathrm{HNO_3} \longrightarrow 3\mathrm{Cu(NO_3)_2} + 4\mathrm{H_2O} + 2\mathrm{NO}
\mathrm{Cu} + 4\mathrm{HNO_3} \longrightarrow \mathrm{Cu(NO_3)_2} + 2\mathrm{H_2O} + 2\mathrm{NO_2}
Nitrogen monoxide is a colourless gas, and nitrogen dioxide is a reddish-brown gas. Nitric acid decomposes easily in light or when heated, so it is kept in brown bottles in a cool, dark place.