Nitrogen is one of the most common elements and it makes up nearly 80% of the Earth's atmosphere. With its unique set of chemical properties, it is a critical component of life as we understand it. Understanding these chemical properties provides some insight into the role that nitrogen plays on Earth and in living organisms.
With an atomic number of 7, nitrogen has five electrons in its outer shell. This leaves three unpaired electrons for bonding and reactions. Because of this, nitrogen is trivalent in most compounds. In nitrogen gas, for instance, two nitrogens form a nearly unbreakable triple bond. Ammonia is a nitrogen saturated with hydrogens and is the primary neutral hydride of nitrogen. Because of it's nature, nitrogen is found universally in organisms, in the form of protein amides and within nucleic acids.
Nitrogen has a strong electro-negativity (~3), causing it to house a partial positive charge and have the properties of bases. Nitrogen, in the form of amines and amides, forms some of the primary functional groups in the backbone of proteins. It is also common in nucleic acids. Amines are functional groups formed by a basic nitrogen atom with a lone pair of electrons. In biological systems, the nitrogen is often available for hydrogen bonding, a stabilizing force for the molecules in these systems. Amines are also critical in many kinds of biological activities. For instance, neurotransmitters such as epinephrine and dopamine are amines that function as cell signaling systems.
Nitrogen in the atmosphere is impossible to utilize directly in biological systems. In order to be useful, plants and animals convert nitrogen to a "fixed" or reduced state. Bacteria and plants generally fix and utilize nitrogen, which is then consumed by higher organisms. Many plants also utilize nitrites or nitrates present in soil, rather than spend precious energy fixing atmospheric nitrogen.
While a critical component in all organisms, nitrogen processing in animal metabolism produces nitrites that must be removed. These nitrites are excreted as urea or uric acid. The decay of organisms generally produces nitrates as well, but most nitrogen is released as atmospheric nitrogen.
The powerful bond formed by reactions with nitrogen is critical to understanding nitrogen chemistry. Breaking down nitrogen gas, an incredibly difficult process because of the bond strength, is an important process in industry, as it releases free nitrogen for use in the construction of other compounds or molecules. Liquid nitrogen is an important tool in many industries as well. It is liquefied at extremely low temperatures. Most compounds experience rapid freezing in the presence of liquid nitrogen, making it useful to quickly freeze and store things at low temperatures.