Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.


Nitrogen, symbol N, gaseous element that makes up the largest portion of the earth's atmosphere. The atomic number of nitrogen is 7. Nitrogen is in group 15 (or Va) of the periodic table.

Nitrogen was isolated by the British physician Daniel Rutherford in 1772 and recognized as an elemental gas by the French chemist Antoine Laurent Lavoisier about 1776.

Nitrogen is a colorless, odorless, tasteless, nontoxic gas. It can be condensed into a colorless liquid, which can in turn be compressed into a colorless, crystalline solid. Nitrogen exists in two natural isotopic forms, and four radioactive isotopes have been artificially prepared. Nitrogen melts at -210.01° C (-346.02° F), boils at -195.79° C (-320.42° F), and has a density of 1.251 g/liter at 0° C (32° F) and 1 atmosphere pressure. The atomic weight of nitrogen is 14.007.

Nitrogen is obtained from the atmosphere by passing air over heated copper or iron. The oxygen is removed from the air, leaving nitrogen mixed with inert gases. Pure nitrogen is obtained by fractional distillation of liquid air; because liquid nitrogen has a lower boiling point than liquid oxygen, the nitrogen distills off first and can be collected.

Nitrogen composes about four-fifths (78.03 percent) by volume of the atmosphere. Nitrogen is inert and serves as a diluent for oxygen in burning and respiration processes. It is an important element in plant nutrition; certain bacteria in the soil convert atmospheric nitrogen into a form, such as nitrate, that can be absorbed by plants, a process called nitrogen fixation. Nitrogen in the form of protein is an important constituent of animal tissue. The element occurs in the combined state in minerals, of which saltpeter and Chile saltpeter are commercially important products.

Nitrogen combines with other elements only at very high temperatures or pressures. It is converted to an active form by passing through an electric discharge at low pressure. The nitrogen so produced is very active, combining with alkali metals to form azides; with the vapor of zinc, mercury cadmium, and arsenic to form nitrides; and with many hydrocarbons to form hydrocyanic acid and cyanides, also known as nitriles. Activated nitrogen returns to ordinary nitrogen in about one minute.

Used as a coolant, liquid nitrogen has found widespread application in the field of cryogenics. With the recent advent of ceramic materials that become superconductive at the boiling point of nitrogen, the use of nitrogen as a coolant is increasing (see Superconductivity).