The element that occupies the seventh position on the periodic table is Nitrogen, represented by the symbol N. The atomic number of 7 specifies that a neutral nitrogen atom contains exactly seven protons in its nucleus. Nitrogen is a nonmetal and the lightest member of Group 15, often referred to as the pnictogens. This element makes up approximately 78% of Earth’s atmosphere, making it the most abundant gas in the air we breathe. Its presence is woven into the fabric of life and industry, yet it often goes unnoticed due to its colorless, odorless, and generally unreactive nature in its most common form.
Fundamental Identity and Properties
Nitrogen is positioned in the second row, or Period 2, of the periodic table, sitting between carbon and oxygen. This placement reflects its electron configuration, which results in five valence electrons, giving it a strong tendency to form three covalent bonds to achieve a stable outer shell. The element exists naturally as a diatomic molecule, N₂, a gas at standard temperature and pressure. This molecular form is characterized by an exceptionally strong triple covalent bond linking the two nitrogen atoms together.
This triple bond requires a significant amount of energy, roughly 941 kilojoules per mole, to break. This stability means atmospheric nitrogen does not readily react with other substances, allowing it to coexist in high concentrations with reactive oxygen gas in the air. Nitrogen exists as a gas with an extremely low boiling point of approximately -196°C (-321°F).
When cooled to this cryogenic temperature, it becomes liquid nitrogen (LN₂), a substance with broad applications ranging from medical preservation to industrial cooling. As a nonmetal, nitrogen can form compounds across a vast range of oxidation states, allowing it to participate in diverse chemical reactions, a stark contrast to the inert nature of the N₂ molecule. This dual nature—inert as a gas but highly reactive in its compounds—underpins its widespread utility in both the natural world and human technology.
The Nitrogen Cycle and Biological Necessity
Despite its atmospheric abundance, nitrogen gas is unusable directly by most life forms, leading to the necessity of the complex global process known as the nitrogen cycle. Nitrogen is a foundational component of all life, serving as an essential building block for two major classes of biomolecules: amino acids and nucleic acids. Amino acids link together to form proteins, which are responsible for almost all cellular structure and function, acting as enzymes, transporters, and structural support.
Nitrogen is also incorporated into the nitrogenous bases—adenine, guanine, cytosine, thymine, and uracil—that form the rungs of the double helix in Deoxyribonucleic Acid (DNA) and Ribonucleic Acid (RNA). These nucleic acids carry the genetic instructions for life and its replication, underscoring nitrogen’s role in heredity and cellular control. Without sufficient fixed nitrogen, plants and animals cannot synthesize these fundamental molecules, which often limits the growth of organisms and entire ecosystems.
The conversion of atmospheric N₂ into biologically usable forms, such as ammonia (NH₃) or nitrate (NO₃⁻), is called nitrogen fixation. This process is largely carried out by specialized microorganisms, including nitrogen-fixing bacteria like Rhizobium, which use the enzyme nitrogenase to break the triple bond and reduce N₂ to ammonia.
Once fixed, the nitrogen is modified through nitrification, a two-step process performed by bacteria like Nitrosomonas and Nitrobacter. Plants assimilate these nitrates and ammonium ions from the soil to build organic molecules, which are then transferred to animals through consumption. Eventually, decomposition by fungi and bacteria returns nitrogen to the soil as ammonia (ammonification). Other specialized bacteria complete the cycle through denitrification, converting nitrates back into atmospheric N₂.
Key Industrial and Practical Applications
The challenge of converting inert atmospheric nitrogen into reactive, usable compounds was solved industrially in the early 20th century by the development of the Haber-Bosch process. This chemical synthesis combines nitrogen gas with hydrogen gas under extremely high pressure and temperature, using a metal catalyst to produce ammonia (NH₃). This breakthrough allowed for the mass production of synthetic nitrogen-based fertilizers, a development that dramatically increased global crop yields and supported the rapid growth of the world population.
Modern agriculture relies heavily on these fertilizers, with the nitrogen produced by the Haber-Bosch process responsible for sustaining nearly half of the global population. The process is highly energy-intensive, requiring a significant portion of the world’s industrial energy supply. Ammonia derived from this process is also a precursor for manufacturing other industrial products, including nitric acid and various explosives.
Beyond fertilizers, the extremely low temperature of liquid nitrogen makes it indispensable for cryogenic applications. In medicine, liquid nitrogen is used for cryosurgery to precisely freeze and destroy abnormal tissues like warts and certain skin cancers. It is also the standard for cryopreservation, allowing for the long-term, stable storage of biological materials in medical banks, such as:
- Sperm
- Eggs
- Embryos
- Tissue samples
Gaseous nitrogen is also widely employed for its inert properties, serving as a non-reactive blanket to prevent oxidation and combustion. This is commonly seen in the food industry, where nitrogen gas is flushed into packaging, a process known as Modified Atmosphere Packaging (MAP), to displace oxygen and significantly extend the shelf life of products like chips and coffee. The gas is also used in sensitive industrial processes and in fire suppression systems, where it quickly extinguishes flames by lowering the oxygen concentration in an enclosed area.