Nitrogen is an element found everywhere in the natural world, making up approximately 78% of Earth’s atmosphere in its diatomic gas form (\(\text{N}_2\)). It is the most abundant gas in the air we breathe. This element is a fundamental building block of all known life forms and is continuously cycled through the environment.
The Pnictogen Family
Nitrogen is the first and lightest member of the family of elements located in Group 15 of the Periodic Table. This grouping is officially known as the Pnictogens, a term derived from the Greek word pnígein. The original Greek word translates to “to choke” or “to suffocate,” referencing the discovery of nitrogen gas. Early scientists noted that nitrogen was an inactive component of air that could not support combustion or life. Historically, this group was also referred to by the designation Group VA.
The Pnictogen family is positioned in the p-block of the table. Their location and shared chemical heritage are determined by their electron configuration. This grouping highlights a distinct pattern of reactivity and physical properties that changes predictably as one moves down the column.
Shared Chemical Properties
The elements in Group 15 are united by having a common valence shell electron configuration of \(ns^2np^3\). This configuration means the valence shell consists of a filled s-orbital and a half-filled p-orbital, which contributes to a high degree of stability. To achieve a full, stable octet, these atoms typically seek to gain three additional electrons, often resulting in a \(-3\) oxidation state in compounds.
This stability is why elemental nitrogen exists as a largely unreactive \(\text{N}_2\) gas with an extremely strong triple bond. These elements also commonly form covalent bonds by sharing their five valence electrons. They can exhibit positive oxidation states like \(+3\) and \(+5\).
Other Elements in Group 15
The Pnictogen family includes four other naturally occurring elements below nitrogen: Phosphorus (\(\text{P}\)), Arsenic (\(\text{As}\)), Antimony (\(\text{Sb}\)), and Bismuth (\(\text{Bi}\)). A defining feature of this group is the dramatic transition in physical and chemical properties as the atomic number increases. Nitrogen is a colorless, odorless nonmetal gas at room temperature.
Below nitrogen, Phosphorus is a reactive nonmetal that exists in several solid allotropes. Further down the group, Arsenic and Antimony are classified as metalloids, displaying properties of both metals and nonmetals. Arsenic, for instance, has a metallic appearance but behaves chemically like a nonmetal. The trend culminates with Bismuth, which is a soft, silvery-pink metal, confirming the steady increase in metallic character as one descends the column.
Nitrogen’s Vital Biological Function
The chemical versatility of nitrogen enables it to play an indispensable role in the biology of every living organism. Nitrogen atoms are fundamental components of amino acids, which are the molecular building blocks used to create all proteins. Nitrogen is also a required component of nucleic acids, specifically the nitrogenous bases that form the rungs of the DNA and RNA double helix structures.
Despite its abundance, atmospheric \(\text{N}_2\) is unusable by most organisms due to its strong triple bond. Nitrogen must be converted into biologically available forms, such as ammonia and nitrate, through nitrogen fixation. Specialized bacteria, such as Rhizobium, possess the enzyme nitrogenase that performs this conversion. This process is the first and most limiting step in the Nitrogen Cycle, which continuously moves the element through the air, soil, and living systems.