Nitrogen ($\mathrm{N}$)

Nitrogen is the chemical element with atomic number 7 and symbol $\mathrm{N}$. Its familiar elemental form is molecular nitrogen, ${\mathrm{N}}_2$, whose strong triple bond contributes to its low reactivity under many ordinary conditions. Nitrogen in ammonia, nitrate, and biological molecules occupies chemically different forms.^[ref]

Dinitrogen is the diatomic molecule ${\mathrm{N}}_2$, consisting of two nitrogen atoms joined by a strong triple bond. It is the principal molecular form of nitrogen in Earth's atmosphere. A nitrogen atom incorporated in a protein belongs to the same chemical element, but is not a dinitrogen molecule.^[ref]

The reactionillustrates the bookkeeping: one dinitrogen molecule supplies the two nitrogen atoms in two Ammonia molecules. Breaking and replacing its strong bond requires suitable reaction pathways and conditions. Its abundance therefore does not mean that organisms can automatically use it as a source of chemically available nitrogen.^[ref]

Nitrogen-15 is a stable isotope with seven protons and eight neutrons. Its nuclear spin is one-half, making it useful in isotope-labeling and nuclear-magnetic-resonance studies.

Replacing nitrogen-14 by nitrogen-15 changes mass and nuclear-spin properties while preserving closely related electronic chemistry. Enrichment can therefore make specific nitrogen-containing sites easier to observe in a complex biological molecule.

The nitrogen isotope with seven protons and seven neutrons.