— a chemical element, atomic number $9$, atomic mass $18.99840$. Light yellow gas with a pungent odor, density $1.693\ \mathrm{kg} / \mathrm{m}^{3}$, boiling point $−188.12\ ^{\circ} \! {\mathrm{C}}$, malting poimt $−219.6\ ^{\circ} \! {\mathrm{C}}$.