Ammonia

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Ammonia is a pungent gas that smells of 'sal ammoniac' in everyday life. In space, complex molecules are built from it, and on Earth it is needed for fertilizers. Its molecule resembles a pyramid of nitrogen with three hydrogens on top.

History

Ammonia in pure form was obtained by Joseph Priestley in 1774, and its composition was deciphered by Claude Berthollet in 1785. In astronomy, it was first detected in 1968 in the interstellar medium via radio emission at a wavelength of 1.3 cm.

How it works

The ammonia molecule is a triangular pyramid. The nitrogen sits at the top, and the three hydrogens are at the base. This shape makes the molecule slightly 'flip', emitting radio waves of a specific frequency—astronomers use these 'flips' to find ammonia in space.

💡 In Jupiter's clouds, ammonia creates brown and white bands.

Ammonia (NH₃) is an inorganic compound, a colorless gas with a characteristic pungent odor. The molecule has a pyramidal structure (point group C₃v), the nitrogen atom is sp³ hybridized, one of the hybrid orbitals is occupied by a lone pair of electrons. Liquid ammonia is a good ionizing solvent. In space, ammonia is an important component of interstellar ice and gas, participating in the synthesis of amino acids and other prebiotic molecules. It is observed in molecular clouds, comets, and the atmospheres of giant planets. Industrially, it is synthesized by the Haber-Bosch method at high pressure and temperature.

History

Although ammonia was known to the ancient Egyptians in the form of sal ammoniac (NH₄Cl) from camel dung, gaseous ammonia was first isolated by Joseph Priestley in 1774, who called it 'alkaline air'. Its composition was determined by Claude Berthollet in 1785. In astrophysics, the first reliable detection of interstellar ammonia was made in 1968 by the group of Cheung and Welch via the inversion transition at 23.7 GHz in the direction of the Galactic center. This discovery confirmed the existence of dense molecular clouds. The spectroscopic parameters of ammonia were later refined in laboratory experiments.

How it works

Ammonia exhibits the quantum effect of inversion—tunneling of the nitrogen atom through the plane of the three hydrogens. The potential curve of the molecule has two minima separated by a barrier of ~2020 cm⁻¹. The tunneling splitting frequency of the ground vibrational state is 23.7 GHz (1.26 cm), which is used in astronomical observations. Inversion transitions are sensitive to gas temperature and density, making ammonia a thermometer for molecular clouds. In the interstellar medium, ammonia is formed mainly on the surface of dust grains by sequential hydrogenation of nitrogen. In planetary atmospheres, it participates in cycles, forming clouds (on Jupiter and Saturn) and contributing to coloration. Symmetric and asymmetric bending modes lie in the infrared region (950–1100 cm⁻¹). For astrophysics, rotational and inversion-rotational transitions in the microwave range are important. Spectroscopically, ammonia is described by quantum numbers J, K, where K is the projection of the angular momentum on the symmetry axis.

\Delta E_{inv} = h \nu_{inv}

ΔE_inv — energy difference between the two lowest inversion levels, h — Planck's constant (h ≈ 6.626×10⁻³⁴ J·s), ν_inv — frequency of the inversion transition (ν_inv ≈ 23.7 GHz for the ground state)

\nu_{J \to J-1} = 2B J

ν — transition frequency, B — rotational constant (B = h/(8π²I) for NH₃), J — rotational quantum number of the upper level; I — moment of inertia of the molecule about an axis perpendicular to the symmetry axis

X(NH_3) = \frac{N(NH_3)}{N(H_2)}

X(NH₃) — relative abundance of ammonia, N(NH₃) — column density of NH₃ molecules, N(H₂) — column density of hydrogen molecules

Open problems & fun facts

Formation of ammonia in the diffuse interstellar medium (gas-phase reactions are too slow); The role of ammonia in prebiotic chemistry as a source of nitrogen; Detailed spectroscopy of highly excited states for exoplanetary atmospheres

💡 Ammonia is one of the few compounds whose maser emission in space is more powerful than that of water.

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Ammonia

History

How it works

History

How it works

History

How it works

Open problems & fun facts

Related articles

Molecular Rope in the Cradle of Planets

Comet Spice: How a Late Influx of Matter Reshaped the Outer Solar System