Ammonia

Ball-and-stick Stick Space-filling

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4 atoms, 3 single bonds, trigonal pyramidal.

IUPAC name: azane
Molar mass: 17.0310(16) g/mol
Atoms: 4
Shape: trigonal pyramidal
Bond angle: about 107.3°
Polarity: polar
SMILES: N
InChIKey: QGZKDVFQNNGYKY-UHFFFAOYSA-N
PubChem CID: 222

Made of

Structural formula of ammonia. Every atom in the molecule is drawn. It shows which atoms are joined to which, not the shape. The model above shows the shape.

Smelling salts owe their sting to ammonia, a gas whose odour PubChem’s property data liken to drying urine. Its molecule joins one nitrogen atom to three hydrogen atoms, and IUPAC names it azane. Nitrogen brings five valence electrons. Three of them pair with hydrogen electrons in covalent bonds, completing nitrogen’s octet, a full set of eight outer electrons. The remaining two stay on the nitrogen as a lone pair.

A mole of the compound weighs 17.031 g, and at room temperature it is a gas. Going by the trend among similar hydrides in its group, ammonia would be expected to boil near −120 °C. It actually boils at about −33 °C, because of hydrogen bonding between its molecules. NIOSH notes that the gas liquefies easily under pressure, and it is shipped as a compressed liquid.

Structure and bonding

Ammonia is a squat tripod, nitrogen on top and three hydrogen atoms spread in a triangle beneath it: the shape OpenStax calls trigonal pyramidal. Each H–N–H angle in PubChem’s computed model is 106.0°; the observed angle is 107.3°. Both fall short of a regular tetrahedron’s 109.5°. Nitrogen holds four electron pairs; the fourth, a lone pair the viewer does not draw, fills the open space above the nitrogen. A lone pair repels bonding pairs more strongly than they repel each other, so the three bonds are pushed closer together.

The Stick view reduces the molecule to three equal bonds, and Labels marks one N and three H. In the Space-filling view the hydrogen spheres cluster on one side of the larger nitrogen, leaving its far side bare. That lopsided pyramid is why ammonia is polar: its three identical bond moments, the uneven sharing of charge along each N–H bond, do not cancel. The model shows the neutral molecule, which stays the main form in water; only a small share takes a proton and becomes ammonium, NH₄⁺.

Where it turns up

Ammonia forms naturally wherever nitrogen-containing organic material decomposes without air, though the amounts are small. Household ammonia is a solution holding 5 to 10 per cent of the compound, according to PubChem’s manufacturing data. A products database there lists 181 consumer products that contain it. Cleaners for homes and industry are a common place to meet it. Bottles labelled ammonium hydroxide hold mostly dissolved ammonia molecules, with little actual ammonium hydroxide present.

Far larger quantities go to farms, where the liquid is applied straight to fields as a nitrogen fertiliser. Refrigeration uses a grade sold at 99.97 per cent purity. The compound also serves in the purification of water supplies. Pulp and paper mills, metallurgy, rubber, food and beverage plants, textiles and leather tanning all take a share.

Why it matters

Los Alamos calls ammonia the most important commercial compound of nitrogen. About 80 per cent of it goes into fertilisers, by the ATSDR’s count, either applied directly or first turned into ammonium nitrate, ammonium sulfate or urea. Burning ammonia is how industry makes nitric oxide, the start of the path to nitric acid, and the compound also feeds plastics, synthetic fibres and dyes.

Most of it comes from the Haber-Bosch process, which reacts atmospheric nitrogen with hydrogen on an iron catalyst. PubChem’s manufacturing data list smaller routes as well, among them recovery as a by-product of coal distillation. The reaction gives out heat, so cool conditions would favour ammonia, but they leave it far too slow. Plants run at roughly 150 to 250 atm, which pushes the equilibrium towards ammonia, with a catalyst at a moderate 400 to 500 °C. Two thirds of all hydrogen made worldwide ends up in this one compound.

Properties

Properties of ammonia Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 222.
Property	Value	Unit	Source
Molecular formula (Hill order)	H₃N
Molar mass	17.0310(16) g/mol	g/mol	IUPAC/CIAAW 2024
Nitrogen atoms	1
Hydrogen atoms	3
Bonds	3
PubChem molecular weight	17.031 g/mol	g/mol	PubChem

Safety

Ammonia is corrosive and toxic by inhalation, and EU rules give the anhydrous gas the signal word Danger. Its hazard statements are flammable gas, causes severe skin burns and eye damage, toxic if inhaled, and very toxic to aquatic life. The eyes, skin and respiratory system are the organs at risk, and even dilute household solutions cause irritation. The PubChem Laboratory Chemical Safety Summary and the NIOSH Pocket Guide entry for ammonia are the authoritative sheets.

Questions

What shape is an ammonia molecule?

Trigonal pyramidal. Four electron pairs surround the nitrogen atom, so its electron-pair geometry is tetrahedral, but only three of those pairs bond to hydrogen. The fourth is a lone pair, and a molecular shape names only the positions of atoms. What remains is a pyramid: nitrogen at the apex, three hydrogen atoms at the corners of its triangular base. The model on this page draws exactly those four atoms.

Why is ammonia a base?

The lone pair on its nitrogen atom can accept a proton. In water, ammonia takes a hydrogen ion from a water molecule and becomes ammonium, NH₄⁺, leaving hydroxide, OH⁻, behind. It is a weak base, with a base constant of 1.8 × 10⁻⁵, so most dissolved molecules stay neutral. Even so, PubChem's property data give a pH of 11.6 for one test solution and 11.1 for another ten times more dilute.

Why is ammonia used as a refrigerant?

Evaporating liquid ammonia soaks up a great deal of heat. Hydrogen bonding between its molecules gives it an enthalpy of vaporisation, the heat a liquid absorbs as it turns to gas, which OpenStax ranks above every liquid but water. It also boils at about −33 °C, and NIOSH notes that it liquefies easily under pressure. PubChem's manufacturing data list a refrigerant grade of 99.97 per cent purity.

Why does ammonia form hydrogen bonds?

Nitrogen's electronegativity, the pull an atom exerts on shared electrons, is 3.04 against hydrogen's 2.2. That large gap, together with the tiny size of the hydrogen atom, concentrates partial charges on each N–H bond. The hydrogen of one molecule is then drawn to the nitrogen of the next. These attractions are only 5 to 10 per cent as strong as covalent bonds, yet much stronger than ordinary dipole attractions.

Appears in these articles

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Why water is bent and carbon dioxide is straight: the VSEPR model Water's two bonds meet at 104.5°; carbon dioxide's lie in a straight line.