CID 962

H₂O

Water

Ball-and-stick Stick Space-filling

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3 atoms, 2 single bonds, bent.

IUPAC name: oxidane
Molar mass: 18.0150(14) g/mol
Atoms: 3
Shape: bent
Bond angle: about 104.5°
Polarity: polar
SMILES: O
InChIKey: XLYOFNOQVPJJNP-UHFFFAOYSA-N
PubChem CID: 962

Made of

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

The name hydrogen means “water-forming”, from the Greek hydro and genes, because the gas burns in air to form water. That product is the most abundant compound of hydrogen on Earth, by OpenStax’s account, and about 89 per cent of its mass is combined oxygen. Its IUPAC name is oxidane and its molar mass 18.015 g/mol. Pure, it is a clear liquid with no colour, smell or taste.

A molecule that light might be expected to boil far below room temperature. Following the trend of related hydrogen compounds, OpenStax estimates that water would boil at about −80 °C, and the real figure is dramatically higher. The difference is hydrogen bonding, the strong attraction between an O–H group and similar groups on nearby molecules. Every water molecule takes part in several of these attractions with its neighbours at once.

Structure and bonding

The model places the oxygen atom in the middle with a hydrogen atom on either side; the readout gives 3 atoms, 2 single bonds. In Ball-and-stick, the two bonds meet at an angle instead of forming a line. That V is the bent shape, 104.5° in OpenStax, though PubChem’s computed geometry measures about 104°.

What bends it is not drawn. Oxygen carries four regions of electron density, places where electrons gather: two bonds and two lone pairs, arranged tetrahedrally. Lone pairs take up more room than the bonding pairs, so the H–O–H angle closes below the tetrahedral 109.5°; nucleus–electron forces also play a part, OpenStax notes.

The Stick view strips it to a bare V. Oxygen’s electronegativity, its pull on bonding electrons, is 3.44 to hydrogen’s 2.2, so each O–H bond has a bond moment, a charge separation. Because the shape is bent, those two moments do not cancel, and water is polar.

Space-filling shows two small hydrogen spheres sunk into the larger oxygen, both on one side, and Labels names them. Hydrogen bonds form between molecules, beyond a one-molecule model. The neutral molecule drawn is the real form: at 25 °C only about two in a billion are ionised.

Where it turns up

Water vapour in the air cools against a cold glass and condenses as liquid on the outside, and fog is another case of the same condensation. A paper towel soaks up a spill because water molecules form hydrogen bonds with the −OH groups on its cellulose fibres and cling to one another as they climb. In xylem, the water-carrying cells of plants, this capillary action, a liquid drawn through porous material, is part of how plants lift water, not all of it.

At the base of a glacier, the weight of the ice above raises the pressure enough to melt some of it, which partly explains how glaciers move. At −10 °C, ice held at a pressure below 0.20 kPa sublimes, passing straight from solid to vapour, and freeze-dried ice cream is dried that way. In the United States about 286 million people receive their tap water from a community system, and about 45 million rely on private wells.

Why it matters

The Hazardous Substances Data Bank (HSDB), quoted by PubChem, says water dissolves at least a little of almost any solid or gas it meets. It names polarity as an important factor in that solvent power. Water is also a raw material of photosynthesis, in which plants turn carbon dioxide and water into glucose and oxygen using the energy of light.

Hydrogen bonding also sets two properties that matter to living things, OpenStax Biology explains. As water freezes, the way its hydrogen bonds line up pushes the molecules farther apart than in the liquid, so ice is less dense and floats. On lakes and ponds that floating layer insulates the plants and animals beneath. Water also has the highest specific heat of any common liquid, 4.184 joules per gram per degree Celsius, about five times that of sand. That is the reason land cools faster than the sea.

For industry, OpenStax ranks water as the least costly and most plentiful hydrogen source. Electrolysis, splitting it with an electric current, releases hydrogen at the cathode and oxygen at the anode.

Properties

Properties of water Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 962.
Property	Value	Unit	Source
Molecular formula (Hill order)	H₂O
Molar mass	18.0150(14) g/mol	g/mol	IUPAC/CIAAW 2024
Hydrogen atoms	2
Oxygen atoms	1
Bonds	2
PubChem molecular weight	18.015 g/mol	g/mol	PubChem

Safety

Water is essentially harmless in its everyday forms. Of 1,876 notifications to the European Chemicals Agency (ECHA), 1,866, or 99.5%, report that it does not meet the hazard criteria of the Globally Harmonized System (GHS). The CAMEO Chemicals database calls it nontoxic. The PubChem Laboratory Chemical Safety Summary for water is the authoritative sheet.

Questions

Why is the water molecule bent and not straight?

If oxygen bonded through two plain p orbitals, the angle between the bonds would be 90°, but experiment gives 104.5°. OpenStax explains the difference with hybrid orbitals: oxygen is sp³ hybridised, its four hybrid orbitals, each a mixture of one s and three p orbitals, point towards the corners of a tetrahedron. Two hold lone pairs and two form the O–H bonds. The model draws only the two bonds and three atoms; the lone pairs are not shown.

Why is water a liquid when its molecules are so light?

Nitrosyl fluoride, ONF, has a mass of 49 atomic mass units (amu) and is a gas, while water, at 18 amu, is a liquid. ONF is heavier and larger, so it should feel stronger dispersion forces, the attractions every molecule has; and both molecules are polar with comparable dipole moments. OpenStax traces the gap to a particularly strong dipole–dipole attraction: hydrogen bonding between the O–H groups of neighbours.

How can insects walk on water?

Hydrogen bonding between its molecules gives water a distinctly high surface tension, the tendency of a liquid's surface to resist being stretched, 71.97 dyne/cm at 25 °C in HSDB's figure. Water striders are denser than water, yet that tension holds them up on the surface, and a steel needle laid carefully on water floats for the same reason. Soap and grease films weaken the effect, as Agnes Pockels showed in a paper of 1891.

Is pure water ever split into ions?

Slightly. Water is amphiprotic, able to act as an acid towards ammonia and as a base towards hydrogen fluoride. In pure water a tiny share of molecules ionises to hydronium, H₃O⁺, and hydroxide, OH⁻. The ion product, K_w, is 1.0 × 10⁻¹⁴ at 25 °C, and it grows as the water warms because the ionisation is endothermic, taking in heat.

Made of the same elements

H₂O₂ Hydrogen peroxide 4 atoms, 3 single bonds.

Same shape (bent)

O₃ Ozone 3 atoms, 2 bonds, bent.

Appears in these articles

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A polar bond is not a polar molecule: carbon dioxide against water Carbon dioxide's two bonds are polar, yet the molecule is not.
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.