CID 5234

NaCl

Sodium chloride

Na 1
Cl 1

Formula unit Lattice

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2 atoms in one formula unit of an ionic lattice.

The drawing shows one formula unit of an ionic lattice, not a molecule; sodium chloride has no molecular shape.

IUPAC name: sodium chloride
Molar mass: 58.440(11) g/mol
Atoms: 2
SMILES: [Na+].[Cl-]
InChIKey: FAPWRFPIFSIZLT-UHFFFAOYSA-M
PubChem CID: 5234

Made of

Na Cl

Rock salt cut from an underground bed and sea salt left behind by an evaporating pond are one substance, sodium chloride. HSDB, the Hazardous Substances Data Bank quoted on PubChem, notes that it is sold under the everyday names table salt, rock salt and sea salt. It forms colourless cubic crystals with a salty taste, and Los Alamos names it the commonest compound of sodium.

OpenStax classes it as a binary ionic compound. The metal, sodium, supplies the cations, Na⁺, and the nonmetal, chlorine, supplies the anions, Cl⁻, in a one-to-one ratio with a molar mass of 58.44 g/mol. Pulling one mole of the solid apart into gaseous ions takes 769 kJ, its lattice energy, and a larger lattice energy marks a more stable ionic compound. Full charges attract each other far more strongly than partial charges do, which is why salt stays solid until about 801 °C.

Structure and bonding

The drawing holds one sodium ion, charge +1, and one chloride ion, charge −1, as two separate pieces. It is one formula unit of the lattice, not a particle that exists on its own. In OpenStax’s words, no single ionic bond joins any particular sodium ion to any particular chloride ion.

In the crystal each ion is attracted equally to all the nearby ions of opposite charge. Those attractions draw the ions into a tightly bound, three-dimensional lattice in which every ion has six neighbours of the other kind. The chloride ions, radius 181 pm, form a face-centred cubic cell. The smaller sodium ions, radius 102 pm, fill its octahedral holes, gaps each ringed by six chloride ions. The holes lie at each edge’s midpoint and the cell’s centre.

One unit cell, the crystal’s smallest repeating block, holds four sodium ions and four chloride ions. OpenStax links the arrangement to size: ionic compounds whose anions are much larger than their cations usually take this structure. The crystal is hard but brittle. Enough force makes one layer of ions slide over the next, the slip runs along a plane, the attraction between the layers weakens, and the crystal cleaves.

Where it turns up

Seawater carries chloride ions at 0.54 moles per litre, and chloride is also found in the Dead Sea and the Great Salt Lake. Large salt beds lie underground in many parts of the world and are mined, among them the Kłodawa salt mine in Poland. Most rock salt carries 1 to 4 per cent calcium sulfate as its main impurity, but some deposits hold clear, flawless crystals about 99.8 per cent pure.

Mines reach it by the room-and-pillar method or by solution mining, in which water pumped into the deposit returns as brine. Solar salt forms in diked earthen ponds called condensers, where seawater or brine evaporates until a layer 10 to 25 cm thick settles out. From intake to harvest can take up to two years.

Iodised table salt carries 0.023 per cent potassium iodide, and a US product database lists 1861 consumer goods containing sodium chloride.

Why it matters

HSDB counts over 14,000 identified uses for salt. Making chlorine, caustic soda (sodium hydroxide) and soda ash takes about 57 per cent of all usage. Rock salt spread on roads accounts for about 19 per cent of the tonnage, and a 23 per cent solution stays liquid down to −20.5 °C.

Salt is the starting material for sodium hydroxide because, OpenStax notes, it is less expensive than the oxide. Its electrolysis runs down two branches. A current through brine gives sodium hydroxide, chlorine and hydrogen: hydrogen, not sodium, forms at the negative electrode because hydrogen ions are reduced more easily than sodium ions. Metallic sodium therefore comes from molten salt in a Downs cell, where added calcium chloride lowers the melting point to 600 °C.

Sodium is the main cation outside cells and accounts for half of the osmotic pressure gradient between cells and their surroundings. Chloride, the main anion there, balances those cations and keeps the fluid electrically neutral. In some people, OpenStax adds, excess sodium appears to play a major part in high blood pressure.

Properties

Properties of sodium chloride Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 5234.
Property	Value	Unit	Source
Formula (Hill order)	ClNa
Molar mass	58.440(11) g/mol	g/mol	IUPAC/CIAAW 2024
Sodium atoms	1
Chlorine atoms	1
PubChem molecular weight	58.44 g/mol	g/mol	PubChem

Safety

In its everyday forms, the grains in a salt cellar or on a winter road, sodium chloride is essentially harmless. Of 2063 reports to the European Chemicals Agency, 89.2% found it met no hazard criteria of the Globally Harmonized System (GHS). The caution that remains concerns the eyes, which some of those reports say it seriously irritates. The PubChem Laboratory Chemical Safety Summary gathers these classifications.

Questions

Is table salt made of molecules?

No. Salt is a lattice of sodium ions and chloride ions, and OpenStax calls the word molecule incorrect for it, since no ionic bond links one particular sodium ion to one particular chloride ion. Every ion pulls on all the oppositely charged ions around it, six of them. The drawing on this page therefore shows a formula unit, one ion of each kind, rather than a particle that could exist alone.

What happens when salt dissolves in water?

The lattice comes apart and water molecules crowd around each freed ion, forming what OpenStax Biology calls spheres of hydration. Around a sodium ion the water molecules turn their oxygen ends inward, because that end carries a partial negative charge. Around a chloride ion they turn the other way, with the partly positive hydrogen atoms facing the anion. The ions stay separated in the solution rather than pairing up again.

Why does salt have such a high melting point?

Its particles carry full charges, +1 and −1, and OpenStax explains that full charges pull on each other far harder than the partial charges in polar molecular compounds. Breaking them takes heat: 800.7 °C in the HSDB figure quoted on PubChem, 801 °C in OpenStax. The same attractions show up as lattice energy, the 769 kJ needed to turn one mole of the solid into separate gaseous ions.

Does salt water conduct electricity?

Yes, although dry crystals do not. In the solid every ion is locked in place, and OpenStax notes that ionic solids conduct only when molten or dissolved, because then the ions are free to move. In brine the sodium ions drift one way and the chloride ions the other. Industry exploits exactly this: a direct current through a salt solution sends chloride ions to the positive electrode, where chlorine gas forms.

Why does salt melt ice on roads?

Salt water stays liquid below 0 °C, where pure water freezes. OpenStax explains that the drop depends on the number of dissolved particles, not on what they are, and each formula unit of sodium chloride splits into two ions. One mole of salt in a kilogram of water should lower the freezing point by 3.7 °C; the measured drop is 3.4 °C, because the ions do not separate completely. Calcium chloride yields three ions and a larger drop.

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