Sodium - Na
Sodium (Na) is a soft, silvery alkali metal in group 1. In water and food it occurs in compounds and as sodium ions (Na+), rather than as the highly reactive metal.
Properties and occurrence · Uses · Health and safety · Environment and water
| Atomic number | 11 |
|---|---|
| Standard atomic weight | 22.98976928(2) |
| Electronegativity (Pauling scale) | 0.93 |
| Density at 20 °C | Approximately 0.97 g cm−3 |
| Melting point | 97.794 °C |
| Boiling point | 882.940 °C |
| Non-bonded atomic radius | 0.227 nm |
| Common oxidation state | +1 |
| Stable isotope | 23Na |
| Electron configuration | [Ne] 3s1 |
| First ionisation energy | 495.845 kJ mol−1 |
| Discovery | Humphry Davy, 1807 |
Property sources: Royal Society of Chemistry and CIAAW. Standard atomic weight is dimensionless; the corresponding molar mass is approximately 22.98977 g mol−1. Parentheses indicate uncertainty in the final digits.
Properties and occurrence of sodium
Natural sodium is dominated by its single stable isotope, 23Na; radioactive isotopes also exist. The symbol Na comes from natrium. Humphry Davy first isolated the metal in 1807 by electrolysis of sodium hydroxide.
Sodium occurs in mineral deposits and dissolved salts, including sodium chloride in seawater and halite. Its reactivity prevents it from persisting as free metal in ordinary natural waters. Freshly exposed metal quickly tarnishes in air.
With water, sodium metal produces sodium hydroxide, hydrogen and heat:
2 Na + 2 H2O → 2 NaOH + H2
Uses of sodium and its compounds
The metal is used as a chemical reagent and as a heat-transfer medium in some nuclear reactors. Sodium compounds have much wider everyday uses: sodium chloride is used in food and as an industrial raw material, while sodium carbonate is used in glass manufacture and water treatment.
In water treatment, sodium-cycle ion-exchange resins exchange sodium for calcium and magnesium to reduce hardness. Regeneration restores the resin's sodium form and produces a saline waste stream. This use of sodium ions is distinct from the handling of sodium metal.
Health effects and safe handling
Dietary sodium
Sodium is an essential nutrient involved in fluid balance and nerve signalling. Excess dietary sodium raises blood pressure and increases cardiovascular risk. For adults, WHO recommends less than 2000 mg of sodium per day, equivalent to less than 5 g of salt. These are different quantities: 1 g of table salt contains approximately 400 mg of sodium.
The recommendation covers total intake from food and drinks, not added salt alone. People prescribed a sodium-restricted diet should discuss drinking-water sodium with their healthcare professional, particularly if the supply is saline or softened.
Sodium metal
Sodium metal reacts violently with water and moisture. The reaction releases heat, flammable hydrogen and corrosive sodium hydroxide. It can cause fires, severe skin burns and eye damage. These hazards do not describe ordinary dissolved sodium ions in drinking water.
Storage, handling and emergency response require trained personnel, appropriate controls and the material's safety data sheet. Water and carbon dioxide are unsuitable extinguishing agents for a sodium-metal fire. Follow specialist emergency procedures; do not attempt an improvised clean-up or demonstration.
Sodium in the environment and water
Related page: Sodium in water (Na + H₂O) — reactions, water quality and treatment.
Dissolved sodium contributes to salinity. Weathering, seawater intrusion and human inputs can raise concentrations in water. Environmental effects depend on concentration, the accompanying ions and the receiving environment; sodium-containing substances cannot be assigned one universal toxicity value.
In irrigation, high sodium relative to calcium and magnesium can disperse soil particles and reduce infiltration. Assess sodium adsorption ratio (SAR) together with salinity, soil characteristics and crop requirements. A single sodium concentration is insufficient to determine irrigation suitability.
Drinking-water quality
WHO has not set a health-based drinking-water guideline value for sodium. Its fact sheet notes that concentrations above approximately 200 mg/L may cause unacceptable taste. This is a taste consideration, not a universal health limit or a substitute for local drinking-water requirements. Taste varies with the associated salts and the consumer.
Removing sodium from water
A sodium-cycle softener does not remove sodium. It replaces calcium and magnesium with sodium and can increase the sodium content of the treated water. Reduced hardness therefore does not mean reduced salinity.
Reverse osmosis or distillation can reduce dissolved sodium. For process-water applications, select treatment against the feed-water composition and required product quality, and verify performance for the intended operating conditions. Reverse osmosis produces a concentrate stream; softener regeneration also produces saline wastewater. These streams require appropriate management.
Learn more about sodium in water, mineral-water composition and water pollution. Related reading: dietary reference information and the element pages for nitrogen and carbon.
Sodium page last reviewed: by Lenntech.
Back to the periodic table of elements.
Sources and further reading
- RSC: sodium properties, uses and history.
- CIAAW: sodium atomic weight and isotopes.
- ILO/WHO: International Chemical Safety Card 0717 — sodium metal.
- WHO: dietary sodium and salt reduction.
- WHO: sodium in drinking water.
- EPA WaterSense: water softening and purification (PDF).
- Australian Drinking Water Guidelines: sodium occurrence and treatment.
- USDA NRCS: soil salinity and sodicity (PDF).