Magnesium - Mg
Magnesium (Mg) is a lightweight, silvery-white alkaline earth metal in group 2. In natural waters it occurs mainly as dissolved magnesium ions (Mg2+), which contribute to hardness alongside calcium.
Properties and occurrence · Uses · Health and safety · Environment and water
| Atomic number | 12 |
|---|---|
| Standard atomic weight | [24.304, 24.307] |
| Electronegativity (Pauling scale) | 1.31 |
| Density at 20 °C | Approximately 1.74 g cm−3 |
| Melting point | 650 °C |
| Boiling point | 1090 °C |
| Non-bonded atomic radius | 0.173 nm |
| Common oxidation state | +2 |
| Stable isotopes | 24Mg, 25Mg and 26Mg |
| Electron configuration | [Ne] 3s2 |
| First ionisation energy | 737.75 kJ mol−1 |
| Second ionisation energy | 1450.683 kJ mol−1 |
| Early isolation of metal | Humphry Davy, 1808 |
Property sources: Royal Society of Chemistry and CIAAW. The standard atomic-weight interval reflects natural isotope variation; 24.305 is a commonly used conventional value, corresponding to a molar mass of approximately 24.305 g mol−1. Atomic weight itself is dimensionless.
Properties and occurrence of magnesium
Magnesium has three stable isotopes: 24Mg, 25Mg and 26Mg. Radioactive isotopes also exist, so a count of stable isotopes should not be confused with the total number known.
It occurs in minerals including magnesite, dolomite, olivine and serpentine, and in seawater and brines. The element is normally present in compounds rather than as free metal. Joseph Black recognised magnesium as distinct from calcium in 1755; Humphry Davy obtained metal in 1808, and Antoine-Alexandre Brutus Bussy prepared pure metal in 1831.
Uses of magnesium and its compounds
Magnesium alloys are used where low weight is valuable, including transport components and equipment housings. Magnesium is also used in metallurgical processing and pyrotechnics. Its compounds have different properties: magnesium oxide is used in refractory materials, while magnesium hydroxide is used in selected industrial and water-treatment applications.
In treatment systems, magnesium-based materials can contribute to pH adjustment or conditioning. Their suitability depends on the material's purity, dissolution behaviour, dosing and the required product-water quality. Adding magnesium compounds can also increase dissolved magnesium or create solids that need removal.
Health effects and safe handling
Magnesium as a nutrient
Magnesium is essential for normal muscle and nerve function and many enzyme reactions. Requirements vary with age, sex and life stage; a single minimum intake is not appropriate for everyone. Foods such as nuts, legumes, whole grains and leafy vegetables provide magnesium, and drinking water can contribute to intake.
High doses from supplements or medicines can cause diarrhoea, nausea and abdominal cramping. Excessive intake can cause serious toxicity, especially when kidney function is impaired. Supplement use and suspected toxicity should be assessed by a healthcare professional; the nutritional role of magnesium does not make every magnesium-containing material safe to ingest.
Metal, powder and fumes
Bulk metal, fine magnesium powder and dissolved magnesium salts have different hazards. Fine powder can create dust explosions, and pyrophoric powder may ignite on exposure to air. Contact with water or acids can release flammable hydrogen. Handling and emergency procedures must match the material and its safety data sheet.
Burning or hot-work operations can generate magnesium oxide fume. Inhalation can irritate the respiratory tract and cause metal fume fever, with symptoms such as cough, chest discomfort and flu-like fever. Engineering controls, exposure assessment and suitable protection are needed for the task.
Magnesium-metal fires need specialist response and an extinguishing agent suitable for the material; water must not be used on burning magnesium powder. For an exposure or ingestion incident, contact emergency medical services or a poison-information service and follow the product's safety data sheet. Do not induce vomiting unless specifically directed by a medical professional.
Magnesium in the environment and water
Related page: Magnesium in water (Mg + H₂O) — water chemistry, occurrence and treatment.
Weathering of magnesium-bearing rocks and minerals introduces magnesium into water. Concentrations vary with geology, seawater influence, groundwater conditions and human inputs. Magnesium is a naturally occurring nutrient, but the effects of a release depend on its chemical form, concentration and the receiving environment. The hazards of reactive metal powder should not be equated with those of dissolved Mg2+.
Hardness, drinking water and process water
Dissolved calcium and magnesium are the main contributors to hardness. Magnesium hardness is often expressed as a calcium-carbonate equivalent: 1 mg/L of magnesium corresponds to approximately 4.12 mg/L as CaCO3. This reporting convention does not mean that the magnesium is actually present as calcium carbonate.
WHO does not propose a health-based guideline value for hardness. This does not establish one ideal magnesium concentration for every supply or person. Drinking-water conditioning should consider mineral composition, stability and applicable local requirements.
For industrial systems, the acceptable magnesium concentration depends on the application. Magnesium can contribute to deposits under suitable chemical and temperature conditions; hardness alone does not describe all scaling or corrosion risks.
Removing magnesium from water
- Sodium-cycle ion exchange: exchanges calcium and magnesium for sodium, reducing hardness without necessarily reducing total dissolved solids.
- Precipitative softening: controlled chemical treatment can precipitate magnesium hydroxide, followed by solids separation and appropriate pH conditioning.
- Reverse osmosis: reduces dissolved magnesium and other salts. Performance depends on the membrane, feed composition and operating conditions.
Ordinary sediment filtration does not remove dissolved magnesium ions. Treatment selection should consider the full water analysis, flow and required quality. Softening creates regeneration waste or sludge, and RO produces concentrate; these residual streams need appropriate management.
Read more about magnesium in water and dietary reference information. Related elements: aluminium, iron, manganese, zirconium, zinc and thorium.
Magnesium page last reviewed: by Lenntech.
Back to the periodic table of elements.
Sources and further reading
- RSC: magnesium properties, uses and history.
- CIAAW: magnesium atomic weight and isotopes.
- NIH: magnesium nutrition, supplements and toxicity.
- ILO/WHO: International Chemical Safety Card 0289 — pyrophoric magnesium powder.
- NIOSH: magnesium oxide fume and occupational health.
- WHO: hardness in drinking water (PDF).
- EPA: water purification and softening (PDF).
- EPA: precipitative softening and treatment residuals (PDF).