Magnesium – facts and properties
- Symbol
- Mg
- Atomic number
- 12
- Atomic mass
- 24.305 u
- Period
- 3
- Group
- 2
- Category
- alkaline earth metal
- Electron configuration
- [Ne] 3s²
- Electronegativity
- 1.31
- Atomic radius
- 145 pm
- Valence electrons
- 2
- State at standard conditions
- solid
- Melting point
- 650 °C
- Boiling point
- 1090 °C
- Density
- 1.74 g/cm³
- Discovered
- 1808
Magnesium is a light, silvery-grey metal with atomic number 12. It is noticeably lighter than aluminium, which makes it valuable wherever weight matters, and it is famous for burning with a dazzling white flame. Magnesium is just as important in living things. A magnesium ion sits at the centre of every chlorophyll molecule, and the human body relies on it for hundreds of chemical reactions.
Position in the periodic table
Occurrence and extraction
Magnesium ranks among the ten most abundant elements in the Earth's crust. Being a reactive metal, it occurs only in compounds. The main ores are dolomite, a calcium magnesium carbonate that makes up entire mountain ranges such as the Dolomites in Italy, and magnesite, which is magnesium carbonate. Silicate minerals such as olivine and talc also contain a great deal of magnesium. The oceans are a vast reserve too: after sodium, magnesium is the most common metal ion dissolved in seawater, and salt deposits contain magnesium chloride and magnesium sulfate.
Most magnesium today is made by the Pidgeon process. Roasted dolomite is heated strongly with ferrosilicon under vacuum, silicon reduces the magnesium oxide, and magnesium vapour condenses in a cooler part of the retort. The alternative is electrolysis of molten magnesium chloride obtained from seawater or brines. Joseph Black recognised magnesium as an element in the eighteenth century, and Humphry Davy isolated it by electrolysis in 1808. The name comes from Magnesia, a district in Greece known for magnesium-bearing minerals.
Properties and reactions
Magnesium has the electron configuration [Ne] 3s² and two valence electrons. It belongs to period 3 and group 2, the alkaline earth metals. By giving up both outer electrons it forms the doubly charged magnesium ion, whose electron arrangement matches that of neon. Its electronegativity is 1.31.
With a density of 1.74 g/cm³, magnesium is the lightest metal used widely in engineering. It melts at 650 °C and boils at 1090 °C. In air it quickly gains a thin oxide film that dulls its shine and slows further attack. Solid magnesium barely reacts with cold water, but it reacts with hot water, steam and acids, releasing hydrogen. Once ignited, it burns with an intensely bright white light to form magnesium oxide. The flame is extremely hot and rich in ultraviolet light, which can damage the eyes.
Burning magnesium is so reactive that it continues to burn in carbon dioxide and pulls oxygen out of water. A magnesium fire must therefore never be fought with water, because the reaction produces hydrogen that makes the fire far more violent. Dry sand or a special metal fire powder is used to smother it.
In biology, magnesium is essential. In chlorophyll, a magnesium ion holds the centre of the ring-shaped molecule that captures light for photosynthesis, and many enzymes need magnesium ions to work, especially those that handle the energy carrier ATP.
Uses in everyday life and technology
The largest single use of magnesium is as an alloying element in aluminium, where it adds strength and corrosion resistance; drinks cans are a familiar example. Magnesium alloys on their own are chosen where every gram counts, in gearbox housings and steering wheel frames, laptop and camera bodies, bicycle parts and aircraft components. Steelmakers use magnesium to remove sulfur from molten iron, and it is the reducing agent that turns titanium tetrachloride into titanium metal. Magnesium sacrificial anodes protect water heaters and boat hulls, because the magnesium corrodes in place of the steel.
The brilliant light of burning magnesium is used in signal flares and fireworks, and early photographers relied on magnesium powder for flash. Magnesium compounds are common at home as well. Magnesium oxide lines furnaces because it withstands extreme heat, magnesium hydroxide neutralises excess stomach acid, and magnesium sulfate is sold as Epsom salt. Gymnasts and climbers dust their hands with magnesium carbonate, known as chalk. Nuts, wholegrain foods and green vegetables are good dietary sources.
Molecules containing this element
- Magnesium oxide MgO
- Magnesium hydroxide Mg(OH)2
- Magnesium chloride MgCl2
Quick question
How many occupied electron shells does magnesium (Mg) have in the ground state?
FAQ
Why can't you put out a magnesium fire with water?
Burning magnesium is hot enough to strip oxygen from water molecules. This releases hydrogen, which burns or explodes at once and feeds the fire instead of cooling it. Carbon dioxide extinguishers fail as well. Magnesium fires are smothered with dry sand or a special class D metal fire powder.
What does magnesium do in the body?
Magnesium ions take part in hundreds of enzyme reactions, including those that release energy from food, and they help muscles and nerves to work properly. A large share of the body's magnesium is stored in the bones. A varied diet with nuts, wholegrains and vegetables usually covers what we need.
Why does magnesium burn so brightly?
Burning magnesium releases a great deal of energy, and the magnesium oxide it forms is a solid that does not vaporise at flame temperature. These tiny particles glow white-hot and give off intense light across the visible spectrum, together with a lot of ultraviolet.