Atomic number 12

Magnesium

Mg
+ phase
− phase

Shells Orbitals 3s

Drag to rotate; pinch or Ctrl + scroll to zoom. Arrow keys rotate, + − zoom, R resets.

12 electrons in 3 shells: 2, 8, 2.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Alkaline earth metal
Period: 3
Group: 2
Block: s
Standard atomic weight: 24.305(2) Da
Electron configuration: [Ne] 3s²
Phase at 25 °C: solid
Melting point: 650 °C
Boiling point: 1090 °C
Density: 1.74 g/cm³
Electronegativity (Pauling): 1.31
Discovered: 1808, Humphry Davy; recognised as an element by Joseph Black in 1755

Neighbours in the table

Na Al

Everything magnesium does chemically comes down to two electrons. Twelve electrons, arranged 2, 8 and 2, leave one pair on the outside, in the 3s subshell, the part of a shell whose electrons share the same energy. Group 2 metals lose that pair to form the Mg²⁺ ion. Magnesium carries one proton more than sodium, its neighbour in period 3, and that extra charge pulls the atom smaller. It also raises magnesium’s first ionisation energy, the energy it costs to take one electron off an atom, so magnesium is the less reactive of the two.

Air blunts what reactivity is left. A thin, tightly bonded coat of magnesium oxycarbonate grows on the surface and blocks further attack, so a strip on a shelf tarnishes only slightly. The metal underneath is silvery-white and fairly tough, and turns malleable and ductile when hot. Its density, 1.74 g/cm³, is the lowest of the structural metals in wide use.

The name comes from Magnesia, a district in the Greek region of Thessaly. Joseph Black, a Scottish chemist, recognised magnesium as a separate element in 1755. Humphry Davy obtained the first metal in 1808, by passing a current through magnesium oxide mixed with mercuric oxide. That metal was impure; Antoine-Alexandre Brutus Bussy, a French pharmacist, prepared it in coherent form in 1831.

Where it is found

Magnesium is never found free in nature: it reacts too readily with water and air. It occurs as silicates, carbonates and sulfates, in large deposits of magnesite, dolomite and other minerals. Magnesium accounts for about 2.3 per cent of the crust by mass, 2.33 × 10⁴ milligrams per kilogram. Jefferson Lab ranks it seventh among the elements of the crust.

Water is a source as well as rock, and a vast one. Seawater is about 0.5 per cent magnesium chloride; a litre holds 1.29 × 10³ milligrams of magnesium, about 1.3 billion kilograms per cubic kilometre. In the United States it is won from fused magnesium chloride drawn from brines, wells and seawater.

Magnesium ions are hard to reduce, hard to hand electrons back to, so the classical route to the metal is electrolysis. Calcium hydroxide precipitates magnesium hydroxide from seawater, hydrochloric acid converts that to magnesium chloride, and a current through the molten chloride yields the metal and chlorine gas. Over 75 per cent of world production, mostly in China, has left electrolysis behind for the Pidgeon process, which drives magnesium vapour out of the oxide with silicon.

Why it matters

Magnesium’s lightness is why most of what is produced goes into lightweight alloys. It is one third lighter than aluminium, and its alloys are counted essential to aircraft construction. Alloyed into aluminium it also makes that metal easier to roll, extrude and weld. Magnesium alloys also go into tyre rims, cameras, horseshoes, snowshoes and the masks baseball catchers wear.

The same readiness to corrode is put to work as protection. A sacrificial anode is a block of a metal that gives up its electrons more readily, attached to the metal that needs protecting. Corrosion then happens at the block instead of at the thing being protected. Of the three usual choices, zinc, aluminium and magnesium, magnesium’s reduction potential is the most negative, and that number tracks the readiness. It therefore serves best in freshwater, where there are few dissolved ions to carry the current; zinc and aluminium work better in saltwater.

Magnesium’s compounds are as widespread as its alloys. Magnesium oxide, also called magnesia, goes into antacids, crucibles, insulating materials and some cements, and dead-burned magnesite is made into the brick and liners of furnaces. Magnesium carbonate keeps table salt from caking and is used in paints and inks. A thin film of magnesium fluoride on an optical lens reduces glare and reflections.

Properties

Properties of magnesium Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	12
Standard atomic weight	24.305(2) Da	Da	IUPAC/CIAAW 2024
Melting point	650 °C	°C	Los Alamos: 650 °C
Boiling point	1090 °C	°C	PubChem: 1363 K
Density	1.74 g/cm³	g/cm³
Electronegativity (Pauling)	1.31
Electron configuration	1s² 2s² 2p⁶ 3s²
Phase at 25 °C	solid

The standard atomic weight of this element is an interval in nature, from 24.304 to 24.307 Da; the value shown is IUPAC's conventional abridged value.

Safety

Magnesium’s recorded hazard is fire, and the finer the metal, the more readily it catches: powder and dust before ribbon and shavings. PubChem’s GHS entries carry the signal word Danger for magnesium powder, which catches fire in air, and for magnesium itself, which releases flammable gas in contact with water. A solid piece is difficult to ignite and must first be heated past its melting point, while the dust irritates eyes and airway. Magnesium fume can cause metal fume fever, which makes this a workplace matter, set out in the PubChem Laboratory Chemical Safety Summary among the sources.

Questions

Why does magnesium form a 2+ ion?

Magnesium's twelve electrons leave just two in the outer shell, both in the 3s subshell. Group 2 metals give up both in reactions, and the ion left behind carries a charge of 2+. Many common salts of the alkaline earth metals, unlike most alkali metal salts, are insoluble in water, which OpenStax puts down to the high lattice energy of a crystal built on 2+ ions.

Why is magnesium used in flares and fireworks?

Finely divided magnesium ignites readily when heated in air, and the flame is a dazzling white. That white light is what flares, fireworks and old photographic flashbulbs were built around. The metal's pull on oxygen is strong enough that a burning piece takes the oxygen out of carbon dioxide, leaving carbon behind. Industry uses that reducing power to take uranium and other metals out of their salts.

What does magnesium do in the human body?

Organic magnesium matters in both plant and animal life, Los Alamos says. In plants, chlorophylls are magnesium-centred porphyrins: each is built around one magnesium atom. For people, the same laboratory says the amount an adult needs each day varies with weight and size. The same page lists the hydroxide sold as milk of magnesia, the chloride, the sulfate sold as Epsom salts and the citrate among compounds used in medicine.

Compounds containing magnesium

MgCl₂ Magnesium chloride 3 atoms in one formula unit of an ionic lattice.
Mg(OH)₂ Magnesium hydroxide 5 atoms in one formula unit of an ionic lattice.
MgSO₄ Magnesium sulfate 6 atoms in one formula unit of an ionic lattice.