Magnesium

Magnesium is a light, silvery-white metal that burns with one of the most dazzling white flames in all of chemistry. It is inside your bones and muscles, inside every green leaf on Earth, inside racing car components and aircraft seats, and inside the bright flares that light up emergencies at sea.

  • Atomic Number1212 protons, 12 electrons
  • Atomic Mass24.305 uAbout 24× heavier than hydrogen
  • State at Room TempSolidlight silvery metal
  • Density1.74 g/cm³Lighter than aluminium
  • Melting / Boiling649.9°C / 1089.8°CBurns at over 3,000°C
  • Discovered1808Humphry Davy, 1808

How light is magnesium compared to common metals?

Magnesium is one of the lightest structural metals, just over half the density of aluminium.

Density Comparison (g/cm³)
Magnesium1.74
Aluminium2.70
Titanium4.51
Iron7.87
Lead11.34

Magnesium (1.74 g/cm³) is about one-third the density of iron and even lighter than aluminium. This makes magnesium alloys attractive wherever reducing weight matters, from aircraft to laptops to wheelchairs.

What is magnesium?

Magnesium is an alkaline earth metal in Group 2 of the periodic table. It has 12 protons and two electrons in its outer shell, which it gives away readily to form Mg²⁺ ions. It is lighter than aluminium but does not burst into flame in air at room temperature. When ignited, however, it burns intensely at temperatures above 3,000°C and is almost impossible to extinguish, water makes it worse, not better.

Magnesium gets its name from Magnesia, a region of Greece where a white mineral called magnesia alba (magnesium carbonate, MgCO₃) was found and used as a medicine since at least the 1600s. A different mineral from the same region, magnetite, gave us the word magnet, though magnesium itself is not magnetic. The symbol Mg comes from the element's name.

Fact Every green leaf on Earth owes its colour to magnesium. The chlorophyll molecule that captures sunlight and powers photosynthesis has a magnesium atom right at its centre, in the same way that haemoglobin, which carries oxygen in blood, has an iron atom at its centre. Without magnesium, plants cannot photosynthesise.

Where you find magnesium

In space

Magnesium is the ninth most abundant element in the universe, produced in the cores of large stars. It is a major component of Earth's mantle: the rocky layer between the core and the crust. Many meteorites contain significant amounts of magnesium silicate minerals, and the surface of Mars is rich in magnesium-containing rocks.

On Earth

Magnesium is the eighth most abundant element in the Earth's crust and is widespread in rocks, seawater and living things.

  • Dolomite and magnesite. These carbonate minerals are major magnesium ores. Dolomite forms entire mountain ranges: the Italian Dolomites are named after it.
  • Seawater. Magnesium is the third most abundant dissolved element in seawater. Some magnesium is commercially extracted directly from the sea.
  • Olivine and pyroxene. These magnesium silicate minerals make up most of the Earth's mantle and are common in basalt rock and meteorites.

How we use magnesium

  • Lightweight alloys. Magnesium alloys are among the lightest structural metals. Mixed with aluminium, they are used in the frames of laptops, aircraft seats, camera bodies and car components, saving weight and fuel.
  • Fireworks and flares. Magnesium powder burns with a brilliantly bright white flame, producing a light almost as bright as the Sun. Used in emergency flares, military illumination rounds and traditional flash photography.
  • Supplements and medicine. Magnesium is essential for over 300 enzyme reactions in the body. Magnesium tablets treat muscle cramps, and milk of magnesia (magnesium hydroxide) is a common antacid.
  • Steel making. Pure magnesium is used to remove sulfur from molten iron before it is made into steel, improving quality.
Did you know? A magnesium fire cannot be put out with water. Water reacts with burning magnesium to produce hydrogen gas and magnesium oxide, making the fire more violent and dangerous. Burning magnesium must be smothered with sand or dry powder. Even carbon dioxide fire extinguishers do not work, magnesium is hot enough to split the CO₂ molecule.

How it was discovered

Magnesium was first recognised as a distinct element by the Scottish chemist Joseph Black in 1755, who showed that magnesia alba was chemically different from calcium oxide. However, Black could not isolate the metal. Humphry Davy produced impure magnesium metal in 1808 using electrolysis, and Antoine Bussy obtained a purer sample in 1831 by reducing magnesium chloride with potassium metal.

Deeper dive: magnesium in biology and combustion chemistry

Magnesium is the fourth most abundant mineral in the human body, with roughly 25 grams in the average adult. It is essential for ATP synthesis, ATP is the molecule that stores and releases energy in cells, powering almost everything your body does. Magnesium ions (Mg²⁺) are required for ATP to be biologically active. Without magnesium, cells could not use energy at all. Magnesium also regulates hundreds of enzymes and is critical for DNA replication and protein synthesis.

When magnesium burns, the reaction is so energetic that it reaches temperatures above 3,000°C. The white product, magnesium oxide (MgO), is extremely stable and has a very high melting point (2,852°C) of its own. This is why burning magnesium cannot be smothered with water or CO₂: magnesium is hot enough to split water molecules (producing hydrogen that then catches fire) and react with CO₂ (producing black carbon soot and magnesium oxide).

Magnesium alloys are typically 30-40% lighter than equivalent aluminium alloys and have been used in performance cars, aircraft and spacecraft. The Volkswagen Beetle had a magnesium alloy crankcase: one reason early Beetles were so light and economical. Modern Formula 1 cars use magnesium components wherever regulations permit.

Magnesium is a light metal that burns brilliantly and powers life through photosynthesis. Moving one step to the right on the same row brings us to aluminium, the most abundant metal in the Earth's crust.