Magnesium (Mg)

Magnesium is the lightest structural metal in ordinary use, about a third lighter than aluminium, and it is the fourth most abundant element in the Earth as a whole. A magnesium atom sits at the centre of every chlorophyll molecule, so the colour of every leaf on the planet depends on it. Burning magnesium reaches around 3,100 °C and keeps burning inside carbon dioxide or steam, which is why a magnesium fire in a machine shop cannot be fought with an ordinary extinguisher.

Properties

Atomic number

12

Symbol

Mg

Standard atomic weight

24.305 u

Category

Alkaline earth metal

Group

2

Period

3

Block

s-block

Electron configuration

[Ne] 3s²

Electrons per shell

2, 8, 2

Melting point

923 K — 649.9 °C — 1201.7 °F

Boiling point

1,363 K — 1089.8 °C — 1993.7 °F

Density

1.74 g/cm³

Electronegativity

1.31 (Pauling)

Oxidation states

+2

Discovered

1808 by Humphry Davy

Electron configuration

Full

1s² 2s² 2p⁶ 3s²

Noble gas shorthand

[Ne] 3s²

Shells

K: 2 · L: 8 · M: 2

What it is used for

• Lightweight alloy castings for gearboxes, laptops and camera bodies
• Sacrificial anodes in water heaters and buried steel pipelines
• Epsom salts and milk of magnesia
• Magnesium chloride for de-icing roads
• Flares, fireworks and emergency fire-starters

Hazards

• Burning magnesium cannot be extinguished with water, foam or carbon dioxide
• The flame emits enough ultraviolet to damage eyes at close range
• Fine swarf and dust are an explosion hazard in machine shops

IUPAC publishes an interval, [24.304, 24.307]; 24.305 is the conventional value.

Neighbours in the table

Group 2

Elements in the same group share their outer electron count, which is why they behave similarly in reactions. Magnesium sits in group 2 with beryllium, calcium, strontium, barium, radium.