Average mass of an atom in unified atomic mass units (1 u is 1/12 of the mass of a carbon-12 atom). Accounts for the natural mix of the element’s isotopes.
Number of the vertical column, 1 to 18 in IUPAC numbering. Elements in a group share the same outer-shell layout, so their properties are alike.
Number of the horizontal row. Usually equal to the number of electron shells in the atom; palladium is the exception, with four shells in period 5.
The subshell (s, p, d or f) that receives the last electrons in the usual filling order; it divides the table into four blocks. Real atoms sometimes differ: lanthanum starts the f-block but has no f electrons.
[Ne] 3s²
How electrons are arranged in subshells. The bracket stands for the previous noble gas; the superscript is the number of electrons in a subshell.
2, 8, 2
Number of electrons in each shell, from the nucleus outwards.
650 °C (923 K)
Temperature at which the solid turns liquid at standard pressure. Helium, carbon and arsenic do not melt at normal pressure: helium needs about 25 atm, carbon and arsenic turn straight into vapour.
1,090 °C (1,363 K)
Temperature at which the liquid boils at standard pressure. For carbon and arsenic, the temperature at which the solid turns straight into vapour (sublimation).
1.74 g/cm³
Mass of one cubic centimetre of the substance. For gases, at 0 °C and standard pressure.
1.31 Pauling
How strongly an atom pulls electrons towards itself in a chemical bond. On the Pauling scale it runs from 0.79 for caesium to 3.98 for fluorine.
7.646 eV
Energy needed to remove the most loosely bound electron from an atom. The lower it is, the more easily the element gives up an electron, which is typical of metals.
no stable negative ion
Energy released when a neutral atom gains an electron. Highest for the halogens. Noble gases, nitrogen and some metals form no stable negative ion.
141 pm
Half the length of a single covalent bond between two atoms of the element. It shows the size of an atom inside a molecule or crystal: 76 pm for carbon, 132 pm for iron.
+2
Charges the atom would have in its compounds if all its bonds were ionic; the main ones are listed.
Magnesium (Mg) is a chemical element with atomic number 12. It sits in group 2 and period 3 of the periodic table, in the s-block. It is an alkaline earth metal. At room temperature it is a solid. Its standard atomic weight is 24.305.
Origin of the name
Magnesium is named after Magnesia, a district of Thessaly in Greece, rich in magnesium minerals. Magnetite and manganese take their names from the same region, so chemists long had to tell these substances apart. Joseph Black recognised magnesium as an element in 1755, and Humphry Davy obtained the metal in 1808.
Where is magnesium found?
Magnesium never occurs free, but it is the eighth most abundant element in the Earth’s crust, about 2%. Its main minerals are dolomite, magnesite and carnallite. It is also the third most abundant element dissolved in sea water, after chlorine and sodium, and much of the metal is extracted from sea water and brines.
What is magnesium used for?
Magnesium is the lightest metal used in construction: alloyed with aluminium, it goes into aircraft, cars and the cases of laptops and cameras. It burns with a dazzling white light, so it is used in flares and fireworks. It also frees titanium and uranium from their compounds. Milk of magnesia (magnesium hydroxide) soothes heartburn, and Epsom salt (magnesium sulfate) is a bath salt and laxative.
Interesting facts
In 1618 a farmer in Epsom, England, saw that his cows refused to drink from a bitter well: its water contained magnesium sulfate, ever since called Epsom salt.
At the centre of every chlorophyll molecule, the green pigment of plants, sits a magnesium atom.
Burning magnesium cannot be put out with water: it reacts with it and burns even more fiercely.