Magnesium is
a chemical element with the symbol "Mg" and atomic number 12.
Its common oxidation number is +2. It is an alkaline earth metal and the eighth most abundant element in
the Earth's crust and
ninth in the known universe as a whole. Magnesium
is the fourth most common element in the Earth as a whole (behind iron, oxygen
and silicon), making up 13% of the planet's mass and a large fraction of the
planet's mantle. The relative abundance of magnesium is related to the fact
that it easily builds up in supernova stars
from a sequential addition of three helium nuclei to carbon (which in turn is
made from three helium nuclei). Due to magnesium ion's high solubility in
water, it is the third most abundant element dissolved in seawater.
The free element (metal) is not
found naturally on Earth, as it is highly reactive (though once produced, it is
coated in a thin layer of oxide (see passivation), which partly masks this
reactivity). The free metal burns with a characteristic brilliant white light,
making it a useful ingredient in flares. The metal is now mainly obtained
by electrolysis of
magnesium salts obtained from brine. Commercially, the chief use for the metal is as an alloying agent to
make aluminium-magnesium
alloys, sometimes called magnalium or magnelium.
Since magnesium is less dense than aluminium, these alloys are prized for their
relative lightness and strength.
In human biology,
magnesium is the eleventh most abundant element by mass in the human body.
Its ions are essential to all living cells,
where they play a major role in manipulating important biological poly-phosphate compounds
like ATP, DNA, and RNA. Hundreds of enzymes thus
require magnesium ions to function. Magnesium compounds are used medicinally as
common laxatives,
antacids (e.g., milk of magnesia), and in a number of
situations where stabilization of abnormal nerve excitation and
blood vessel spasm is required (e.g., to treat eclampsia).
Magnesium ions are sour to the taste, and in low concentrations they help to
impart a natural tartness to fresh mineral waters.
