A magnesium atom has 12 electrons around its nucleus. There are 2 in the K shell, 8 in the L shell and 2 in the M shell.
By giving up the two electrons in its outermost M shell, magnesium becomes a doubly charged positive ion, \mathrm{Mg}^{2+}.
The elements of group 2 of the periodic table all have two electrons in their outer shell, and they are called the alkaline earth metals. Beryllium and magnesium differ somewhat in their properties from the other group 2 elements.
When a magnesium ribbon is set alight, it burns fiercely with a dazzling white light and turns into white magnesium oxide.
2\mathrm{Mg} + \mathrm{O_2} \longrightarrow 2\mathrm{MgO}
This light is so intense that it was once used for the flash in photography. Looking straight at burning magnesium can damage the eyes.
Burning magnesium keeps on burning even in carbon dioxide, forming magnesium oxide and carbon. For this reason, a magnesium fire cannot be put out with a carbon dioxide extinguisher.
Magnesium hardly reacts with cold water, but it reacts slowly with hot water, giving off hydrogen.
\mathrm{Mg} + 2\mathrm{H_2O} \longrightarrow \mathrm{Mg(OH)_2} + \mathrm{H_2}
The magnesium hydroxide formed dissolves only slightly in water, but it makes the water weakly basic. If phenolphthalein has been added, the water turns pale pink.
Calcium and barium, which are also in group 2, react even with cold water.
When a platinum wire dipped in a magnesium compound is held in the flame of a gas burner, the colour of the flame does not change. This is because magnesium gives out almost no visible light in a gas burner flame.
Other group 2 elements do colour a flame: calcium turns it orange-red, strontium crimson and barium yellowish green. Giving no colour to a flame is one of the ways in which beryllium and magnesium differ from the other group 2 elements.
The density of magnesium is about 1.74 g/cm³, which is about two-thirds that of aluminium (about 2.70 g/cm³) and less than a quarter that of iron (about 7.87 g/cm³). Its alloys with aluminium and other metals are light and strong, and they are used for the cases of laptop computers and for car parts.
The liquid left after salt has been taken from seawater is called bittern, and its main component is magnesium chloride, \mathrm{MgCl_2}. Bittern is used to set soy milk when making tofu.
Chlorophyll, the substance that makes leaves green, also has a magnesium atom at its centre.