Since magnesium is less dense than aluminium, these alloys are prized for their relative lightness and strength. Commercially, the chief use for the metal is as an alloying agent to make aluminium-magnesium alloys, sometimes called " magnalium" or "magnelium". The metal is now mainly obtained by electrolysis of magnesium salts obtained from brine. The free metal burns with a characteristic brilliant white light, making it a useful ingredient in flares. 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, which partly masks this reactivity). Due to the magnesium ion's high solubility in water, it is the third most abundant element dissolved in seawater. It is relatively abundant because it is easily built up in supernova stars by sequential additions of three helium nuclei to carbon (which in turn is made from three helium nuclei). 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. It is the eighth most abundant element in the Earth's crust and the ninth in the known universe as a whole. Magnesium (symbol Mg) is an alkaline earth metal and has common oxidation number +2. The octet rule generally applies to period 3 in the same way as to period 2 elements, because the 3d subshell is normally non-acting. This makes all eight elements analogs of the period 2 elements in the same exact sequence. There is a 3d subshell, but-in compliance with the Aufbau principle-it is not filled until period 4. In a quantum mechanical description of atomic structure, this period corresponds to the buildup of electrons in the third ( n = 3) shell, more specifically filling its 3s and 3p subshells. All of the period 3 elements occur in nature and have at least one stable isotope. The first two, sodium and magnesium, are members of the s-block of the periodic table, while the others are members of the p-block. The third period contains eight elements: sodium, magnesium, aluminium, silicon, phosphorus, sulfur, chlorine and argon. The periodic table is laid out in rows to illustrate recurring (periodic) trends in the chemical behaviour of the elements as their atomic number increases: a new row is begun when the periodic table skips a row and a chemical behaviour begins to repeat, meaning that elements with similar behaviour fall into the same vertical columns. A period 3 element is one of the chemical elements in the third row (or period) of the periodic table of the chemical elements.
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