The third period contains eight elements: sodium, magnesium, aluminium, silicon, phosphorus, sulfur, chlorine and argon.
[1] 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.
Due to the magnesium ion's high solubility in water, it is the third most abundant element dissolved in seawater.
Magnesium ions are sour to the taste, and in low concentrations help to impart a natural tartness to fresh mineral waters.
Aluminium is remarkable for the metal's low density and for its ability to resist corrosion due to the phenomenon of passivation.
It is most widely distributed in dusts, sands, planetoids and planets as various forms of silicon dioxide (silica) or silicates.
Silicate goes into Portland cement for mortar and stucco, and combined with silica sand and gravel, to make concrete.
Because of wide use of silicon in integrated circuits, the basis of most computers, a great deal of modern technology depends on it.
Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8.
Elemental sulfur crystals are commonly sought after by mineral collectors for their brightly colored polyhedron shapes.
It was needed to make the best quality of black gunpowder, and the bright yellow powder was hypothesized by alchemists to contain some of the properties of gold, which they sought to synthesize from it.
Elemental sulfur was once extracted from salt domes, where it sometimes occurs in nearly pure form, but this method has been obsolete since the late 20th century.
Today, almost all elemental sulfur is produced as a byproduct of removing sulfur-containing contaminants from natural gas and petroleum.
The element's commercial uses are primarily in fertilizers, because of the relatively high requirement of plants for it, and in the manufacture of sulfuric acid, a primary industrial chemical.
Hydrogen sulfide produced by living organisms imparts the characteristic odor to rotting eggs and other biological processes.
The suggestion that this newly discovered gas was a simple element was made in 1809 by Joseph Louis Gay-Lussac and Louis-Jacques.
This was confirmed in 1810 by Sir Humphry Davy, who named it chlorine, from the Greek word χλωρός (chlōros), meaning "green-yellow".
The great oxidizing power of chlorine led it to its bleaching and disinfectant uses, as well as being an essential reagent in the chemical industry.
The name "argon" is derived from the Greek neuter adjective ἀργόν, meaning "lazy" or "the inactive one", as the element undergoes almost no chemical reactions.
The complete octet (eight electrons) in the outer atomic shell makes argon stable and resistant to bonding with other elements.
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 polyphosphate compounds like ATP, DNA, and RNA.
[11] 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).
It is much more important to the metabolism of plants, particularly many grasses, and silicic acid (a type of silica) forms the basis of the striking array of protective shells of the microscopic diatoms.
Today, the most important commercial use of phosphorus-based chemicals is the production of fertilizers, to replace the phosphorus that plants remove from the soil.
In metabolic reactions, sulfur compounds serve as both fuels and respiratory (oxygen-replacing) materials for simple organisms.
Disulfide bonds are largely responsible for the mechanical strength and insolubility of the protein keratin, found in outer skin, hair, and feathers, and the element contributes to their pungent odor when burned.