Sodium

Chemistry

Sodium atoms carry 11 electrons, one beyond the stable electron arrangement of neon. Its first ionization energy is 495.8 kJ/mol, and its second is 4562 kJ/mol, which explains why it typically forms ionic compounds as the Na+ cation. As a metal, sodium is less reactive than potassium but more reactive than lithium. It acts as a strong reducing agent, with a standard reduction potential of −2.71 volts for the Na+/Na pair, though both potassium and lithium have more negative potentials.

Salts and oxides

Sodium compounds are vital to commerce, especially in making glass, paper, soap, and textiles. Important examples include table salt, soda ash, baking soda, caustic soda, sodium nitrate, di- and tri-sodium phosphates, sodium thiosulfate, and borax. In its compounds, sodium bonds ionically to water and anions and is regarded as a hard Lewis acid. Most soaps are sodium salts of fatty acids; these soaps have higher melting points and feel harder than potassium soaps.

Like all alkali metals, sodium reacts with water exothermically, producing caustic soda and flammable hydrogen gas. High-speed camera footage suggests the resulting explosion is a Coulomb explosion. When burned in air, it mainly forms sodium peroxide along with some sodium oxide. Sodium tends to form water-soluble compounds, including halides, sulfates, nitrates, carboxylates, and carbonates.

Aqueous solutions

In water, it exists as aquo complexes [Na(H2O)n]+, with n ranging from 4 to 8; X-ray diffraction and computer models point to n = 6 as most common. Direct precipitation of sodium salts from water is uncommon because these salts have a strong affinity for water, though sodium bismuthate is an exception—it does not dissolve in cold water and breaks down in hot water. Sodium salts are typically isolated by evaporating the water or by adding an organic antisolvent like ethanol.

A crown ether such as 15-crown-5 can serve as a phase-transfer catalyst. Sodium levels in samples are measured using atomic absorption spectrophotometry or potentiometry with ion-selective electrodes.

Occurrence

Sodium makes up 2.27% of the Earth's crust, ranking as the sixth most common element and the fourth most abundant metal, trailing only aluminum, iron, calcium, and magnesium, while surpassing potassium. In the ocean, it is found at roughly 10.8 grams per liter. Due to its extreme reactivity, sodium never occurs in its pure form naturally.

It appears in a variety of minerals; some, like halite and natron, dissolve easily, while others, such as amphibole and zeolite, are far less soluble. The low solubility of minerals like cryolite and feldspar comes from their polymeric anions—in feldspar, this is a polysilicate. Across the universe, sodium is the 15th most abundant element, with an abundance of 20,000 parts per billion, meaning it accounts for 0.002% of all atoms.

Astronomical observations

Atomic sodium produces a very strong spectral line in the yellow-orange range, the same line used in sodium-vapor street lamps. This line appears as an absorption feature in many stars, including the Sun. It was first examined in 1814 by Joseph von Fraunhofer while he studied the lines in the solar spectrum, now called Fraunhofer lines. He labeled it the D line, though it is now understood to be a cluster of closely spaced lines split by fine and hyperfine structure.

The D line's strength makes it detectable in many astronomical settings. In stars, it shows up when surface temperatures are low enough for sodium to remain in atomic form rather than becoming ionized, which happens in stars of roughly F-type and cooler. Many other stars display a sodium absorption line, but this often comes from gas in the foreground interstellar medium instead. High-resolution spectroscopy can tell them apart because interstellar lines are much narrower than those broadened by stellar rotation.

Sodium has been spotted in various parts of the Solar System, including the exospheres of Mercury and the Moon, as well as other bodies. Some comets possess a sodium tail, first observed in Comet Hale–Bopp in 1997. It has also been detected in the atmospheres of certain extrasolar planets through transit spectroscopy.

Uses

Sodium in its pure metallic form has its uses, but the element is far more commonly encountered in compounds. Every year, millions of tons of sodium chloride, hydroxide, and carbonate are made. Sodium chloride is a go-to for de-icing roads and preserving food. Sodium bicarbonate shows up in baking as a leavening agent and in cleaning via sodablasting.

Many medications include sodium to help the body absorb them better, even though potassium is often the superior ion; sodium wins out because it is cheaper and lighter. Sodium hydride serves as a strong base in organic chemistry, for instance in the aldol reaction. The metal itself is chiefly used to manufacture sodium borohydride, sodium azide, indigo, and triphenylphosphine. It was once common for producing tetraethyllead and titanium metal, but production of sodium dropped after 1970 as those uses faded.

It also works as an alloying metal, an anti-scaling agent, and a reducing agent for metals when nothing else will do. The free element does not act as a scaling agent; instead, water softeners exchange other ions for sodium ions. Sodium vapor lamps light up many city streets, glowing yellow-orange at low pressure and shifting toward peach as pressure rises. Alone or mixed with potassium, sodium acts as a drying agent; it turns an intense blue when mixed with benzophenone if the material is dry.

In the lab, sodium is key to the Birch reduction and the sodium fusion test for analyzing compounds. It reacts with alcohols to form alkoxides, and when dissolved in liquid ammonia, it can turn alkynes into trans-alkenes. Lasers tuned to the sodium D line create artificial guide stars that help ground-based telescopes adjust their optics for clearer views.

Biological role

Sodium is a mineral the human body needs to manage blood volume, blood pressure, pH balance, and osmotic equilibrium. Newborns require a minimum of about 120 milligrams daily, while people over ten need at least 500 milligrams. Table salt, or sodium chloride, is the main dietary source, used for seasoning and preserving foods like pickles and jerky. In the United States, most sodium chloride comes from processed items.

Sodium also appears naturally in food and in additives such as monosodium glutamate, sodium nitrite, sodium saccharin, baking soda, and sodium benzoate. The U.S. Institute of Medicine sets the tolerable upper intake at 2.3 grams per day, though the average American consumes 3.4 grams. The American Heart Association advises no more than 1.5 grams daily. Because research has not provided enough evidence for an Estimated Average Requirement or Recommended Dietary Allowance, the National Academies committee established Adequate Intake levels instead.

For infants up to six months, the AI is 110 milligrams per day; for seven to twelve months, 370 milligrams; for children one to three years, 800 milligrams; four to eight years, 1,000 milligrams; for adolescents nine to thirteen years, 1,200 milligrams; fourteen to eighteen years, 1,500 milligrams; and for all adults, regardless of age or sex, 1,500 milligrams. Sodium chloride is about 39.34 percent elemental sodium by mass. This means that 3,812.91 milligrams of table salt contain the 1,500 milligrams of elemental sodium recommended as the adult AI.

Did You Know?

• Sodium at standard temperature and pressure is a soft silvery metal that combines with oxygen in the air, forming sodium oxides; bulk sodium is usually stored in oil or an inert gas.
• The name sodium is thought to originate from the Arabic suda, meaning headache, because sodium carbonate was known for alleviating headaches.
• About 100,000 tonnes of metallic sodium are produced annually, first commercially by carbothermal reduction of sodium carbonate at 1100 °C.