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Sodium
Sodium (Na, atomic number 11) is the third alkali metal — a soft, silvery-white BCC metal with a melting point of 97.7 °C, density of 0.97 g/cm³ (lighter than water), and a standard electrode potential of –2.71 V vs. SHE, making it one of the strongest common reducing agents. Sodium reacts vigorously with water (2Na + 2H₂O → 2NaOH + H₂↑), ignites in moist air, and must be stored under mineral oil or in an inert atmosphere; it is less violent than potassium but more so than lithium. It is the sixth most abundant element in Earth's crust (~2.3 wt%), occurring ubiquitously in halite (NaCl), trona (Na₂CO₃·NaHCO₃·2H₂O), and feldspar minerals, but never as the free metal. Elemental sodium is produced industrially by the Downs process — molten electrolysis of NaCl at ~600 °C — at ~100,000 tonnes/year, primarily for chemical synthesis and nuclear reactor coolant applications.
The largest single use of elemental sodium metal is as a liquid-metal coolant in sodium-cooled fast reactors (SFRs), exploiting its high thermal conductivity (~70 W/m·K liquid), low neutron moderation cross-section, and low melting point relative to other liquid metals. Sodium coolant operates at 350–550 °C in the primary circuit of fast breeder reactors (BN-800/BN-1200 in Russia, Superphénix in France, EBR-II in the US) and experimental systems; a major engineering challenge is the violent reaction of Na with air and water, requiring fully inert-gas (Ar) atmospheres for all sodium-circuit maintenance. In organic chemistry, sodium metal is the classic reductant for the Birch reduction (Na in liquid NH₃, yielding solvated electrons that reduce arenes), for synthesis of sodium alkoxides (NaOR, strong bases for ester condensations and transesterifications), and for production of sodium hydride (NaH, a non-nucleophilic base widely used in deprotonation reactions). NaK alloy (typically 78 wt% K, mp –12.6 °C) extends liquid metal applicability to near-ambient temperatures for secondary coolant loops and heat transfer systems.
Sodium-ion batteries (SIBs) represent the most rapidly growing new application for sodium chemistry — though SIBs use sodium compounds (hard carbon anodes, layered oxide or Prussian blue cathodes with NaPF₆/NaFSI electrolytes) rather than sodium metal, the technology directly parallels lithium-ion battery chemistry and exploits the far greater natural abundance and lower cost of Na vs. Li. Commercial SIB cells (CATL Naxtra, HiNa, Faradion/Reliance) reached market in 2023–2024 targeting grid storage, low-speed EVs, and two-wheelers where energy density is less critical than cost (~$50–70/kWh SIB target vs. ~$100/kWh LFP). The sodium D-line doublet (D₁ 589.6 nm / D₂ 589.0 nm) — the intense yellow emission of sodium vapor — is produced by the 3p→3s electronic transition and has historically been the primary emission of high-pressure sodium (HPS) street lamps; it is also the standard wavelength reference for optical dispersion measurements (Abbe number, refractive index nd) and the basis of laser guide star systems used in adaptive optics at major observatories.
General Properties
| Property | Value | Notes |
|---|---|---|
| Atomic Number | 11 | Group 1 (alkali metals), Period 3; 3s¹; only stable oxidation state +1. Na⁺ (ionic radius 102 pm) is the dominant extracellular cation in biology (~145 mM plasma), maintained against the concentration gradient by Na⁺/K⁺-ATPase at the cost of ~25% of neuronal ATP. |
| Atomic Mass | 22.990 u | Monoisotopic — ²³Na is the only stable isotope (100% natural abundance). This makes Na one of the 22 monoisotopic elements and simplifies ICP-MS and NAA analysis; ²³Na(n,γ)²⁴Na activation is used to determine Na content in geological and biological samples. |
| Density (20 °C) | 0.968 g/cm³ | Lighter than water — Na floats before reacting violently. The low density arises from the large BCC unit cell (a = 4.225 Å) with only 2 atoms per cell. |
| Melting Point | 97.7 °C (370.9 K) | Low melting point makes Na a liquid at modest temperatures; liquid Na density is ~0.93 g/cm³ at 100 °C and ~0.83 g/cm³ at 500 °C. The liquid range (97.7–883 °C) at atmospheric pressure is well-suited to nuclear reactor primary coolant operation at 350–550 °C. |
| Boiling Point | 883 °C (1,156 K) | Moderate boiling point for a metal. Na vapor pressure reaches ~1 Pa at ~440 °C and ~1 kPa at ~600 °C; sodium vapor lamps operate with Na vapor at ~10–100 Pa (low-pressure) or ~10–100 kPa (high-pressure), giving the narrow D-line doublet or broadened yellow-white spectrum respectively. |
| Thermal Conductivity | 140 W/m·K (solid, 20 °C); ~70 W/m·K (liquid, 400 °C) | High thermal conductivity — among the highest of any liquid metal at reactor operating temperatures (~70 W/m·K at 400 °C vs. ~20 W/m·K for water), underpinning its use as a fast reactor coolant. The solid value (140 W/m·K) drops substantially on melting. |
| Electrical Resistivity | 47.0 nΩ·m (20 °C) | The source lists "4.70 × 10⁻⁸ Ω·m" — converted to SI base units: 47.0 nΩ·m. Low resistivity consistent with a good metallic conductor; increases linearly with temperature in the solid phase. |
| Crystal Structure | BCC; a = 4.225 Å (stable to –237 °C, then transitions to FCC/HCP) | BCC stable at room temperature; transforms to FCC below ~–237 °C (36 K) and to HCP below ~–268 °C (5 K) under some conditions. At high pressure (>65 GPa), Na adopts a transparent insulating phase — an unusual pressure-induced metal-to-insulator transition. |
Mechanical Properties
| Property | Value | Notes |
|---|---|---|
| Young's Modulus | ~10 GPa | Very low modulus — Na is extremely soft and deformable, comparable to other alkali metals. It can be cut with a knife and pressed into shape at room temperature. |
| Hardness | Mohs ~0.5 | One of the softest metals — softer than lead (Mohs 1.5). The low hardness reflects the weak metallic bonding from a single 3s valence electron per atom. |
Chemical Properties
| Property | Value / Behavior | Notes |
|---|---|---|
| Oxidation State | +1 (dominant); –1 in sodides (rare) | Na⁺ is the universal form in aqueous chemistry and biology. Rare Na⁻ (sodide) occurs in cryptand complexes where Na⁺ is sequestered, leaving Na⁻ with a filled 3s² shell; Cs⁺Na⁻ (cesium sodide) is a room-temperature stable ionic salt. |
| Reactivity with water | Vigorous: 2Na + 2H₂O → 2NaOH + H₂↑; ΔH = –368 kJ/mol Na | Less violent than K (H₂ does not usually ignite spontaneously with Na at room temperature, unlike K), but still hazardous — larger pieces of Na can ignite the evolved H₂. Na must be stored under mineral oil or dry inert gas. |
| Reactivity in air | Rapid tarnishing; forms Na₂O and NaOH in moist air; Na₂O₂ in dry O₂ | Unlike K (which forms KO₂ superoxide), Na forms predominantly Na₂O₂ (sodium peroxide) in excess dry O₂ — used industrially as an oxidizing bleach and oxygen source. Na₂O₂ reacts exothermically with water, creating a fire hazard with water-based extinguishers. |
| Standard Electrode Potential | –2.71 V vs. SHE | Strong reducing agent, though less so than Li (–3.04 V) and K (–2.93 V). Na dissolves in liquid ammonia to give deep blue solvated electron solutions (lower concentration) used for Birch reductions. |
| Identifier | Value |
|---|---|
| Symbol | Na |
| Atomic Number | 11 |
| CAS Number | 7440-23-5 |
| UN Number | UN1428 (sodium, metal) |
| EINECS Number | 231-132-9 |
| Isotope | Type | Notes |
|---|---|---|
| ²³Na | Stable | 100% natural abundance; I = 3/2, NMR-active. ²³Na MAS-NMR characterizes Na coordination in glasses, zeolites, and solid electrolytes (Na-β-alumina, NASICON). The large quadrupole moment (Q = 0.104 barn) causes line broadening in disordered environments, making ²³Na NMR sensitive to local site symmetry. |
| ²²Na | Radioactive | t½ = 2.602 yr; β⁺ (Emax = 545.8 keV, 89.8%) + 1,274.5 keV gamma. Produced by ²⁴Mg(p,α)²²Na or ²²Ne(p,n)²²Na in cyclotrons. Used as a long-lived positron source for positron annihilation lifetime spectroscopy (PALS) to probe vacancy defects in metals, polymers, and porous materials; also used as a PET calibration phantom source. |
| ²⁴Na | Radioactive | t½ = 14.96 hr; β⁻ (Emax = 5.51 MeV) + 1,368.6 keV and 2,754.0 keV gammas. Produced by ²³Na(n,γ)²⁴Na (thermal neutron activation, σ = 0.53 barn) — used in neutron activation analysis (NAA) for Na determination in geological, environmental, and biological samples, and as a short-lived radiotracer for studying Na transport in plants and soils. |
Scientific & Research Applications
| Use Case | Form Typically Used | Description |
|---|---|---|
| Birch Reduction & Organic Synthesis | Na metal pieces/cubes (99.9%), liquid NH₃ solvent | Na dissolves in liquid ammonia (–33 °C) to give blue solvated electron solutions used for Birch reductions (arene→1,4-cyclohexadiene, 1,4-selectivity for electron-poor rings), alkyne→trans-alkene reduction, and reductive cleavage of C–O bonds. Also used to prepare NaH, NaNH₂, and sodium alkoxides (NaOEt, NaOtBu) as strong bases for deprotonation and condensation reactions. |
| Sodium-Ion Battery Research | Na metal foil/disc (99.9%) as counter/reference electrode | Na metal foil is used as the counter and reference electrode in half-cell testing of SIB anode and cathode materials — the standard method for measuring capacity, rate capability, and cycling stability of hard carbon anodes, Prussian blue cathodes, and layered oxide (NaMO₂) materials. Challenges include Na dendrite formation and the instability of the Na metal/electrolyte SEI compared to Li. |
| Laser Guide Stars & Spectroscopy | Na vapor (from Na metal heated in sealed cells or Na resonance lamps) | The Na D-line doublet (D₁ 589.6 nm / D₂ 589.0 nm) is excited by tunable dye or solid-state lasers in laser guide star (LGS) adaptive optics systems — a 589 nm laser beam excites mesospheric Na atoms (~90 km altitude) to create an artificial reference star for wavefront correction. The D-lines are also the primary wavelength standard for optical dispersion (refractive index nd, Abbe number) measurements in optics. |
| ²²Na Positron Source (PALS) | ²²NaCl sealed source (~10 µCi) | ²²Na (t½ = 2.6 yr) is the standard positron source for positron annihilation lifetime spectroscopy (PALS), which measures vacancy defect concentration and size in metals, semiconductors, and polymers from the positronium lifetime distribution. The simultaneous 1,274.5 keV gamma provides the start signal for timing electronics. |
Industrial & Commercial Applications
| Sector | Form / Grade Used | Description |
|---|---|---|
| Sodium-Cooled Fast Reactors | High-purity Na metal (>99.9%), oxygen <1 ppm specification | Liquid sodium is the primary coolant in sodium-cooled fast reactors (SFRs) including Russia's BN-800/BN-1200, the planned Natrium (TerraPower) and ARC-100 designs. Na coolant purity is critical — dissolved oxygen above ~10 ppm causes corrosion of stainless steel cladding; cold traps precipitate Na₂O to maintain O <1 ppm. The Na/air and Na/water reaction hazard requires double-walled piping and inert Ar cover gas throughout. |
| Metal Reduction (Ti, Zr) | Na metal (99.9%) | The Hunter process uses Na metal to reduce TiCl₄ to titanium sponge (4Na + TiCl₄ → Ti + 4NaCl), competing with the Kroll process (Mg reduction). Na reduction also produces Zr and Ta metal. Though largely displaced by Mg-based Kroll for Ti, Na reduction remains used for specialty Ti and for production of reactive metal powders. |
| Sodium Vapor Lamps | Na metal pellets in lamp envelope | Low-pressure sodium (LPS) lamps produce near-monochromatic 589 nm yellow light (highest luminous efficacy of any lamp, ~200 lm/W) used in highway and tunnel lighting where color rendering is not required. High-pressure sodium (HPS) lamps broaden the spectrum to warm white (~2,200 K, ~150 lm/W) for street and industrial lighting. Both technologies are being displaced by LED but remain in large installed bases. |
| Chemical Industry (NaH, NaOH, Na₂O₂) | Na metal (99.9%) as feedstock | Na metal is the feedstock for sodium hydride (NaH, from Na + H₂ at 200–300 °C, a widely used strong base in pharma synthesis), sodium peroxide (Na₂O₂, from Na combustion in O₂, used as a bleach and O₂ source in closed environments), and for production of sodium azide (NaN₃, from Na + N₂O or Na + NH₃ routes, used in automotive airbag inflators and as a pharmaceutical intermediate). |
| Purity | Main Use |
|---|---|
| 99.9% (3N) | Standard grade for all primary applications: organic synthesis (Birch reductions, NaH/NaOR preparation), sodium-ion battery half-cell electrodes, metal reduction (Hunter process for Ti/Zr), sodium vapor lamp manufacture, NaH/Na₂O₂/NaN₃ chemical production, and sodium-cooled reactor coolant where further oxygen control is achieved by cold-trap purification in-circuit |
| Synonym / Alternative Name | Context |
|---|---|
| Na | Chemical symbol; from Latin/Medieval Latin Natrium (derived from Arabic natrun / Greek nitron, a naturally occurring sodium carbonate mineral). The symbol Na was established by Berzelius; the element was first isolated by Humphry Davy in 1807 by electrolysis of molten NaOH. |
| Sodium metal | Commercial designation distinguishing the elemental metal from sodium compounds (NaCl, NaOH, Na₂CO₃, etc.) in supply chain and safety documentation; used in ASTM standards, UN hazard classifications, and commodity trading. |
| Natrium | The element name in German (Natrium), as well as in several other European languages; the source of the chemical symbol Na. Used throughout German, Dutch, Scandinavian, and Eastern European scientific literature and industrial standards. |
| Natrium metal | German-language commercial designation for elemental sodium metal; used in German-language safety data sheets (Sicherheitsdatenblatt), technical specifications, and chemical supply documentation across German-speaking markets. |