Atomic number 11

Sodium

Na
+ phase
− phase

Shells Orbitals 3s

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11 electrons in 3 shells: 2, 8, 1.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Alkali metal
Period: 3
Group: 1
Block: s
Standard atomic weight: 22.990(1) Da
Electron configuration: [Ne] 3s¹
Phase at 25 °C: solid
Melting point: 97.8 °C
Boiling point: 883 °C
Density: 0.97 g/cm³
Electronegativity (Pauling): 0.93
Discovered: 1807, Humphry Davy

Neighbours in the table

Ne Mg

Sodium keeps one 3s electron outside a filled neon core: eleven electrons in three shells of 2, 8 and 1. Group 1 atoms have the largest radius in their period and the lowest first ionisation energy, the energy needed to strip one electron from a neutral atom. Sodium’s outer electron therefore leaves easily, and what is left is Na⁺, a stable ion with a charge of 1+. Its Pauling electronegativity, 0.93, is among the lowest on the scale; electronegativity measures how strongly an atom pulls on the electrons of a bond it shares.

So sodium gives the electron away rather than sharing it. It reacts with every nonmetal except the noble gases, and each product is an ionic compound of 1+ metal ions. A metal joined to a nonmetal makes a salt, an ionic compound of positive and negative ions, so sodium chloride is a lattice of ions, not a molecule. The family name follows from that: group 1 metals and their oxides turn water alkaline, meaning strongly basic.

Los Alamos calls it a soft, bright, silvery metal. At 0.97 g/cm³ sodium is lighter than water and floats on it. It melts at 97.80 °C, just below the temperature at which water boils.

Where it is found

Sodium is found in salt deposits left where ancient seas evaporated, in silicates, and in minerals such as soda niter, cryolite and zeolite. A kilogram of crust holds 2.36 × 10⁴ milligrams of sodium on PubChem’s element record, 2.36 per cent of its mass. Seawater carries another 1.08 × 10⁴ milligrams per litre. Jefferson Lab ranks it sixth most abundant on Earth; Los Alamos puts it first among the alkali metals.

Sodium never occurs as the free metal. Group 1 ions are very difficult to reduce, to add electrons to, so an electric current has to free the metal. Sodium compounds were in use long before anyone saw the element, so the metal dates only from 1807.

Industry uses a Downs cell. It electrolyses molten sodium chloride, with calcium chloride added to bring the melt from 801 °C down to 600 °C. Sodium ions reach the negative cathode and pick up electrons there, turning back into metal. The liquid sodium, lighter than the molten salt, floats up and flows into a collector. The chlorine leaving the positive anode is a valuable product in its own right.

Why it matters

Sodium is too soft and too reactive for the jobs other metals do, so industry uses it for what it reacts with. It reduces titanium and zirconium compounds in the making of those metals. Chemists use it to prepare esters, the compounds an acid and an alcohol make together, and other organic compounds. The manufacture of certain dyes, drugs and perfumes depends on reduction by sodium or lithium.

Sodium and its compounds colour a flame bright yellow, and an electrical discharge through sodium vapour gives the same colour. Low-pressure sodium streetlamps give about 200 lumens per watt, as much visible light per watt as almost any electric lamp. Their light is nearly a single wavelength, 589 nm, and the eye resolves one colour more sharply than white light, whose colours come to focus almost two dioptres apart. Sodium marks sunlight too: its D lines are among the most prominent features of the solar spectrum.

Most people meet the element in its compounds. According to PubChem, common ones include table salt, soda ash, baking soda, caustic soda, Chile saltpeter and borax. Ordinary soap is a sodium salt of fatty acids, and sodium compounds run through the paper, glass, textile, petroleum and metal industries. Refining aluminium begins with hot sodium hydroxide dissolving bauxite into soluble sodium aluminate.

Properties

Properties of sodium Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	11
Standard atomic weight	22.990(1) Da	Da	IUPAC/CIAAW 2024
Melting point	97.8 °C	°C	Los Alamos: 97.8 °C
Boiling point	883 °C	°C	Los Alamos: 883 °C
Density	0.97 g/cm³	g/cm³
Electronegativity (Pauling)	0.93
Electron configuration	1s² 2s² 2p⁶ 3s¹
Phase at 25 °C	solid

Safety

Sodium metal is classified Danger on PubChem’s record, for two hazards that both follow from how readily the atom loses its outer electron. In contact with water it releases a flammable gas that may ignite by itself, and it burns skin and eyes. Part of that burn is chemical: the metal reacts with moisture in air or in tissue to form caustic sodium hydroxide. That concerns people who handle the metal rather than anyone meeting sodium in salt or baking soda, and the PubChem Laboratory Chemical Safety Summary below is the sheet to consult.

Questions

Why does sodium react so violently with water?

Sodium keeps one electron outside a filled core, and group 1 atoms have the lowest first ionisation energy in their period, so that electron leaves with very little encouragement. Water takes it. The products are hydrogen gas and a basic solution of sodium hydroxide, and international safety data call the reaction violent. Whether the metal also catches fire depends on how much bare metal and oxide meet the water.

Why is sodium stored under oil?

Sodium reacts with the moisture in air and with oxygen, so both have to be kept off it. Jefferson Lab puts the reason plainly: because the metal can ignite on contact with water, it belongs in a moisture-free environment. The safety card's storage line is dry, under mineral oil, in a well-closed container. The same PubChem record notes that chemistry laboratories may hold one-pound bricks or smaller pieces, usually kept under oil.

Is sodium metal the same as the sodium in table salt?

No. The metal is the element on its own, uncombined, and it reacts with water and air. In salt that electron has gone to chlorine, leaving a sodium ion with a charge of 1+, which is stable and unreactive by comparison. Sodium chloride is a lattice of those ions, not a molecule.

Who discovered sodium?

Humphry Davy first isolated it in 1807, by passing an electric current through caustic soda, which is sodium hydroxide. The substance was not new: sodium compounds had long been recognised, and the value of common salt in animal nutrition goes back to prehistory. What was new was the current. Group 1 ions resist chemical reduction, so the free element had to wait for electrolysis.

Why is sodium's symbol Na?

The symbol comes from the Latin natrium, for natron, the word English renders as soda. The English name has the same root, soda, with the Medieval Latin sodanum, a headache remedy, standing behind it. Name and symbol therefore point at the same old substance through two different languages, which is why the letters of Na appear nowhere in the word sodium.

Compounds containing sodium

NaCl Sodium chloride 2 atoms in one formula unit of an ionic lattice.
NaOH Sodium hydroxide 3 atoms in one formula unit of an ionic lattice.
NaHCO₃ Sodium bicarbonate 6 atoms in one formula unit of an ionic lattice.
Na₂CO₃ Sodium carbonate 6 atoms in one formula unit of an ionic lattice.
CH₃COONa Sodium acetate 8 atoms in one formula unit of an ionic lattice.
C₅H₈NO₄Na Monosodium glutamate 19 atoms in one formula unit of an ionic lattice.

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