Sodium

Sodium is an alkali metal so reactive that it must be stored under oil to keep it away from air and water. You may never have seen the pure metal, but you almost certainly ate some today, every grain of table salt contains sodium bonded to chlorine, and sodium ions fire every nerve impulse in your body.

  • Atomic Number1111 protons, 11 electrons
  • Atomic Mass22.9897693 uAbout 23× heavier than hydrogen
  • State at Room TempSolidsoft, silvery metal
  • Density0.97 g/cm³Just less dense than water
  • Melting / Boiling97.8°C / 882.9°CMelts at just 98°C
  • Discovered1807Humphry Davy, 1807

How does sodium compare to other alkali metals?

Sodium sits in the middle of Group 1. Each alkali metal is heavier and more reactive going down the group.

Atomic Mass Comparison
Lithium6.9 u
Sodium23 u
Potassium39.1 u
Iron55.8 u
Caesium133 u

Sodium at 23 atomic mass units is more than three times heavier than lithium (7 u) and about half the mass of caesium (133 u). All share the single outer electron that drives their reactivity.

What is sodium?

Sodium is an alkali metal in Group 1 of the periodic table. It has 11 protons and a single electron in its outer shell, which it gives away so readily that sodium reacts instantly with water, fizzing and hissing as it releases hydrogen gas and sometimes catching fire from the heat produced. Pure sodium is a soft, silver-white metal that can be cut with a table knife and tarnishes to a dull grey within seconds in air.

The English word sodium comes from the medieval Latin sodanum. The chemical symbol Na comes from natrium, the Latin word for a natural soda mineral (sodium carbonate) found in dry lake beds in Egypt. Both names have been used historically in different countries, which is why the element has two names in common use even today.

Fact Sodium metal dropped in water fizzes vigorously and slides across the surface on a cushion of hydrogen gas it produces. Larger pieces burst into a yellow-orange flame from the sodium vapour. Despite this violence, the sodium compound in your food, table salt, is perfectly stable and essential for life.

Where you find sodium

In space

Sodium is found across the solar system. Mercury has a thin exosphere containing sodium atoms blasted off its surface by the solar wind. Jupiter's moon Io ejects clouds of sodium into space from its volcanoes, creating a vast glowing cloud.

On Earth

Sodium is the sixth most abundant element in the Earth's crust, but because it reacts so easily, it is never found as a free metal, always bonded to other elements in compounds.

  • Rock salt and salt mines. Sodium chloride (NaCl), table salt, is found in vast underground deposits formed when ancient seas dried up. Major salt mines exist in the UK, USA, Germany, China, India and many other countries.
  • The oceans. Seawater contains approx. 3.5% dissolved salts, mostly sodium chloride. The oceans hold an almost unimaginable quantity of sodium.
  • Other minerals. Sodium is found in borax, sodium carbonate (soda ash) and dozens of other minerals formed in dry lake beds and volcanic regions.

How we use sodium

  • Table salt. Sodium chloride (NaCl) is the most familiar sodium compound. It seasons food, preserves meat and fish, and is used in enormous quantities in the chemical industry as a starting material for other chemicals.
  • Street lighting. Sodium vapour lamps produce a distinctive yellow-orange glow that cuts through fog and mist better than white light. Low-pressure sodium lamps were once the most common type of street light.
  • Soap and glass making. Sodium hydroxide (caustic soda) is used to make soap from fats and oils. Sodium carbonate (soda ash) is a key ingredient in glass making.
  • Biology. Sodium ions (Na⁺) are essential for life. Nerves send signals using a flow of sodium and potassium ions across cell membranes. Your kidneys carefully regulate blood sodium levels at all times.
Did you know? Humans have been mining and trading salt for thousands of years. The Roman word salarium: the payment given to Roman soldiers to buy salt, is where we get the English word "salary". Salt was so valuable in ancient times that wars were fought over it, and some societies used it as currency.

How it was discovered

Sodium was isolated for the first time in 1807 by the British chemist Humphry Davy at the Royal Institution in London. He passed an electric current through molten sodium hydroxide (caustic soda) and small silvery globules of sodium metal appeared at the negative electrode. Davy was reportedly so excited that he danced around his laboratory. He discovered potassium on the same evening using the same method: one of the most productive evenings in the history of chemistry.

Deeper dive: sodium's role in biology and the nerve impulse

Sodium is central to how your nervous system works. Every nerve impulse, including the signals that make your heart beat, your muscles contract and your brain think, relies on sodium ions rushing across cell membranes. Cell membranes have protein channels that open to let sodium ions (Na⁺) flood in. This sudden shift in electrical charge, called an action potential, travels along the nerve fibre as an impulse, at speeds up to 120 metres per second.

After each impulse, sodium-potassium pumps in the membrane use energy from ATP to push three sodium ions out and pull two potassium ions back in, resetting the system. This pumping uses about one third of all the energy your brain consumes, showing how critical maintaining sodium balance is to life.

Too much sodium in the diet raises blood pressure by making the body retain extra water to dilute the excess salt in the blood. This extra fluid puts strain on the heart and blood vessels. Public health guidelines in most countries recommend limiting salt intake to reduce the risk of heart disease and stroke. The World Health Organisation recommends less than 5 grams of salt per day for adults.

Sodium is one of the most reactive metals and one of the most important elements for life. Moving along the same row of the periodic table to 12 protons brings you to magnesium, a metal that burns with a brilliant white flame.