Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal. It is an alkali metal, being in group 1 of the periodic table, because it has a single electron in its outer shell, which it readily donates, creating a positively charged ion—the Na+ cation. Its only stable isotope is 23Na. The free metal does not occur in nature, and must be prepared from compounds. It is the sixth most abundant element in the Earth’s crust and exists in numerous minerals such as feldspars, sodalite, and rock salt (NaCl). Many salts of Na are highly water-soluble, Na ions have been leached by the action of water from the Earth’s minerals over eons, and thus Na and chlorine are the most common dissolved elements by weight in the oceans.
It was first isolated by Humphry Davy in 1807 by the electrolysis of Na hydroxide. Among many other useful sodium compounds, sodium hydroxide is used in soap manufacture, and sodium chloride is a de-icing agent and a nutrient for animals including humans. It is an essential element for all animals and some plants.
Characteristics of Sodium
- Sodium at standard temperature and pressure is a soft silvery metal that combines with oxygen in the air and forms grayish white sodium oxide unless immersed in oil or inert gas, which are the conditions it is usually stored in.
- Sodium metal can be easily cut with a knife and It is a good conductor of electricity and heat because it has only one electron in its valence shell, resulting in weak metallic bonding and free electrons, which carry energy.
- Due to having low atomic mass and large atomic radius, sodium is third-least dense of all elemental metals and is one of only three metals that can float on water, the other two being lithium and potassium.
- The melting (98 °C) and boiling (883 °C) points of sodium are lower than those of lithium but higher than those of the heavier alkali metals potassium, rubidium, and caesium, following periodic trends down the group.
- These properties change dramatically at elevated pressures: at 1.5 Mbar, the color changes from silvery metallic to black; at 1.9 Mbar the material becomes transparent with a red color; and at 3 Mbar, sodium is a clear and transparent solid. All of these high-pressure allotropes are insulators and electrides.
- In a flame test, sodium and its compounds glow yellow because the excited 3s electrons of sodium emit a photon when they fall from 3p to 3s; the wavelength of this photon corresponds to the D line at about 589.3 nm. Spin-orbit interactions involving the electron in the 3p orbital split the D line into two, at 589.0 and 589.6 nm; hyperfine structures involving both orbitals cause many more lines.