Sodium

Sodium was an element. It was a vital nutrient and was necessary for living things to grow. (TV: The Waters of Mars)

Sodium in space when lit up by the sun appeared yellow, it was used by the Moonbase in 2070 as a distress rocked signal. (TV: The Moonbase ^[+]Loading...["The Moonbase (TV story)"])

Sodium chloride was the chemical name for salt. (TV: The Sensorites, Image of the Fendahl, Four to Doomsday et al.) Rex Matheson once had low sodium levels in his blood and needed some salt. (TV: Rendition)

Captain Maitland's spectrograph of minerals in the vicinity of the Sense Sphere showed emission wavelengths of "ordinary" oxygen, hydrogen, and sodium, as well as the rare molybdenum. (TV: The Sensorites)

Sodium ethoxide (C₂H₅ONa) was the derivative of sodium acting on ethyl alcohol. (TV: The Power of the Daleks)