Chemical element symbols (Fe, Au, Na)

How to read it

The IUPAC Green Book says the symbols are in most cases taken from the elements’ Latin names, are printed upright, and have no full stop after them. OpenStax’s Chemistry 2e lists the common ones that come from another name: Fe from ferrum (iron), Au from aurum (gold), Na from natrium (sodium), K from kalium (potassium), Cu from cuprum (copper), Ag from argentum (silver), Pb from plumbum (lead), Hg from hydrargyrum (mercury) and Sn from stannum (tin). A symbol can stand for one atom or for any amount of the element: OpenStax’s example is Hg on a jar of mercury, and the Green Book’s are Cl for a chlorine atom and Fe for a sample of iron.

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Why only the first letter is a capital

OpenStax gives the reason: Co is the element cobalt, but CO, with two capitals, is carbon monoxide, a compound of carbon (C) and oxygen (O). The same rule separates the unit cd from the element cadmium, Cd.

Numbers around the symbol

NIST’s Guide for the Use of the SI gives each corner its own job. The mass number goes top left (²⁸Si), the atomic number bottom left (₂₉Cu), the number of atoms in a molecule bottom right (¹H₂), and the charge or state of excitation top right.

Who names the elements

IUPAC does. Until it recognises a name, OpenStax explains, a new element is called after the Latin for its atomic number: element 106 was unnilhexium (Unh), and three-letter symbols have been used for elements above 112. On 28 November 2016 IUPAC approved the names and symbols of the four most recent: nihonium (Nh) for element 113, moscovium (Mc) for 115, tennessine (Ts) for 117 and oganesson (Og) for 118. IUPAC said that, keeping with tradition, they were named after a place or region or a scientist; nihonium comes from Nihon, one of the two ways of saying “Japan” in Japanese.