Did you know? An atom is made up of protons, neutrons and electrons. The position of an element in the periodic table is based only on the number of protons in its nucleus, which is known as the atomic number.
Isotopes, molecules and ions are indicated using various superscript and subscript notations.
The mass of an atom includes both the protons and neutrons, and is usually the same for each atom of an element; however, some variants (isotopes) occur, which have different numbers of neutrons. A familiar example is the carbon-14 isotope of carbon, which is used for carbon dating of ancient organic remains (the common form of carbon is carbon-12).
When referring to an isotope in general text, use the full name with a hyphen and the mass number:
In technical texts, tables and figures, use the element symbol with the mass number of the atom in the left superscript position:
12C 14C 226Ra 32S
When the isotopic symbol is used as a descriptor, place it immediately before the part of the chemical name it describes in square brackets, with no space before the chemical name:
Include any modifiers in the square brackets before the isotopic symbol:
l-[methyl-14C]methionine [2,3-3H]serine
Brackets are not used when the isotopic mass symbol is attached to the name of a compound that does not normally contain the element concerned:
or to a word that is not a specific chemical compound:
14C-amino acids 125I-labelled protein
If quoting both the mass number and the atomic number, use the following format (note that the numbers are right-aligned):
