Noun

(chemistry) The smallest particle of a specific element or compound that retains the chemical properties of that element or compound; two or more atoms held together by chemical bonds.

* {{quote-magazine, year=2013, month=September-October, author= Katie L. Burke

, magazine=(American Scientist), title= In the News , passage=The critical component of the photosynthetic system is the “water-oxidizing complex”, made up of manganese atoms and a calcium atom. This system splits water molecules' and delivers some of their electrons to other ' molecules that help build up carbohydrates.}}

A tiny amount.