Colour indicators are far from being limited to acid-base reactions. Organic reducing or oxidising agents act as indicators in redox reactions and show the respective equivalence point of the reaction in redox titration procedures (redox indicators). Organic complexing agents are also used as so-called metal indicators in complexometry.
An element is characterised by atoms of the same nuclear charge number. All atoms that contain the same number of protons in the nucleus belong to the same element.
The French chemist Antoine de laVoisier (1743-1794) introduced the term "chemical element" for substances that cannot be further decomposed by chemical processes and cannot be transformed into each other. He thus replaced the four-element theory (fire, water, air, earth) of Aristoteles (384-332 BC).
Today, a chemical element is a pure substance consisting only of atoms with the same number of nuclear charges. The elements differ in the number of protons in their atomic nucleus.
Such elements that consist of a single type of atom, i.e. in which all atoms have the same number of protons in the nucleus and also contain the same number of neutrons in the nucleus, are called pure elements (or anisotopic elements). Examples include aluminium or fluorine.