|Definition:|| species of atom characterized by its proton number|
NOTE 1 Atoms in a chemical element may differ by their neutron numbers, i.e. being different isotopes, e.g. hydrogen and deuterium.
NOTE 2 Substances constituted of the same chemical element may differ by their structure, e.g. graphite, diamond, and fullerene, or oxygen O2 and ozone O3.
NOTE 3 A chemical element cannot be transformed into another chemical element by chemical processes.