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.
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