= Chemical element
{wiki}
A <chemical element> is an identity shared by all <chemical atoms> whose <atomic nuclei> contain the same number of <protons>. That number is its <atomic number>, $Z$: six means <carbon>, eleven means <sodium>. The identity survives differences in <neutron> count, which distinguish <isotopes>, and differences in <electron> count, which distinguish neutral <chemical atoms> from <ions>. See the https://goldbook.iupac.org/terms/view/C01022[IUPAC Gold Book, chemical element].
The same name can also denote a pure <chemical substance> containing only that <chemical element>. These two uses answer different questions. <Molecular oxygen>, $\mathrm O_2$, is a <chemical substance> made entirely of <oxygen>; <water>, $\mathrm H_2\mathrm O$, is a <chemical compound> containing both <hydrogen> and <oxygen>. A <water> <molecule> contains three <chemical atoms> but only two <chemical elements>. Joining <chemical atoms> through <chemical bonds> changes their arrangement without changing their <atomic numbers>.
This distinction makes <stoichiometry> possible: ordinary <chemical reactions> rearrange <chemical atoms>, so one can count each <chemical element> before and after a transformation. Changing the <proton> count instead changes the <chemical element> and requires a <nuclear reaction>. The <periodic table> organizes these identities into a useful map, while <atomic number> bookkeeping keeps identity, <isotope>, and <electric charge> separate.
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