atomic number (proton number), \(Z\)

https://doi.org/10.1351/goldbook.A00499
The number of protons in the atomic nucleus.
Sources:
Green Book, 2nd ed., p. 20 (https://dev.goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
PAC, 1996, 68, 957. 'Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)' on page 962 (https://doi.org/10.1351/pac199668040957)
Red Book, 3rd ed., p. 35 (https://doi.org/10.1515/pac-2014-0718)