Isotope ¹⁸F⁻ contains:
1) p⁺ = 9; number of protons.
Fluorine has atomic number Z = 9 (total number of protons).
2) e⁻ = 10; number of electrons.
In element number of electrons and protons are the same, because element has neutral charge, but because in this example, fluorine is anion with negative charge, it has one electron more.
3) n° = 9; number of neutrons.
Mass number
A = 18 is total number of protons and neutrons in a nucleus, so number of neutrons is A-Z = 18-9=9.
