List the numbers of protons, neutrons, and electrons in Chlorine-18.

Answer 1

the number of protons is the atomic number of an element
... its number in the periodic table

atoms are neutral ... so the number of electrons is the same as protons

elements (atoms) have isotopes
... same number of protons , but different number of neutrons

the 18 after the chlorine is not standard nomenclature
... but it probably indicates the number of neutrons