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Chlorine-35

Chlorine-35: atomic number 17, mass number 35, with 17 protons and 18 neutrons.

IsotopeCl
3517Cl
Protons17
Neutrons18
Electrons (neutral)17

The proton count fixes the element: 17 protons identify Chlorine. Mass number 35 is the total number of protons and neutrons.

A neutral Chlorine-35 atom also has 17 electrons. Changing neutrons changes the isotope; changing electrons changes the charge.