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Carbon-13

Carbon-13: atomic number 6, mass number 13, with 6 protons and 7 neutrons.

IsotopeC
136C
Protons6
Neutrons7
Electrons (neutral)6

The proton count fixes the element: 6 protons identify Carbon. Mass number 13 is the total number of protons and neutrons.

A neutral Carbon-13 atom also has 6 electrons. Changing neutrons changes the isotope; changing electrons changes the charge.