25Mn-55 (the naturally occurring isotope) has 25 protons, 25 electrons and 30 neutrons, 25Mn-54 has 29 neutrons instead but is not a stable isotope. 25Mn-55 (the naturally occurring isotope) has 25 protons, 25 electrons and 30 neutrons, 25Mn-54 has 29 neutrons instead but is not a stable isotope.