The superscript represents the mass number which is made up of neutrons and protons The value of the superscript is 25 . Number of protons + Number of neutrons =25 The subscript represents the atomic number which is the number of electrons in a neutral atom. In a neutral atom, the number of electrons is equal to the number of protons. Atomic number =12 Atomic number = Number of electrons Number of electrons = Number of protons Number of electrons =12 Number of protons =12 Protons + Neutrons =25 12+ Neutrons =25 Neutrons =25−12 Neutrons =13 13 neutrons and 12 protons