Write the atomic symbols for two isotopes of carbon, one with the mass number of 12 the other with the mass number of 13.

Isotopes are substances that belong to the same element. They have the same number of protons designated as the atomic number, but they differ in their mass numbers. The mass number is the sum of the neutrons and protons in the nucleus. As a conventional nomenclature, isotopes are written using the symbol of the element, with a subscript as the atomic number, and mass number as the superscript. Because isotopes have the same atomic number, the subscript is usually not included. So, for the isotopes, they are written as

¹²C and ¹³C