Final answer:
The mass number of isotopes is the total number of protons and neutrons in their nucleus, not provided on the periodic table, and is indicated by adding a hyphen and the number to the element's name or symbol, such as Carbon-14.
Step-by-step explanation:
When you are finding isotopes and want to determine their mass numbers, you need to know that the mass number is the sum of protons and neutrons in the nucleus. Unlike the atomic number (the number of protons) which is found on the periodic table, the mass number is not and must be deduced or given. Isotopes of an element can be represented by adding a hyphen and the mass number to the chemical name or symbol, for example, Carbon-14 or C-14. In this representation, '14' is the mass number of the isotope, indicating this carbon atom has 6 protons and 8 neutrons (since 14 minus the atomic number 6, which is the number of protons in carbon, gives you the number of neutrons).