Answer:
Subtract the atomic number (the number of protons) from the rounded atomic weight. This gives you the number of neutrons in the most common isotope. Use the interactive periodic table at The Berkeley Laboratory Isotopes Project to find what other isotopes of that element exist.
Step-by-step explanation:
Forms of the same atom that differ only in their number of neutrons are called isotopes. Together, the number of protons and the number of neutrons determine an element's mass number: mass number = protons + neutrons.