Final answer:
An isotope is an atom of an element with a different number of neutrons than protons in its nucleus. Some isotopes can become unstable due to having too many neutrons, and these are called radioactive isotopes. Periodic tables of isotopes provide information on the different isotopes of each element.
Step-by-step explanation:
An isotope is an atom of an element with a different number of neutrons than protons in its nucleus. For example, a typical carbon atom on the periodic table of elements should have six protons and six neutrons in its nucleus, called carbon-12. Some atoms of carbon can have an extra neutron in the nucleus, called carbon-13, and some can have two extra neutrons in the nucleus, called carbon-14. If an atom has too many neutrons in its nucleus, it can become unstable because the nucleus has too much energy; this is called a radioactive isotope. Carbon-14 is a radioactive isotope. Just as the periodic table of elements summarizes the most important information about every element, there are periodic tables of isotopes that include all the possible isotopes for each element and their relative abundance and properties.