Isotopes represent atoms of an element that contain the same atomic number (same number of protons), but different mass numbers (different number of neutrons).
The atomic mass of an element that you find on the periodic table represents the average masses of its isotopes.
Recall that the mass number or atomic mass of an element represents the total number of its protons and neutrons, while the atomic number represents the total number of its protons.
Concept #1: The atomic masses listed for the elements on the periodic table are the average masses of their isotopes.
Isotopes are elements with the same number of protons, but different number of neutrons.
Concept #2: Calculating the average mass of an element
Example #1: Antimony has two common isotopes. If one of the isotopes 121Sb has an isotopic mass of 120.9038 amu and a natural abundance of 57.25%, what is the isotopic mass (to 4 significant figures) of the other isotope? The atomic mass of antimony is 121.8 g/mol.
Example #2: The atomic mass of an imaginary element A is 251.7 amu. If element A consists of two isotopes that have atomic masses of 250 and 253 respectively, what is the natural abundance of each isotope?
A lesser known term referred to as isotones involves atoms that have the same number of neutrons, but different number of protons.