Answer: isotopes
An isotope is an atom of the same chemical element that has the same number of protons in its nucleus, but not the same number of neutrons.
Therefore, this isotope atom will have the same atomic number and will be located in the same place of the periodic table of the main chemical element, but different mass number.
In other words, the isotopes of a chemical element have a different mass although they have the same atomic number, which is directly related to the number of protons in the nucleus of the atom.
